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Purpose: Aim of this article is to describe a new knowledge-based planning (KBP) methodology using volumetric modulated arc therapy (VMAT) for stereotactic radiosurgery (SRS) and radiotherapy (SRT) assisted by an ensemble mapping technique for use in a Monte Carlo planning system., Methods: Libraries of 121 stereotactic patients were assembled on the basis of eight different parameters (a) tumor laterality, (b) whether planning target volume (PTV) dose coverage challenged by the presence of the organ at risk (OAR), (c) prescription dose and number of fractions, (d) number of PTVs, (e) tumor volume, (f) shortest distance between OAR and PTV (edge to edge distance, or EED), (g) center to center distance between OARs and PTV (CCD), and (h) lateral dimension of external contour (brain). For new patients, the most appropriate library plan was selected on the basis of the above categorization. A KBP plan was created based on this selected library plan with all parameters unchanged keeping the isocenter at the center of PTV. Using the same beam configuration, another independent treatment plan (IP) was generated by an experienced dosimetrist for comparison. IP and KBP were compared for 76 new patients., Results: Of 197 patients (121 library and 76 new), 103 (52.3%) were placed in the OAR-challenged category and 94 were placed in the OAR unchallenged category. The ensemble mapping technique shows that, for an OAR-challenged patient, picking up the library plan is appropriate. IP was marginally better than KBP in PTV coverage and dose conformity (PCI). Library plans, IP, and KBP offer a mean PCI of 0.77 ± 0.2, 0.79 ± 0.2, and 0.78 ± 0.4, mean PTV-V99% of 97.3 ± 22.0%, 98.9 ± 14.1%, and 98.2 ± 13.2%, and mean MU of 2403.8 ± 2403.8, 2344.0 ± 2423.6, and 2473.6 ± 2296.8, respectively. Statistically significant differences were observed in the planning time between the IP and KBP plans for both OAR-challenged (P < 0.001) and -unchallenged (P < 0.002) categories. Comparison of optimization and dose calculation time showed a much lower average planning time of 111.0 ± 84.1 min for KBP as against 248.2 ± 96.6 min for IP., Conclusion: Validation results for KBP plans indicate the multidimensional ensemble mapping mechanism can accurately pick up the most appropriate library plan. KBP plans, although slightly inferior in their dosimetric quality, fulfill all the required clinical conditions and dose constraints. KBP plans save considerable planning time and are nearly independent of the skill and knowledge of the treatment planner. KBP works well with a Monte Carlo planning system like Monaco., (© 2019 American Association of Physicists in Medicine.)

Purpose: Multileaf collimator (MLC) tracking is being clinically pioneered to continuously compensate for thoracic and pelvic motion during radiotherapy. The purpose of this work was to characterize the performance of two MLC leaf-fitting algorithms, direct optimization and piecewise optimization, for real-time motion compensation with different plan complexity and tumor trajectories., Methods: To test the algorithms, both in silico and phantom experiments were performed. The phantom experiments were performed on a Trilogy Varian linac and a HexaMotion programmable motion platform. High and low modulation VMAT plans for lung and prostate cancer cases were used along with eight patient-measured organ-specific trajectories. For both MLC leaf-fitting algorithms, the plans were run with their corresponding patient trajectories. To compare algorithms, the average exposure errors, i.e., the difference in shape between ideal and fitted MLC leaves by the algorithm, plan complexity and system latency of each experiment were calculated., Results: Comparison of exposure errors for the in silico and phantom experiments showed minor differences between the two algorithms. The average exposure errors for in silico experiments with low/high plan complexity were 0.66/0.88 cm 2 for direct optimization and 0.66/0.88 cm 2 for piecewise optimization, respectively. The average exposure errors for the phantom experiments with low/high plan complexity were 0.73/1.02 cm 2 for direct and 0.73/1.02 cm 2 for piecewise optimization, respectively. The measured latency for the direct optimization was 226 ± 10 ms and for the piecewise algorithm was 228 ± 10 ms. In silico and phantom exposure errors quantified for each treatment plan demonstrated that the exposure errors from the high plan complexity (0.96 cm 2 mean, 2.88 cm 2 95% percentile) were all significantly different from the low plan complexity (0.70 cm 2 mean, 2.18 cm 2 95% percentile) (P < 0.001, two-tailed, Mann-Whitney statistical test)., Conclusions: The comparison between the two leaf-fitting algorithms demonstrated no significant differences in exposure errors, neither in silico nor with phantom experiments. This study revealed that plan complexity impacts the overall exposure errors significantly more than the difference between the algorithms., (© 2019 American Association of Physicists in Medicine.)

Purpose: Proton dose distribution is sensitive to uncertainties in range estimation and patient positioning. Currently, the proton robustness is managed by worst-case scenario optimization methods, which are computationally inefficient. To overcome these challenges, we develop a novel intensity-modulated proton therapy (IMPT) optimization method that integrates dose fidelity with a sensitivity term that describes dose perturbation as the result of range and positioning uncertainties., Methods: In the integrated optimization framework, the optimization cost function is formulated to include two terms: a dose fidelity term and a robustness term penalizing the inner product of the scanning spot sensitivity and intensity. The sensitivity of an IMPT scanning spot to perturbations is defined as the dose distribution variation induced by range and positioning errors. To evaluate the sensitivity, the spatial gradient of the dose distribution of a specific spot is first calculated. The spot sensitivity is then determined by the total absolute value of the directional gradients of all affected voxels. The fast iterative shrinkage-thresholding algorithm is used to solve the optimization problem. This method was tested on three skull base tumor (SBT) patients and three bilateral head-and-neck (H&N) patients. The proposed sensitivity-regularized method (SenR) was implemented on both clinic target volume (CTV) and planning target volume (PTV). They were compared with conventional PTV-based optimization method (Conv) and CTV-based voxel-wise worst-case scenario optimization approach (WC)., Results: Under the nominal condition without uncertainties, the three methods achieved similar CTV dose coverage, while the CTV-based SenR approach better spared organs at risks (OARs) compared with the WC approach, with an average reduction of [Dmean, Dmax] of [4.72, 3.38]  GyRBE for the SBT cases and [2.54, 3.33] GyRBE for the H&N cases. The OAR sparing of the PTV-based SenR method was comparable with the WC method. The WC method, and SenR approaches all improved the plan robustness from the conventional PTV-based method. On average, under range uncertainties, the lowest [D95%, V95%, V100%] of CTV were increased from [93.75%, 88.47%, 47.37%] in the Conv method, to [99.28%, 99.51%, 86.64%] in the WC method, [97.71%, 97.85%, 81.65%] in the SenR-CTV method and [98.77%, 99.30%, 85.12%] in the SenR-PTV method, respectively. Under setup uncertainties, the average lowest [D95%, V95%, V100%] of CTV were increased from [95.35%, 94.92%, 65.12%] in the Conv method, to [99.43%, 99.63%, 87.12%] in the WC method, [96.97%, 97.13%, 77.86%] in the SenR-CTV method, and [98.21%, 98.34%, 83.88%] in the SenR-PTV method, respectively. The runtime of the SenR optimization is eight times shorter than that of the voxel-wise worst-case method., Conclusion: We developed a novel computationally efficient robust optimization method for IMPT. The robustness is calculated as the spot sensitivity to both range and shift perturbations. The dose fidelity term is then regularized by the sensitivity term for the flexibility and trade-off between the dosimetry and the robustness. In the stress test, SenR is more resilient to unexpected uncertainties. These advantages in combination with its fast computation time make it a viable candidate for clinical IMPT planning., (© 2018 American Association of Physicists in Medicine.)

Purpose: The goal of this work was to develop and evaluate a fast inverse direct aperture optimization (FIDAO) algorithm for IMRT treatment planning and plan adaptation., Methods: A previously proposed fluence map optimization algorithm called fast inverse dose optimization (FIDO) was extended to optimize the aperture shapes and weights of IMRT beams. FIDO is a very fast fluence map optimization algorithm for IMRT that finds the global minimum using direct matrix inversion without unphysical negative beam weights. In this study, an equivalent second-order Taylor series expansion of the FIDO objective function was used, which allowed for the objective function value and gradient vector to be computed very efficiently during direct aperture optimization, resulting in faster optimization. To evaluate the speed gained with FIDAO, a proof-of-concept algorithm was developed in MATLAB using an interior-point optimization method to solve the reformulated aperture-based FIDO problem. The FIDAO algorithm was used to optimize four step-and-shoot IMRT cases: on the AAPM TG-119 phantom as well as a liver, prostate, and head-and-neck clinical cases. Results were compared with a conventional DAO algorithm that uses the same interior-point method but using the standard formulation of the objective function and its gradient vector., Results: A substantial gain in optimization speed was obtained with the prototype FIDAO algorithm compared to the conventional DAO algorithm while producing plans of similar quality. The optimization time (number of iterations) for the prototype FIDAO algorithm vs the conventional DAO algorithm was 0.3 s (17) vs 56.7 s (50); 2.0 s (28) vs 134.1 s (57); 2.5 s (26) vs 180.6 s (107); and 6.7 s (20) vs 469.4 s (482) in the TG-119 phantom, liver, prostate, and head-and-neck examples, respectively., Conclusions: A new direct aperture optimization algorithm based on FIDO was developed. For the four IMRT test cases examined, this algorithm executed approximately 70-200 times faster without compromising the IMRT plan quality., (© 2018 American Association of Physicists in Medicine.)

Purpose: The purposes of this study were two-fold: first, to develop a four-axis moving phantom for patient-specific quality assurance (QA) in surrogate signal-based dynamic tumor-tracking intensity-modulated radiotherapy (DTT-IMRT), and second, to evaluate the accuracy of the moving phantom and perform patient-specific dosimetric QA of the surrogate signal-based DTT-IMRT., Methods: The four-axis moving phantom comprised three orthogonal linear actuators for target motion and a fourth one for surrogate motion. The positional accuracy was verified using four laser displacement gauges under static conditions (±40 mm displacements along each axis) and moving conditions [eight regular sinusoidal and fourth-power-of-sinusoidal patterns with peak-to-peak motion ranges (H) of 10-80 mm and a breathing period (T) of 4 s, and three irregular respiratory patterns with H of 1.4-2.5 mm in the left-right, 7.7-11.6 mm in the superior-inferior, and 3.1-4.2 mm in the anterior-posterior directions for the target motion, and 4.8-14.5 mm in the anterior-posterior direction for the surrogate motion, and T of 3.9-4.9 s]. Furthermore, perpendicularity, defined as the vector angle between any two axes, was measured using an optical measurement system. The reproducibility of the uncertainties in DTT-IMRT was then evaluated. Respiratory motions from 20 patients acquired in advance were reproduced and compared three-dimensionally with the originals. Furthermore, patient-specific dosimetric QAs of DTT-IMRT were performed for ten pancreatic cancer patients. The doses delivered to Gafchromic films under tracking and moving conditions were compared with those delivered under static conditions without dose normalization., Results: Positional errors of the moving phantom under static and moving conditions were within 0.05 mm. The perpendicularity of the moving phantom was within 0.2° of 90°. The differences in prediction errors between the original and reproduced respiratory motions were -0.1 ± 0.1 mm for the lateral direction, -0.1 ± 0.2 mm for the superior-inferior direction, and -0.1 ± 0.1 mm for the anterior-posterior direction. The dosimetric accuracy showed significant improvements, of 92.9% ± 4.0% with tracking versus 69.8% ± 7.4% without tracking, in the passing rates of γ with the criterion of 3%/1 mm (p < 0.001). Although the dosimetric accuracy of IMRT without tracking showed a significant negative correlation with the 3D motion range of the target (r = - 0.59, p < 0.05), there was no significant correlation for DTT-IMRT (r = 0.03, p = 0.464)., Conclusions: The developed four-axis moving phantom had sufficient accuracy to reproduce patient respiratory motions, allowing patient-specific QA of the surrogate signal-based DTT-IMRT under realistic conditions. Although IMRT without tracking decreased the dosimetric accuracy as the target motion increased, the DTT-IMRT achieved high dosimetric accuracy.

Purpose: The aim of this work is to search for new metrics that could give more reliable acceptance/rejection criteria on the IMRT verification process and to offer solutions to the discrepancies found among different conventional metrics. Therefore, besides conventional metrics, new ones are proposed and evaluated with new tools to find correlations among them. These new metrics are based on the processing of the dose-volume histogram information, evaluating the absorbed dose differences, the dose constraint fulfillment, or modified biomathematical treatment outcome models such as tumor control probability (TCP) and normal tissue complication probability (NTCP). An additional purpose is to establish whether the new metrics yield the same acceptance/rejection plan distribution as the conventional ones., Methods: Fifty eight treatment plans concerning several patient locations are analyzed. All of them were verified prior to the treatment, using conventional metrics, and retrospectively after the treatment with the new metrics. These new metrics include the definition of three continuous functions, based on dose-volume histograms resulting from measurements evaluated with a reconstructed dose system and also with a Monte Carlo redundant calculation. The 3D gamma function for every volume of interest is also calculated. The information is also processed to obtain ΔTCP or ΔNTCP for the considered volumes of interest. These biomathematical treatment outcome models have been modified to increase their sensitivity to dose changes. A robustness index from a radiobiological point of view is defined to classify plans in robustness against dose changes., Results: Dose difference metrics can be condensed in a single parameter: the dose difference global function, with an optimal cutoff that can be determined from a receiver operating characteristics (ROC) analysis of the metric. It is not always possible to correlate differences in biomathematical treatment outcome models with dose difference metrics. This is due to the fact that the dose constraint is often far from the dose that has an actual impact on the radiobiological model, and therefore, biomathematical treatment outcome models are insensitive to big dose differences between the verification system and the treatment planning system. As an alternative, the use of modified radiobiological models which provides a better correlation is proposed. In any case, it is better to choose robust plans from a radiobiological point of view. The robustness index defined in this work is a good predictor of the plan rejection probability according to metrics derived from modified radiobiological models. The global 3D gamma-based metric calculated for each plan volume shows a good correlation with the dose difference metrics and presents a good performance in the acceptance/rejection process. Some discrepancies have been found in dose reconstruction depending on the algorithm employed. Significant and unavoidable discrepancies were found between the conventional metrics and the new ones., Conclusions: The dose difference global function and the 3D gamma for each plan volume are good classifiers regarding dose difference metrics. ROC analysis is useful to evaluate the predictive power of the new metrics. The correlation between biomathematical treatment outcome models and the dose difference-based metrics is enhanced by using modified TCP and NTCP functions that take into account the dose constraints for each plan. The robustness index is useful to evaluate if a plan is likely to be rejected. Conventional verification should be replaced by the new metrics, which are clinically more relevant.

Purpose: A detailed analysis of 2728 intensity modulated proton therapy (IMPT) fields that were clinically delivered to patients between 2007 and 2013 at Paul Scherrer Institute (PSI) was performed. The aim of this study was to analyze the results of patient specific dosimetric verifications and to assess possible correlation between the quality assurance (QA) results and specific field metrics., Methods: Dosimetric verifications were performed for every IMPT field prior to patient treatment. For every field, a steering file was generated containing all the treatment unit information necessary for treatment delivery: beam energy, beam angle, dose, size of air gap, nuclear interaction (NI) correction factor, number of range shifter plates, number of Bragg peaks (BPs) with their position and weight. This information was extracted and correlated to the results of dosimetric verification of each field which was a measurement of two orthogonal profiles using an orthogonal ionization chamber array in a movable water column., Results: The data analysis has shown more than 94% of all verified plans were within defined clinical tolerances. The differences between measured and calculated dose depend critically on the number of BPs, total thickness of all range shifter plates inserted in the beam path, and maximal range. An increase of the dose difference was observed with smaller number of BPs (i.e., smaller tumor) and smaller ranges (i.e., superficial tumors). The results of the verification do not depend, however, on the prescribed dose, NI correction, or the size of the air gap. There is no dependency of the transversal and longitudinal spot position precision on the beam angle. The value of NI correction depends on the number of spots and number of range shifter plates., Conclusions: The presented study has shown that the verification method used at Centre for Proton Therapy at Paul Scherrer Institute is accurate and reproducible for performing patient specific QA. The results confirmed that the dose discrepancy is dependent on the size and location of the tumor.

The increasing complexity of modern radiation therapy planning and delivery challenges traditional prescriptive quality management (QM) methods, such as many of those included in guidelines published by organizations such as the AAPM, ASTRO, ACR, ESTRO, and IAEA. These prescriptive guidelines have traditionally focused on monitoring all aspects of the functional performance of radiotherapy (RT) equipment by comparing parameters against tolerances set at strict but achievable values. Many errors that occur in radiation oncology are not due to failures in devices and software; rather they are failures in workflow and process. A systematic understanding of the likelihood and clinical impact of possible failures throughout a course of radiotherapy is needed to direct limit QM resources efficiently to produce maximum safety and quality of patient care. Task Group 100 of the AAPM has taken a broad view of these issues and has developed a framework for designing QM activities, based on estimates of the probability of identified failures and their clinical outcome through the RT planning and delivery process. The Task Group has chosen a specific radiotherapy process required for "intensity modulated radiation therapy (IMRT)" as a case study. The goal of this work is to apply modern risk-based analysis techniques to this complex RT process in order to demonstrate to the RT community that such techniques may help identify more effective and efficient ways to enhance the safety and quality of our treatment processes. The task group generated by consensus an example quality management program strategy for the IMRT process performed at the institution of one of the authors. This report describes the methodology and nomenclature developed, presents the process maps, FMEAs, fault trees, and QM programs developed, and makes suggestions on how this information could be used in the clinic. The development and implementation of risk-assessment techniques will make radiation therapy safer and more efficient.

Purpose: The gamma comparison is widely used to evaluate the agreement between measurements and treatment planning system calculations in patient-specific intensity modulated radiation therapy (IMRT) quality assurance (QA). However, recent publications have raised concerns about the lack of sensitivity when employing commonly used gamma criteria. Understanding the actual sensitivity of a wide range of different gamma criteria may allow the definition of more meaningful gamma criteria and tolerance limits in IMRT QA. We present a method that allows the quantitative determination of gamma criteria sensitivity to induced errors which can be applied to any unique combination of device, delivery technique, and software utilized in a specific clinic., Methods: A total of 21 DMLC IMRT QA measurements (ArcCHECK®, Sun Nuclear) were compared to QA plan calculations with induced errors. Three scenarios were studied: MU errors, multi-leaf collimator (MLC) errors, and the sensitivity of the gamma comparison to changes in penumbra width. Gamma comparisons were performed between measurements and error-induced calculations using a wide range of gamma criteria, resulting in a total of over 20 000 gamma comparisons. Gamma passing rates for each error class and case were graphed against error magnitude to create error curves in order to represent the range of missed errors in routine IMRT QA using 36 different gamma criteria., Results: This study demonstrates that systematic errors and case-specific errors can be detected by the error curve analysis. Depending on the location of the error curve peak (e.g., not centered about zero), 3%/3 mm threshold = 10% at 90% pixels passing may miss errors as large as 15% MU errors and ±1 cm random MLC errors for some cases. As the dose threshold parameter was increased for a given %Diff/distance-to-agreement (DTA) setting, error sensitivity was increased by up to a factor of two for select cases. This increased sensitivity with increasing dose threshold was consistent across all studied combinations of %Diff/DTA. Criteria such as 2%/3 mm and 3%/2 mm with a 50% threshold at 90% pixels passing are shown to be more appropriately sensitive without being overly strict. However, a broadening of the penumbra by as much as 5 mm in the beam configuration was difficult to detect with commonly used criteria, as well as with the previously mentioned criteria utilizing a threshold of 50%., Conclusions: We have introduced the error curve method, an analysis technique which allows the quantitative determination of gamma criteria sensitivity to induced errors. The application of the error curve method using DMLC IMRT plans measured on the ArcCHECK® device demonstrated that large errors can potentially be missed in IMRT QA with commonly used gamma criteria (e.g., 3%/3 mm, threshold = 10%, 90% pixels passing). Additionally, increasing the dose threshold value can offer dramatic increases in error sensitivity. This approach may allow the selection of more meaningful gamma criteria for IMRT QA and is straightforward to apply to other combinations of devices and treatment techniques.

Purpose: The aim of this work was to use a multicenter audit of modulated radiotherapy quality assurance (QA) data to provide a practical examination of gamma evaluation criteria and action level selection. The use of the gamma evaluation method for patient-specific pretreatment QA is widespread, with most commercial solutions implementing the method., Methods: Gamma agreement indices were calculated using the criteria 1%/1 mm, 2%/2 mm, 2%/3 mm, 3%/2 mm, 3%/3 mm, and 5%/3 mm for 1265 pretreatment QA measurements, planned at seven treatment centers, using four different treatment planning systems, delivered using three different delivery systems (intensity-modulated radiation therapy, volumetric-modulated arc therapy, and helical tomotherapy) and measured using three different dose measurement systems. The sensitivity of each pair of gamma criteria was evaluated relative to the gamma agreement indices calculated using 3%/3 mm., Results: A linear relationship was observed for 2%/2 mm, 2%/3 mm, and 3%/2 mm. This result implies that most beams failing at 3%/3 mm would also fail for those criteria, if the action level was adjusted appropriately. Some borderline plans might be passed or failed depending on the relative priority (tighter tolerance) used for dose difference or distance to agreement evaluation. Dosimeter resolution and treatment modality were found to have a smaller effect on the results of QA measurements than the number of dimensions (2D or 3D) over which the gamma evaluation was calculated., Conclusions: This work provides a method (and a large sample of results) for calculating equivalent action levels for different gamma evaluation criteria. This work constitutes a valuable guide for clinical decision making and a means to compare published gamma evaluation results from studies using different evaluation criteria. More generally, the data provided by this work support the recommendation that gamma criteria that specifically prioritize the property of greatest clinical importance for each treatment modality of anatomical site should be selected when using gamma evaluations for modulated radiotherapy QA. It is therefore suggested that departments using the gamma evaluation as a QA analysis tool should consider the relative importance of dose difference and distance to agreement, when selecting gamma evaluation criteria.

Purpose: High-quality radiation therapy using highly conformal dose distributions and image-guided techniques requires optimum machine delivery performance. In this work, a monitoring system for multileaf collimator (MLC) performance, integrating semiautomated MLC quality control (QC) tests and statistical process control tools, was developed. The MLC performance monitoring system was used for almost a year on two commercially available MLC models. Control charts were used to establish MLC performance and assess test frequency required to achieve a given level of performance. MLC-related interlocks and servicing events were recorded during the monitoring period and were investigated as indicators of MLC performance variations., Methods: The QC test developed as part of the MLC performance monitoring system uses 2D megavoltage images (acquired using an electronic portal imaging device) of 23 fields to determine the location of the leaves with respect to the radiation isocenter. The precision of the MLC performance monitoring QC test and the MLC itself was assessed by detecting the MLC leaf positions on 127 megavoltage images of a static field. After initial calibration, the MLC performance monitoring QC test was performed 3-4 times/week over a period of 10-11 months to monitor positional accuracy of individual leaves for two different MLC models. Analysis of test results was performed using individuals control charts per leaf with control limits computed based on the measurements as well as two sets of specifications of ± 0.5 and ± 1 mm. Out-of-specification and out-of-control leaves were automatically flagged by the monitoring system and reviewed monthly by physicists. MLC-related interlocks reported by the linear accelerator and servicing events were recorded to help identify potential causes of nonrandom MLC leaf positioning variations., Results: The precision of the MLC performance monitoring QC test and the MLC itself was within ± 0.22 mm for most MLC leaves and the majority of the apparent leaf motion was attributed to beam spot displacements between irradiations. The MLC QC test was performed 193 and 162 times over the monitoring period for the studied units and recalibration had to be repeated up to three times on one of these units. For both units, rate of MLC interlocks was moderately associated with MLC servicing events. The strongest association with the MLC performance was observed between the MLC servicing events and the total number of out-of-control leaves. The average elapsed time for which the number of out-of-specification or out-of-control leaves was within a given performance threshold was computed and used to assess adequacy of MLC test frequency., Conclusions: A MLC performance monitoring system has been developed and implemented to acquire high-quality QC data at high frequency. This is enabled by the relatively short acquisition time for the images and automatic image analysis. The monitoring system was also used to record and track the rate of MLC-related interlocks and servicing events. MLC performances for two commercially available MLC models have been assessed and the results support monthly test frequency for widely accepted ± 1 mm specifications. Higher QC test frequency is however required to maintain tighter specification and in-control behavior.

Purpose: A quantitative 3D intramodality ultrasound (US) imaging system was verified for daily in-room prostate localization, and compared to prostate localization based on implanted fiducial markers (FMs)., Methods: Thirteen prostate patients underwent multiple US scans during treatment. A total of 376 US-scans and 817 matches were used to determine the intra- and interoperator variability. Additionally, eight other patients underwent daily prostate localization using both US and electronic portal imaging (EPI) with FMs resulting in 244 combined US-EPI scans. Scanning was performed with minimal probe pressure and a correction for the speed of sound aberration was performed. Uncertainties of both US and FM methods were assessed. User variability of the US method was assessed., Results: The overall US user variability is 2.6 mm. The mean differences between US and FM are: 2.5 ± 4.0 mm (LR), 0.6 ± 4.9 mm (SI), and -2.3 ± 3.6 mm (AP). The intramodality character of this US system mitigates potential errors due to transducer pressure and speed of sound aberrations., Conclusions: The overall accuracy of US (3.0 mm) is comparable to our FM workflow (2.2 mm). Since neither US nor FM can be considered a gold standard no conclusions can be drawn on the superiority of either method. Because US imaging captures the prostate itself instead of surrogates no invasive procedure is required. It requires more effort to standardize US imaging than FM detection. Since US imaging does not involve a radiation burden, US prostate imaging offers an alternative for FM EPI positioning.

Purpose: The delivery of volumetric modulated arc therapy (VMAT) is more complex than other conformal radiotherapy techniques. In this work, the authors present the feasibility of performing routine verification of VMAT delivery using a dose matrix measured by a gantry mounted 2D ion chamber array and corresponding dose matrix calculated by an inhouse developed algorithm., Methods: Pinnacle, v9.0, treatment planning system (TPS) was used in this study to generate VMAT plans for a 6 MV photon beam from an Elekta-Synergy linear accelerator. An algorithm was developed and implemented with inhouse computer code to calculate the dose matrix resulting from a VMAT arc in a plane perpendicular to the beam at isocenter. The algorithm was validated using measurement of standard patterns and clinical VMAT plans with a 2D ion chamber array. The clinical VMAT plans were also validated using ArcCHECK measurements. The measured and calculated dose matrices were compared using gamma (γ) analysis with 3%/3 mm criteria and γ tolerance of 1., Results: The dose matrix comparison of standard patterns has shown excellent agreement with the mean γ pass rate 97.7 (σ = 0.4)%. The validation of clinical VMAT plans using the dose matrix predicted by the algorithm and the corresponding measured dose matrices also showed good agreement with the mean γ pass rate of 97.6 (σ = 1.6)%. The validation of clinical VMAT plans using ArcCHECK measurements showed a mean pass rate of 95.6 (σ = 1.8)%., Conclusions: The developed algorithm was shown to accurately predict the dose matrix, in a plane perpendicular to the beam, by considering all possible leaf trajectories in a VMAT delivery. This enables the verification of VMAT delivery using a 2D array detector mounted on a treatment head.

Purpose: This work uses repeat images of intensity modulated radiation therapy (IMRT) fields to quantify fluence anomalies (i.e., delivery errors) that can be reliably detected in electronic portal images used for IMRT pretreatment quality assurance., Methods: Repeat images of 11 clinical IMRT fields are acquired on a Varian Trilogy linear accelerator at energies of 6 MV and 18 MV. Acquired images are corrected for output variations and registered to minimize the impact of linear accelerator and electronic portal imaging device (EPID) positioning deviations. Detection studies are performed in which rectangular anomalies of various sizes are inserted into the images. The performance of detection strategies based on pixel intensity deviations (PIDs) and gamma indices is evaluated using receiver operating characteristic analysis., Results: Residual differences between registered images are due to interfraction positional deviations of jaws and multileaf collimator leaves, plus imager noise. Positional deviations produce large intensity differences that degrade anomaly detection. Gradient effects are suppressed in PIDs using gradient scaling. Background noise is suppressed using median filtering. In the majority of images, PID-based detection strategies can reliably detect fluence anomalies of ≥5% in ∼1 mm(2) areas and ≥2% in ∼20 mm(2) areas., Conclusions: The ability to detect small dose differences (≤2%) depends strongly on the level of background noise. This in turn depends on the accuracy of image registration, the quality of the reference image, and field properties. The longer term aim of this work is to develop accurate and reliable methods of detecting IMRT delivery errors and variations. The ability to resolve small anomalies will allow the accuracy of advanced treatment techniques, such as image guided, adaptive, and arc therapies, to be quantified.

Purpose: To investigate using 3D γ analysis for IMRT and VMAT QA., Methods: We explored and studied 3D γ-analysis by comparing TPS computed and EPID back-projection reconstructed doses in patient's CT images. Two 3D γ quantities, γ(PTV) and γ(10), were proposed and studied for evaluating the QA results, and compared to 2D γ (MapCheck composite: γ(MC))., Results: It was found that when 3%(global)/3 mm criteria was used, all IMRT and 90% of VMAT plans passed QA with a γ pass rate ≥90%. A significant statistical correlation was observed between 3D and 2D γ-analysis results for IMRT QA if γ(10) and γ(MC) are concerned, but no significant relation is found between γ(PTV) and γ(MC)., Conclusions: 3D γ analysis based on EPID dose back-projection may provide a feasible tool for IMRT and VMAT pretreatment plan QA., (© 2012 American Association of Physicists in Medicine.)

Purpose: Pretreatment quality assurance data from four centers, members of the European TrueBeam council were analyzed with different verification devices to assess reliability of flattening filter free beam delivery for intensity modulated radiotherapy (IMRT) and RapidArc (RA) techniques., Methods: TrueBeam(®) (Varian Medical System) is a new linear accelerator designed for delivering flattened, as well as flattening filter free beams. Pretreatment dosimetric validation of plan delivery was performed with different verification devices and responses to high dose rates were tested. Treatment planning was done in Eclipse planning system (PRO 8.9, AAA 8.9). γ evaluation was performed with (dose difference) = 3% and (distance to agreement) = 3 mm scoring the gamma agreement index (GAI, % of field area passing the test). Two hundred and twenty-four patients with 1-6 lesions in various anatomical regions and dose per fraction ranging from 1.8 Gy to 25 Gy were included in the study; 88 were treated with 6 MV flattening filter free (X6FFF) beam energy and 136 with 10 MV flattening filter free (X10FFF) beam. Gafchromic films in solid water, delta(4), arccheck, and matrixx phantom were used to verify the dose distributions. Additionally, point measurements were performed using a PinPoint chamber and a Farmer chamber., Results: Dose calculation as well as dose delivery was equally accurate for IMRT and RA delivery (IMRT: GAI = 99.3% (±1.1); RA: GAI = 98.8% (±1.1) as well as for the two beams evaluated (X6FFF: GAI = 99.1% (±1.0); X10FFF: GAI = 98.8% (±1.2). Only small differences were found for the four verification devices. A point dose verification was performed on 52 cases, obtaining a dose deviation of 0.34%. The GAI variations with number of monitor units were statistically significant., Conclusions: The TrueBeam FFF modality, analyzed with a variety of verification devices and planned with Eclipse planning system is dosimetrically accurate (within the specified limits 3 mm/3%) for both X6FFF and X10FFF beam energy.

Purpose: Intensity-modulated proton therapy (IMPT) using spot scanned proton beams relies on the delivery of a large number of beamlets to shape the dose distribution in a highly conformal manner. The authors have developed a 3D system based on liquid scintillator to measure the spatial location, intensity, and depth of penetration (energy) of the proton beamlets in near real-time., Methods: The detector system consists of a 20 × 20 × 20 cc liquid scintillator (LS) material in a light tight enclosure connected to a CCD camera. This camera has a field of view of 25.7 by 19.3 cm and a pixel size of 0.4 mm. While the LS is irradiated, the camera continuously acquires images of the light distribution produced inside the LS. Irradiations were made with proton pencil beams produced with a spot-scanning nozzle. Pencil beams with nominal ranges in water between 9.5 and 17.6 cm were scanned to irradiate an area of 10 × 10 cm square on the surface of the LS phantom. Image frames were acquired at 50 ms per frame., Results: The signal to noise ratio of a typical Bragg peak was about 170. Proton range measured from the light distribution produced in the LS was accurate to within 0.3 mm on average. The largest deviation seen between the nominal and measured range was 0.6 mm. Lateral position of the measured pencil beam was accurate to within 0.4 mm on average. The largest deviation seen between the nominal and measured lateral position was 0.8 mm; however, the accuracy of this measurement could be improved by correcting light scattering artifacts. Intensity of single proton spots were measured with precision ranging from 3 % for the smallest spot intensity (0.005 MU) to 0.5 % for the largest spot (0.04 MU)., Conclusions: Our LS detector system has been shown to be capable of fast, submillimeter spatial localization of proton spots delivered in a 3D volume. This system could be used for beam range, intensity and position verification in IMPT.

Purpose: The purpose of this work is to explore the usefulness of the gamma passing rate metric for per-patient, pretreatment dose QA and to validate a novel patient-dose∕DVH-based method and its accuracy and correlation. Specifically, correlations between: (1) gamma passing rates for three 3D dosimeter detector geometries vs clinically relevant patient DVH-based metrics; (2) Gamma passing rates of whole patient dose grids vs DVH-based metrics, (3) gamma passing rates filtered by region of interest (ROI) vs DVH-based metrics, and (4) the capability of a novel software algorithm that estimates corrected patient Dose-DVH based on conventional phantom QA data are analyzed., Methods: Ninety six unique "imperfect" step-and-shoot IMRT plans were generated by applying four different types of errors on 24 clinical Head∕Neck patients. The 3D patient doses as well as the dose to a cylindrical QA phantom were then recalculated using an error-free beam model to serve as a simulated measurement for comparison. Resulting deviations to the planned vs simulated measured DVH-based metrics were generated, as were gamma passing rates for a variety of difference∕distance criteria covering: dose-in-phantom comparisons and dose-in-patient comparisons, with the in-patient results calculated both over the whole grid and per-ROI volume. Finally, patient dose and DVH were predicted using the conventional per-beam planar data as input into a commercial "planned dose perturbation" (PDP) algorithm, and the results of these predicted DVH-based metrics were compared to the known values., Results: A range of weak to moderate correlations were found between clinically relevant patient DVH metrics (CTV-D95, parotid D(mean), spinal cord D1cc, and larynx D(mean)) and both 3D detector and 3D patient gamma passing rate (3%∕3 mm, 2%∕2 mm) for dose-in-phantom along with dose-in-patient for both whole patient volume and filtered per-ROI. There was considerable scatter in the gamma passing rate vs DVH-based metric curves. However, for the same input data, the PDP estimates were in agreement with actual patient DVH results., Conclusions: Gamma passing rate, even if calculated based on patient dose grids, has generally weak correlation to critical patient DVH errors. However, the PDP algorithm was shown to accurately predict the DVH impact using conventional planar QA results. Using patient-DVH-based metrics IMRT QA allows per-patient dose QA to be based on metrics that are both sensitive and specific. Further studies are now required to analyze new processes and action levels associated with DVH-based metrics to ensure effectiveness and practicality in the clinical setting.

Purpose: With the increased commercial availability of intensity modulated arc therapy (IMAT) comes the need for comprehensive QA programs, covering the different aspects of this newly available technology. This manuscript proposes such a program for the RapidArc (RA) (Varian Medical Systems, Palo Alto) IMAT solution., Methods: The program was developed and tested out for a Millennium120 MLC on iX Clinacs and a HighDefinition MLC on a Novalis TX, using a variety of measurement equipment including Gafchromic film, 2D ion chamber arrays (Seven29 and StarCheck, PTW, Freiburg, Germany) with inclinometer and Octavius phantom, the Delta4 systam (ScandiDos, Uppsala, Sweden) and the portal imager (EPID). First, a number of complementary machine QA tests were developed to monitor the correct interplay between the accelerating/decelerating gantry, the variable dose rate and the MLC position, straining the delivery to the maximum allowed limits. Second, a systematic approach to the validation of the dose calculation for RA was adopted, starting with static gantry and RA specific static MLC shapes and gradually moving to dynamic gantry, dynamic MLC shapes. RA plans were then optimized on a series of artificial structures created within the homogeneous Octavius phantom and within a heterogeneous lung phantom. These served the double purpose of testing the behavior of the optimization algorithm (PRO) as well as the precision of the forward dose calculation. Finally, patient QA on a series of clinical cases was performed with different methods. In addition to the well established in-phantom QA, we evaluated the portal dosimetry solution within the Varian approach., Results: For routine machine QA, the "Snooker Cue" test on the EPID proved to be the most sensitive to overall problem detection. It is also the most practical one. The "Twinkle" and "Sunrise" tests were useful to obtain well differentiated information on the individual treatment delivery components. The AAA8.9 dose calculations showed excellent agreement with all corresponding measurements, except in areas where the 2.5 mm fixed fluence resolution was insufficient to accurately model the tongue and groove effect or the dose through nearly closed opposing leafs. Such cases benefited from the increased fluence resolution in AAA10.0. In the clinical RA fields, these effects were smeared out spatially and the impact of the fluence resolution was considerably less pronounced. The RA plans on the artificial structure sets demonstrated some interesting characteristics of the PRO8.9 optimizer, such as a sometimes unexpected dependence on the collimator rotation and a suboptimal coverage of targets within lung tissue. Although the portal dosimetry was successfully validated, we are reluctant to use it as a sole means of patient QA as long as no gantry angle information is embedded., Conclusions: The all-in validation program allows a systematic approach in monitoring the different levels of RA treatments. With the systematic approach comes a better understanding of both the capabilities and the limits of the used solution. The program can be useful for implementation, but also for the validation of major upgrades.

This report on intensity-modulated radiation therapy (IMRT) is part of a series of white papers addressing patient safety commissioned by the American Society for Radiation Oncology's (ASTRO) Target Safely Campaign. The document has been approved by the ASTRO Board of Directors, endorsed by the American Association of Physicists in Medicine (AAPM) and American Association of Medical Dosimetrists (AAMD), and reviewed and accepted by the American College of Radiology's Commission on Radiation Oncology. This report is related to other reports of the ASTRO white paper series on patient safety which are still in preparation, and when appropriate it defers to guidance that will be published by those groups in future white papers. This document takes advantage of the large body of work on quality assurance and quality control principles within radiation oncology whenever possible. IMRT provides increased capability to conform isodose distributions to the shape of the target(s), thereby reducing dose to some adjacent critical structures. This promise of IMRT is one of the reasons for its widespread use. However, the promise of IMRT is counterbalanced by the complexity of the IMRT planning and delivery processes, and the associated risks, some of which have been demonstrated by the New York Times reports on serious accidents involving both IMRT and other radiation treatment modalities. This report provides an opportunity to broadly address safe delivery of IMRT, with a primary focus on recommendations for human error prevention and methods to reduce the occurrence of errors or machine malfunctions that can lead to catastrophic failures or errors.

Purpose: The report issued by AAPM Task Group No. 119 outlined a procedure for evaluating the effectiveness of IMRT commissioning. The procedure involves measuring gamma pass-rate indices for IMRT plans of standard phantoms and determining if the results fall within a confidence limit set by assuming normally distributed data. As stated in the TG report, the assumption of normally distributed gamma pass rates is a convenient approximation for commissioning purposes, but may not accurately describe the data. Here the authors attempt to better describe gamma pass-rate data by fitting it to different distributions. The authors then calculate updated confidence limits using those distributions and compare them to those derived using TG No. 119 method., Methods: Gamma pass-rate data from 111 head and neck patients are fitted using the TG No. 119 normal distribution, a truncated normal distribution, and a Weibull distribution. Confidence limits to 95% are calculated for each and compared. A more general analysis of the expected differences between the TG No. 119 method of determining confidence limits and a more time-consuming curve fitting method is performed., Results: The TG No. 119 standard normal distribution does not fit the measured data. However, due to the small range of measured data points, the inaccuracy of the fit has only a small effect on the final value of the confidence limits. The confidence limits for the 111 patient plans are within 0.1% of each other for all distributions. The maximum expected difference in confidence limits, calculated using TG No. 119's approximation and a truncated distribution, is 1.2%., Conclusions: A three-parameter Weibull probability distribution more accurately fits the clinical gamma index pass-rate data than the normal distribution adopted by TG No. 119. However, the sensitivity of the confidence limit on distribution fit is low outside of exceptional circumstances.

Intensity modulated radiation therapy (IMRT) poses a number of challenges for properly measuring commissioning data and quality assurance (QA) radiation dose distributions. This report provides a comprehensive overview of how dosimeters, phantoms, and dose distribution analysis techniques should be used to support the commissioning and quality assurance requirements of an IMRT program. The proper applications of each dosimeter are described along with the limitations of each system. Point detectors, arrays, film, and electronic portal imagers are discussed with respect to their proper use, along with potential applications of 3D dosimetry. Regardless of the IMRT technique utilized, some situations require the use of multiple detectors for the acquisition of accurate commissioning data. The overall goal of this task group report is to provide a document that aids the physicist in the proper selection and use of the dosimetry tools available for IMRT QA and to provide a resource for physicists that describes dosimetry measurement techniques for purposes of IMRT commissioning and measurement-based characterization or verification of IMRT treatment plans. This report is not intended to provide a comprehensive review of commissioning and QA procedures for IMRT. Instead, this report focuses on the aspects of metrology, particularly the practical aspects of measurements that are unique to IMRT. The metrology of IMRT concerns the application of measurement instruments and their suitability, calibration, and quality control of measurements. Each of the dosimetry measurement tools has limitations that need to be considered when incorporating them into a commissioning process or a comprehensive QA program. For example, routine quality assurance procedures require the use of robust field dosimetry systems. These often exhibit limitations with respect to spatial resolution or energy response and need to themselves be commissioned against more established dosimeters. A chain of dosimeters, from secondary standards to field instruments, is established to assure the quantitative nature of the tests. This report is intended to describe the characteristics of the components of these systems; dosimeters, phantoms, and dose evaluation algorithms. This work is the report of AAPM Task Group 120.

Purpose: The purpose of this work is to determine the statistical correlation between per-beam, planar IMRT QA passing rates and several clinically relevant, anatomy-based dose errors for per-patient IMRT QA. The intent is to assess the predictive power of a common conventional IMRT QA performance metric, the Gamma passing rate per beam., Methods: Ninety-six unique data sets were created by inducing four types of dose errors in 24 clinical head and neck IMRT plans, each planned with 6 MV Varian 120-leaf MLC linear accelerators using a commercial treatment planning system and step-and-shoot delivery. The error-free beams/plans were used as "simulated measurements" (for generating the IMRT QA dose planes and the anatomy dose metrics) to compare to the corresponding data calculated by the error-induced plans. The degree of the induced errors was tuned to mimic IMRT QA passing rates that are commonly achieved using conventional methods., Results: Analysis of clinical metrics (parotid mean doses, spinal cord max and D1cc, CTV D95, and larynx mean) vs. IMRT QA Gamma analysis (3%/3 mm, 2/2, 1/1) showed that in all cases, there were only weak to moderate correlations (range of Pearson's r-values: -0.295 to 0.653). Moreover, the moderate correlations actually had positive Pearson's r-values (i.e., clinically relevant metric differences increased with increasing IMRT QA passing rate), indicating that some of the largest anatomy-based dose differences occurred in the cases of high IMRT QA passing rates, which may be called "false negatives." The results also show numerous instances of false positives or cases where low IMRT QA passing rates do not imply large errors in anatomy dose metrics. In none of the cases was there correlation consistent with high predictive power of planar IMRT passing rates, i.e., in none of the cases did high IMRT QA Gamma passing rates predict low errors in anatomy dose metrics or vice versa., Conclusions: There is a lack of correlation between conventional IMRT QA performance metrics (Gamma passing rates) and dose errors in anatomic regions-of-interest. The most common acceptance criteria and published actions levels therefore have insufficient, or at least unproven, predictive power for per-patient IMRT QA.

Purpose: As traditional film is phased out in most radiotherapy centers, computed radiography (CR) systems are increasingly being purchased as a replacement. CR plates can be used for patient imaging, but may also be used for a variety of quality assurance (QA) purposes and can be calibrated in terms of dose. This study looks at their suitability for verification of intensity modulated radiation therapy (IMRT) dose distributions., Methods: A CR plate was calibrated in terms of the relative dose and the stability of response over 1 year was studied. The effect of exposing the CR plate to ambient light and of using different time delays before scanning was quantified. The CR plate was used to verify the relative dose distributions for ten IMRT patients and the results were compared to those obtained using a two dimensional (2D) diode array., Results: Exposing the CR plate to 10 s of ambient light between irradiation (174 cGy) and scanning erased approximately 80% of the signal. Changes in delay time between irradiation and scanning also affected the measurement results. The signal on the plate was found to decay at a rate of approximately 3.6 cGy/min in the first 10 min after irradiation. The use of a CR plate for IMRT patient-specific QA resulted in a significantly lower distance to agreement (DTA) and gamma pass rate than when using a 2D diode array for the measurement. This was primarily due to the over-response of the CR phosphor to low energy scattered radiation. For the IMRT QA using the CR plate, the average gamma pass rate was 97.3%. For the same IMRT QA using a diode array, the average gamma pass rate was 99.7%. The gamma criteria used were 4% dose difference and 4 mm DTA for head and neck treatments and 3% dose difference and 3 mm DTA for prostate treatments. The gamma index tolerance was 1. The lowest 10% of the dose distribution was excluded from all gamma and DTA analyses., Conclusions: Although the authors showed that CR plates can be used for patient specific IMRT QA, the practical problems such as the over-response to low energy scatter and signal fading with light exposure and time mean that alternative detectors such as radiochromic film or diode arrays will be a more sensible choice for most radiotherapy departments.

Purpose: Patient specific IMRT QC is performed routinely in many clinics as a safeguard against errors and inaccuracies which may be introduced during the complex planning, data transfer, and delivery phases of this type of treatment. The purpose of this work is to evaluate the feasibility of detecting systematic errors in MLC leaf bank position with patient specific checks., Methods: 9 head and neck (H&N) and 14 prostate IMRT beams were delivered using MLC files containing systematic offsets (+/- 1 mm in two banks, +/- 0.5 mm in two banks, and 1 mm in one bank of leaves). The beams were measured using both MAPCHECK (Sun Nuclear Corp., Melbourne, FL) and the aS1000 electronic portal imaging device (Varian Medical Systems, Palo Alto, CA). Comparisons with calculated fields, without offsets, were made using commonly adopted criteria including absolute dose (AD) difference, relative dose difference, distance to agreement (DTA), and the gamma index., Results: The criteria most sensitive to systematic leaf bank offsets were the 3% AD, 3 mm DTA for MAPCHECK and the gamma index with 2% AD and 2 mm DTA for the EPID. The criterion based on the relative dose measurements was the least sensitive to MLC offsets. More highly modulated fields, i.e., H&N, showed greater changes in the percentage of passing points due to systematic MLC inaccuracy than prostate fields., Conclusions: None of the techniques or criteria tested is sufficiently sensitive, with the population of IMRT fields, to detect a systematic MLC offset at a clinically significant level on an individual field. Patient specific QC cannot, therefore, substitute for routine QC of the MLC itself.

Purpose: To develop an independent and on-line beam monitoring system, which can validate the accuracy of segment-by-segment energy fluence delivery for each treatment field. The system is also intended to be utilized for pretreatment dosimetric quality assurance of intensity modulated radiation therapy (IMRT), on-line image-guided adaptive radiation therapy, and volumetric modulated arc therapy., Methods: The system, referred to as the integral quality monitor (IQM), utilizes an area integrating energy fluence monitoring sensor (AIMS) positioned between the final beam shaping device [i.e., multileaf collimator (MLC)] and the patient. The prototype AIMS consists of a novel spatially sensitive large area ionization chamber with a gradient along the direction of the MLC motion. The signal from the AIMS provides a simple output for each beam segment, which is compared in real time to the expected value. The prototype ionization chamber, with a physical area of 22 x 22 cm2, has been constructed out of aluminum with the electrode separations varying linearly from 2 to 20 mm. A calculation method has been developed to predict AIMS signals based on an elementwise integration technique, which takes into account various predetermined factors, including the spatial response function of the chamber, MLC characteristics, beam transmission through the secondary jaws, and field size factors. The influence of the ionization chamber on the beam has been evaluated in terms of transmission, surface dose, beam profiles, and depth dose. The sensitivity of the system was tested by introducing small deviations in leaf positions. A small set of IMRT fields for prostate and head and neck plans was used to evaluate the system. The ionization chamber and the data acquisition software systems were interfaced to two different types of linear accelerators: Elekta Synergy and Varian iX., Results: For a 10 x 10 cm2 field, the chamber attenuates the beam intensity by 7% and 5% for 6 and 18 MV beams, respectively, without significantly changing the depth dose, surface dose, and dose profile characteristics. An MLC bank calibration error of 1 mm causes the IQM signal of a 3 x 3 cm2 aperture to change by 3%. A positioning error in a single 5 mm wide leaf by 3 mm in 3 X 3 cm2 aperture causes a signal difference of 2%. Initial results for prostate and head and neck IMRT fields show an average agreement between calculation and measurement to within 1%, with a maximum deviation for each of the smallest beam segments to within 5%. When the beam segments of a prostate IMRT field were shifted by 3 mm from their original position, along the direction of the MLC motion, the IQM signals varied, on average, by 2.5%., Conclusions: The prototype IQM system can validate the accuracy of beam delivery in real time by comparing precalculated and measured AIMS signals. The system is capable of capturing errors in MLC leaf calibration or malfunctions in the positioning of an individual leaf. The AIMS does not significantly alter the beam quality and therefore could be implemented without requiring recommissioning measurements.

The concept of in-air output ratio (Sc) was introduced to characterize how the incident photon fluence per monitor unit (or unit time for a Co-60 unit) varies with collimator settings. However, there has been much confusion regarding the measurement technique to be used that has prevented the accurate and consistent determination of Sc. The main thrust of the report is to devise a theoretical and measurement formalism that ensures interinstitutional consistency of Sc. The in-air output ratio, Sc, is defined as the ratio of primary collision water kerma in free-space, Kp, per monitor unit between an arbitrary collimator setting and the reference collimator setting at the same location. Miniphantoms with sufficient lateral and longitudinal thicknesses to eliminate electron contamination and maintain transient electron equilibrium are recommended for the measurement of Sc. The authors present a correction formalism to extrapolate the correct Sc from the measured values using high-Z miniphantom. Miniphantoms made of high-Z material are used to measure Sc for small fields (e.g., IMRT or stereotactic radiosurgery). This report presents a review of the components of Sc, including headscatter, source-obscuring, and monitor-backscattering effects. A review of calculation methods (Monte Carlo and empirical) used to calculate Sc for arbitrary shaped fields is presented. The authors discussed the use of Sc in photon dose calculation algorithms, in particular, monitor unit calculation. Finally, a summary of Sc data (from RPC and other institutions) is included for QA purposes.

AAPM Task Group 119 has produced quantitative confidence limits as baseline expectation values for IMRT commissioning. A set of test cases was developed to assess the overall accuracy of planning and delivery of IMRT treatments. Each test uses contours of targets and avoidance structures drawn within rectangular phantoms. These tests were planned, delivered, measured, and analyzed by nine facilities using a variety of IMRT planning and delivery systems. Each facility had passed the Radiological Physics Center credentialing tests for IMRT. The agreement between the planned and measured doses was determined using ion chamber dosimetry in high and low dose regions, film dosimetry on coronal planes in the phantom with all fields delivered, and planar dosimetry for each field measured perpendicular to the central axis. The planar dose distributions were assessed using gamma criteria of 3%/3 mm. The mean values and standard deviations were used to develop confidence limits for the test results using the concept confidence limit = /mean/ + 1.96sigma. Other facilities can use the test protocol and results as a basis for comparison to this group. Locally derived confidence limits that substantially exceed these baseline values may indicate the need for improved IMRT commissioning.

A high dosimetric accuracy is required for radiotherapy treatments where IMRT in combination with narrow treatment margins is applied to achieve optimally conformal dose distributions. In order to routinely verify the in vivo fluence delivery (i.e., during the actual patient treatment), our method for predicting portal dose images with a patient in the beam was validated. A unique feature of this method is that it is fully based on calibration measurements with an EPID. The portal dose image (PDI) behind a patient is dependent on the transmission of primary radiation through the patient and a contribution of scattered radiation from the patient. To derive both components, the patient geometry as observed in the planning CT scan is converted into an equivalent homogeneous phantom. A limited set of EPID measurements is required to derive the input parameters of this model. The accuracy of the in vivo PDI prediction was verified using measurements behind phantoms and four prostate cancer patients treated with IMRT. Behind homogeneous slab phantoms, the local differences between measured and predicted PDIs were within 2% inside the field, while behind a lung and a pelvic phantom, the agreement was within 3% or within 3 mm in regions with steep gradients. Outside the fields, the PDIs agreed within 2% of the global dose maximum. Evaluation of the in vivo PDI measurements behind patients showed that, on average, 87% of the pixels inside the field fulfilled the 3% local dose and 3 mm distance-to-agreement criteria. For half of the failing pixels the differences occurred due to changes in patient geometry with respect to the planning CT or due to beam attenuation by the treatment couch that was not accounted for. A fully EPID-based method for predicting portal dose images using the planning CT scan has been implemented and validated for phantoms and clinical patients.

The purpose of this study was to determine the accuracy of five commonly used intensity-modulated radiation therapy (IMRT) treatment planning systems (TPSs), 3 using convolution superposition algorithms or the analytical anisotropic algorithm (CSA/AAAs) and 2 using pencil beam algorithms (PBAs), in calculating the absorbed dose within a low-density, heterogeneous region when compared with measurements made in an anthropomorphic thorax phantom. The dose predicted in the target center met the test criteria (5% of the dose normalization point or 3 mm distance to agreement) for all TPSs tested; however, at the tumor-lung interface and at the peripheral lung in the vicinity of the tumor, the CSA/AAAs performed better than the PBAs (85% and 50%, respectively, of pixels meeting the 5%/3-mm test criteria), and thus should be used to determine dose in heterogeneous regions.

Increased use of intensity modulated radiation therapy (IMRT) has resulted in increased efforts in patient quality assurance (QA). Software and detector systems intended to streamline the IMRT quality assurance process often report metrics, such as percent discrepancies between measured and computed doses, which can be compared to benchmark or threshold values. The purpose of this work is to examine the relationships between two different types of IMRT QA processes in order to define, or refine, appropriate tolerances values. For 115 IMRT plans delivered in a 3 month period, we examine the discrepancies between (a) the treatment planning system (TPS) and results from a commercial independent monitor unit (MU) calculation program; (b) TPS and results from a commercial diode-array measurement system; and (c) the independent MU calculation and the diode-array measurements. Statistical tests were performed to assess significance in the IMRT QA results for different disease site and machine models. There is no evidence that the average total dose discrepancy in the monitor unit calculation depends on the disease site. Second, the discrepancies in the two IMRT QA methods are independent: there is no evidence that a better--or worse--monitor unit validation result is related to a better--or worse--diode-array measurement result. Third, there is marginal benefit in repeating the independent MU calculation with a more suitable dose point, if the initial IMRT QA failed a certain tolerance. Based on these findings, the authors conclude at some acceptable tolerances based on disease site and IMRT QA method. Specifically, monitor unit validations are expected to have a total dose discrepancy of 3% overall, and 5% per beam, independent of disease site. Diode array measurements are expected to have a total absolute dose discrepancy of 3% overall, and 3% per beam, independent of disease site. The percent of pixels exceeding a 3% and 3 mm threshold in a gamma analysis should be greater than or equal to 95% for non-head and neck IMRT cases, and 88% for head and neck IMRT cases. The IMRT QA methodology described here is neither unique nor ubiquitous, and the ability to deliver a safe IMRT does not simply require IMRT QA tests to pass a given tolerance; however, the selection of a tolerance should be meaningful when assessing a complex plan. The methodology in defining appropriate tolerances, described in this article, is based on an interpretation of IMRT QA results from IMRT plans deemed safe to deliver.

Radiographic image guidance has emerged as the new paradigm for patient positioning, target localization, and external beam alignment in radiotherapy. Although widely varied in modality and method, all radiographic guidance techniques have one thing in common--they can give a significant radiation dose to the patient. As with all medical uses of ionizing radiation, the general view is that this exposure should be carefully managed. The philosophy for dose management adopted by the diagnostic imaging community is summarized by the acronym ALARA, i.e., as low as reasonably achievable. But unlike the general situation with diagnostic imaging and image-guided surgery, image-guided radiotherapy (IGRT) adds the imaging dose to an already high level of therapeutic radiation. There is furthermore an interplay between increased imaging and improved therapeutic dose conformity that suggests the possibility of optimizing rather than simply minimizing the imaging dose. For this reason, the management of imaging dose during radiotherapy is a different problem than its management during routine diagnostic or image-guided surgical procedures. The imaging dose received as part of a radiotherapy treatment has long been regarded as negligible and thus has been quantified in a fairly loose manner. On the other hand, radiation oncologists examine the therapy dose distribution in minute detail. The introduction of more intensive imaging procedures for IGRT now obligates the clinician to evaluate therapeutic and imaging doses in a more balanced manner. This task group is charged with addressing the issue of radiation dose delivered via image guidance techniques during radiotherapy. The group has developed this charge into three objectives: (1) Compile an overview of image-guidance techniques and their associated radiation dose levels, to provide the clinician using a particular set of image guidance techniques with enough data to estimate the total diagnostic dose for a specific treatment scenario, (2) identify ways to reduce the total imaging dose without sacrificing essential imaging information, and (3) recommend optimization strategies to trade off imaging dose with improvements in therapeutic dose delivery. The end goal is to enable the design of image guidance regimens that are as effective and efficient as possible.

Helical tomotherapy radiation therapy units, due to their particular design and differences from a traditional linear accelerator, require different procedures by which to perform routine quality assurance (QA). One of the principal QA tasks that should be performed daily on any radiation therapy equipment is the output constancy check. The daily output check on a Hi-Art TomoTherapy unit is commonly performed utilizing ionization chambers placed inside a solid water phantom. This provides a good check of output at one point, but does not give any information on either energy or symmetry of the beam, unless more than one point is measured. This also has the added disadvantage that it has to be done by the physics staff. To address these issues, and to simplify the process, such that it can be performed by radiation therapists, we investigated the use of a commercially available daily QA device to perform this task. The use of this device simplifies the task of daily output constancy checks and eliminates the need for continued physics involvement. This device can also be used to monitor the constancy of beam energy and cone profile and can potentially be used to detect gross errors in the couch movement or laser alignment.