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Purpose: The aim of this work was to investigate the use of amorphous silicon electronic portal imaging devices (EPIDs) for regular quality assurance of linear accelerator asymmetric jaw junctioning., Methods: The method uses the beam central axis position on the EPID measured to subpixel accuracy found from two EPID images with 180 degrees opposing collimator angles. Individual zero jaw position ("half-beam blocked") images are then acquired and the jaw position precisely determined for each using penumbra interpolation. The accuracy of determining jaw position with the EPID method was measured by translating a block (simulating a jaw) by known distances, using a translation stage, and then measuring each translation distance with the EPID. To establish the utility of EPID based junction dose measurements, radiographic film measurements of junction dose maxima/minima as a function of jaw gap/overlap were made and compared to EPID measurements. Using the method, the long-term stability of zero jaw positioning was assessed for four linear accelerators over a 1-1.5 yr time period. The stability at nonzero gantry angles was assessed over a shorter time period., Results: The accuracy of determining jaw translations with the method was within 0.14 mm found using the translation stage [standard deviation (SD) of 0.037 mm]. The junction doses measured with the EPID were different from film due to the nonwater equivalent EPID scattering properties and hence different penumbra profile. The doses were approximately linear with gap or overlap, and a correction factor was derived to convert EPID measured junction dose to film measured equivalent. Over a 1 yr period, the zero jaw positions at gantry zero position were highly reproducible with an average SD of 0.07 mm for the 16 collimator jaws examined. However, the average jaw positions ranged from -0.7 to 0.9 mm relative to central axis for the different jaws. The zero jaw position was also reproducible at gantry 90 degrees position with 0.1 mm SD variation with the mean jaw position offset from the gantry zero position consistently by 0.3-0.4 mm for the jaws studied., Conclusions: The EPID based method is efficient and yields more precise data on linear accelerator jaw positioning and reproducibility than previous methods. The results highlight that zero jaw positions are highly reproducible to a level much smaller than the displayed jaw resolution and that there is a need for better methods to calibrate the jaw positioning.

The quality assurance of synthetic CT (sCT) is crucial for safe clinical transfer to an MRI-only radiotherapy planning workflow. The aim of this work is to propose a population-based process assessing local errors in the generation of sCTs and their impact on dose distribution. For the analysis to be anatomically meaningful, a customized interpatient registration method brought the population data to the same coordinate system. Then, the voxel-based process was applied on two sCT generation methods: a bulk-density method and a generative adversarial network. The CT and MRI pairs of 39 patients treated by radiotherapy for prostate cancer were used for sCT generation, and 26 of them with delineated structures were selected for analysis. Voxel-wise errors in sCT compared to CT were assessed for image intensities and dose calculation, and a population-based statistical test was applied to identify the regions where discrepancies were significant. The cumulative histograms of the mean absolute dose error per volume of tissue were computed to give a quantitative indication of the error for each generation method. Accurate interpatient registration was achieved, with mean Dice scores higher than 0.91 for all organs. The proposed method produces three-dimensional maps that precisely show the location of the major discrepancies for both sCT generation methods, highlighting the heterogeneity of image and dose errors for sCT generation methods from MRI across the pelvic anatomy. Hence, this method provides additional information that will assist with both sCT development and quality control for MRI-based planning radiotherapy [ABSTRACT FROM AUTHOR]

Purpose: Previous work on Magnetic Resonance Imaging (MRI) only planning has been applied to limited treatment regions with a focus on male anatomy. This research aimed to validate the use of a hybrid multi-atlas synthetic computed tomography (sCT) generation technique from a MRI, using a female and male atlas, for MRI only radiation therapy treatment planning of rectum, anal canal, cervix and endometrial malignancies. Patients and methods: Forty patients receiving radiation treatment for a range of pelvic malignancies, were separated into male (n = 20) and female (n = 20) cohorts for the creation of gender specific atlases. A multi-atlas local weighted voting method was used to generate a sCT from a T1-weighted VIBE DIXON MRI sequence. The original treatment plans were copied from the CT scan to the corresponding sCT for dosimetric validation. Results: The median percentage dose difference between the treatment plan on the CT and sCT at the ICRU reference point for the male cohort was − 0.4% (IQR of 0 to − 0.6), and − 0.3% (IQR of 0 to − 0.6) for the female cohort. The mean gamma agreement for both cohorts was > 99% for criteria of 3%/2 mm and 2%/2 mm. With dose criteria of 1%/1 mm, the pass rate was higher for the male cohort at 96.3% than the female cohort at 93.4%. MRI to sCT anatomical agreement for bone and body delineated contours was assessed, with a resulting Dice score of 0.91 ± 0.2 (mean ± 1 SD) and 0.97 ± 0.0 for the male cohort respectively; and 0.96 ± 0.0 and 0.98 ± 0.0 for the female cohort respectively. The mean absolute error in Hounsfield units (HUs) within the entire body for the male and female cohorts was 59.1 HU ± 7.2 HU and 53.3 HU ± 8.9 HU respectively. Conclusions: A multi-atlas based method for sCT generation can be applied to a standard T1-weighted MRI sequence for male and female pelvic patients. The implications of this study support MRI only planning being applied more broadly for both male and female pelvic sites. Trial registration This trial was registered in the Australian New Zealand Clinical Trials Registry (ANZCTR) (www.anzctr.org.au) on 04/10/2017. Trial identifier ACTRN12617001406392. [ABSTRACT FROM AUTHOR]

Introduction: Assessing the use of a radiation therapy (RT) planning MRI performed in the treatment position (pMRI) on target volume delineation and effect on organ at risk dose for oropharyngeal cancer patients planned with diagnostic MRI (dMRI) and CT scan. Methods: Diagnostic MRI scans were acquired for 26 patients in a neutral patient position using a 3T scanner (dMRI). Subsequent pMRI scans were acquired on the same scanner with a flat couch top and the patient in their immobilisation mask. Each series was rigidly registered to the patients planning CT scan and volumes were first completed with the CT/dMRI. The pMRI was then made available for volume modification. For the group with revised volumes, two IMRT plans were developed to demonstrate the impact of the modification. Image and registration quality was also evaluated. Results: The pMRI registration led to the modification of target volumes for 19 of 26 participants. The pMRI target volumes were larger in absolute volume resulting in reduced capacity for organ sparing. Predominantly, modifications occurred for the primary gross tumour volume (GTVp) with a mean Dice Similarity Coefficient (DSC) of 0.7 and the resulting high risk planning target volume, a mean DSC of 0.89. Both MRIs scored similarly for image quality, with the pMRI demonstrating improved registration quality and efficiency. Conclusions: A pMRI provides improvement in registration efficiency, quality and a higher degree of oncologist confidence in target delineation. These results have led to a practice change within our department, where a pMRI is acquired for all eligible oropharyngeal cancer patients. [ABSTRACT FROM AUTHOR]

The era of real‐time radiotherapy is upon us. Robotic and gimbaled linac tracking are clinically established technologies with the clinical realization of couch tracking in development. Multileaf collimators (MLCs) are a standard equipment for most cancer radiotherapy systems, and therefore MLC tracking is a potentially widely available technology. MLC tracking has been the subject of theoretical and experimental research for decades and was first implemented for patient treatments in 2013. The AAPM Task Group 264 Safe Clinical Implementation of MLC Tracking in Radiotherapy Report was charged to proactively provide the broader radiation oncology community with (a) clinical implementation guidelines including hardware, software, and clinical indications for use, (b) commissioning and quality assurance recommendations based on early user experience, as well as guidelines on Failure Mode and Effects Analysis, and (c) a discussion of potential future developments. The deliverables from this report include: an explanation of MLC tracking and its historical development; terms and definitions relevant to MLC tracking; the clinical benefit of, clinical experience with and clinical implementation guidelines for MLC tracking; quality assurance guidelines, including example quality assurance worksheets; a clinical decision pathway, future outlook and overall recommendations. [ABSTRACT FROM AUTHOR]

Abstract: Beam steering is the process of calibrating the angle and translational position with which a linear accelerator's (linac's) electron beam strikes the x‐ray target with respect to the collimator rotation axis. The shape of the dose profile is highly dependent on accurate beam steering and is essential for ensuring correct delivery of the radiotherapy treatment plan. Traditional methods of beam steering utilize a scanning water tank phantom that makes the process user‐dependent. This study is the first to provide a methodology for both beam angle steering and beam translational position steering based on EPID imaging of the beam and does not require a phantom. Both the EPID‐based beam angle steering and beam translational steering methods described have been validated against IC Profiler measurement. Wide field symmetry agreement was found between the EPID and IC Profiler to within 0.06 ± 0.14% (1 SD) and 0.32 ± 0.11% (1 SD) for flattened and flattening‐filter‐free (FFF) beams, respectively. For a 1.1% change in symmetry measured by IC Profiler the EPID method agreed to within 0.23%. For beam translational position steering, the EPID method agreed with IC Profiler method to within 0.03 ± 0.05 mm (1 SD) at isocenter. The EPID‐based methods presented are quick to perform, simple, accurate and could easily be integrated with the linac, potentially via the MPC application. The methods have the potential to remove user variability and to standardize the process of beam steering throughout the radiotherapy community. [ABSTRACT FROM AUTHOR]

Introduction: In-room MRI is a promising image guidance strategy in external beam radiotherapy to acquire volumetric information for moving targets. However, limitations in spatio-temporal resolution led several authors to use 2D orthogonal images for guidance. The aim of this work is to present a method to concurrently compensate for non-rigid tumour motion and provide an approach for 3D reconstruction from 2D orthogonal cine-MRI slices for MRI-guided treatments.Methods: Free-breathing sagittal/coronal interleaved 2D cine-MRI were acquired in addition to a pre-treatment 3D volume in two patients. We performed deformable image registration (DIR) between cine-MRI slices and corresponding slices in the pre-treatment 3D volume. Based on an extrapolation of the interleaved 2D motion fields, the 3D motion field was estimated and used to warp the pre-treatment volume. Due to the lack of a ground truth for patients, the method was validated on a digital 4D lung phantom.Results: On the phantom, the 3D reconstruction method was able to compensate for tumour motion and compared favourably to the results of previously adopted strategies. The difference in the 3D motion fields between the phantom and the extrapolated motion was 0.4 ± 0.3 mm for tumour and 0.8 ± 1.5 mm for whole anatomy, demonstrating feasibility of performing a 3D volumetric reconstruction directly from 2D orthogonal cine-MRI slices. Application of the method to patient data confirmed the feasibility of utilizing this method in real world scenarios.Conclusion: Preliminary results on phantom and patient cases confirm the feasibility of the proposed approach in an MRI-guided scenario, especially for non-rigid tumour motion compensation. [ABSTRACT FROM AUTHOR]

Purpose: Breathing management can reduce breath‐to‐breath (intrafraction) and day‐by‐day (interfraction) variability in breathing motion while utilizing the respiratory motion of internal and external surrogates for respiratory guidance. Audiovisual (AV) biofeedback, an interactive personalized breathing motion management system, has been developed to improve reproducibility of intra‐ and interfraction breathing motion. However, the assumption of the correlation of respiratory motion between surrogates and tumors is not always verified during medical imaging and radiation treatment. Therefore, the aim of the study was to test the hypothesis that the correlation of respiratory motion between surrogates and tumors is the same under free breathing without guidance (FB) and with AV biofeedback guidance for voluntary motion management. Methods: For 13 lung cancer patients receiving radiotherapy, 2D coronal and sagittal cine‐MR images were acquired across two MRI sessions (pre‐ and mid‐treatment) with two breathing conditions: (a) FB and (b) AV biofeedback, totaling 88 patient measurements. Simultaneously, the external respiratory motion of the abdomen was measured. The internal respiratory motion of the diaphragm and lung tumor was retrospectively measured from 2D coronal and sagittal cine‐MR images. The correlation of respiratory motion between surrogates and tumors was calculated using Pearson's correlation coefficient for: (a) abdomen to tumor (abdomen‐tumor) and (b) diaphragm to tumor (diaphragm‐tumor). The correlations were compared between FB and AV biofeedback using several metrics: abdomen‐tumor and diaphragm‐tumor correlations with/without ≥5 mm tumor motion range and with/without adjusting for phase shifts between the signals. Results: Compared to FB, AV biofeedback improved abdomen‐tumor correlation by 11% (p = 0.12) from 0.53 to 0.59 and diaphragm‐tumor correlation by 13% (p = 0.02) from 0.55 to 0.62. Compared to FB, AV biofeedback improved abdomen‐tumor correlation by 17% (p = 0.01) and diaphragm‐tumor correlation by 15% (p < 0.01) while correcting 0.3 s (p = 0.54) and 0.2 s (p = 0.19) phase shifts, respectively. In addition, AV biofeedback with ≥5 mm tumor motion range, compared to FB improved abdomen‐tumor correlation by 14% (p = 0.18) and diaphragm‐tumor correlation by 17% (p = 0.01). The highest abdomen‐tumor and diaphragm‐tumor correlations were found using ≥5 mm tumor motion range and phase shifts, resulting in a 12% improvement in AV biofeedback. Conclusions: Our results demonstrated that AV biofeedback improves the correlation of respiratory motion between surrogates and the tumor. This suggests a need for AV biofeedback for respiratory guidance utilizing respiratory surrogates during image‐guided and MRI‐guided radiotherapy in thoracic regions. [ABSTRACT FROM AUTHOR]

Background: MOSFET dosimetry is a method that has been used to measure in-vivo doses during brachytherapy treatments and during linac based radiotherapy treatment. Rectal displacement devices (RDDs) allow for safe dose escalation for prostate cancer treatment. This study used dual MOSkin detectors to assess real-time in vivo rectal wall dose in patients with an RDD in place during a high dose prostate stereotactic body radiation therapy (SBRT) boost trial.Methods: The PROMETHEUS study commenced in 2014 and provides a prostate SBRT boost dose with a RDD in place. Twelve patients received two boost fractions of 9.5-10 Gy each delivered to the prostate with a dual arc volumetric modulated arc therapy (VMAT) technique. Two MOSkins in a face-to-face arrangement (dual MOSkin) were used to decrease angular dependence. A dual MOSkin was attached to the anterior surface of the Rectafix and read out at 1 Hz during each treatment. The planned dose at each measurement point was exported from the planning system and compared with the measured dose. The root mean square error normalised to the total planned dose was calculated for each measurement point and treatment arc for the entire course of treatment.Results: The average difference between the measured and planned doses over the whole course of treatment for all arcs measured was 9.7% with a standard deviation of 3.6%. The cumulative MOSkin reading was lower than the total planned dose for 64% of the arcs measured. The average difference between the final measured and final planned doses for all arcs measured was 3.4% of the final planned dose, with a standard deviation of 10.3%.Conclusions: MOSkin detectors were an effective tool for measuring dose delivered to the anterior rectal wall in real time during prostate SBRT boost treatments for the purpose of both ensuring the rectal doses remain within acceptable limits during the treatment and for the verification of final rectal doses. [ABSTRACT FROM AUTHOR]

Purpose Due to increasing complexity, modern radiotherapy techniques require comprehensive quality assurance (QA) programmes, that to date generally focus on the pre-treatment stage. The purpose of this paper is to provide a method for an individual patient treatment QA evaluation and identification of a "quality gap" for continuous quality improvement. Design/methodology/approach A statistical process control (SPC) was applied to evaluate treatment delivery using in vivo electronic portal imaging device (EPID) dosimetry. A moving range control chart was constructed to monitor the individual patient treatment performance based on a control limit generated from initial data of 90 intensity-modulated radiotherapy (IMRT) and ten volumetric-modulated arc therapy (VMAT) patient deliveries. A process capability index was used to evaluate the continuing treatment quality based on three quality classes: treatment type-specific, treatment linac-specific, and body site-specific. Findings The determined control limits were 62.5 and 70.0 per cent of the χ pass-rate for IMRT and VMAT deliveries, respectively. In total, 14 patients were selected for a pilot study the results of which showed that about 1 per cent of all treatments contained errors relating to unexpected anatomical changes between treatment fractions. Both rectum and pelvis cancer treatments demonstrated process capability indices were less than 1, indicating the potential for quality improvement and hence may benefit from further assessment. Research limitations/implications The study relied on the application of in vivo EPID dosimetry for patients treated at the specific centre. Sampling patients for generating the control limits were limited to 100 patients. Whilst the quantitative results are specific to the clinical techniques and equipment used, the described method is generally applicable to IMRT and VMAT treatment QA. Whilst more work is required to determine the level of clinical significance, the authors have demonstrated the capability of the method for both treatment specific QA and continuing quality improvement. Practical implications The proposed method is a valuable tool for assessing the accuracy of treatment delivery whilst also improving treatment quality and patient safety. Originality/value Assessing in vivo EPID dosimetry with SPC can be used to improve the quality of radiation treatment for cancer patients. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this study is to investigate the performance and limitations of a real-time transit electronic portal imaging device (EPID) dosimetry system for error detection during dynamic intensity modulated radiation therapy (IMRT) treatment delivery. Sites studied are prostate, head and neck (HN), and rectal cancer treatments.Methods: The system compares measured cumulative transit EPID image frames with predicted cumulative image frames in real-time during treatment using a χ comparison with 4 %, 4 mm criteria. The treatment site-specific thresholds (prostate, HN and rectum IMRT) were determined using initial data collected from 137 patients (274 measured treatment fractions) and a statistical process control methodology. These thresholds were then applied to data from 15 selected patients including 5 prostate, 5 HN, and 5 rectum IMRT treatments for system evaluation and classification of error sources.Results: Clinical demonstration of real-time transit EPID dosimetry in IMRT was presented. For error simulation, the system could detect gross errors (i.e. wrong patient, wrong plan, wrong gantry angle) immediately after EPID stabilisation; 2 seconds after the start of treatment. The average rate of error detection was 7.0 % (prostate = 5.6 %, HN= 8.7 % and rectum = 6.7 %). The detected errors were classified as either clinical in origin (e.g. patient anatomical changes), or non-clinical in origin (e.g. detection system errors). Classified errors were 3.2 % clinical and 3.9 % non-clinical.Conclusion: An EPID-based real-time error detection method for treatment verification during dynamic IMRT has been developed and tested for its performance and limitations. The system is able to detect gross errors in real-time, however improvement in system robustness is required to reduce the non-clinical sources of error detection. [ABSTRACT FROM AUTHOR]

Purpose: The feasibility of radiation therapy treatment planning using substitute computed tomography (sCT) generated from magnetic resonance images (MRIs) has been demonstrated by a number of research groups. One challenge with an MRI-alone workflow is the accurate identification of intraprostatic gold fiducial markers, which are frequently used for prostate localization prior to each dose delivery fraction. This paper investigates a template-matching approach for the detection of these seeds in MRI. Methods: Two different gradient echo T1 and T2* weighted MRI sequences were acquired from fifteen prostate cancer patients and evaluated for seed detection. For training, seed templates from manual contours were selected in a spectral clustering manifold learning framework. This aids in clustering "similar" gold fiducial markers together. The marker with the minimum distance to a cluster centroid was selected as the representative template of that cluster during training. During testing, Gaussian mixture modeling followed by a Markovian model was used in automatic detection of the probable candidates. The probable candidates were rigidly registered to the templates identified from spectral clustering, and a similarity metric is computed for ranking and detection. Results: A fiducial detection accuracy of 95% was obtained compared to manual observations. Expert radiation therapist observers were able to correctly identify all three implanted seeds on 11 of the 15 scans (the proposed method correctly identified all seeds on 10 of the 15). Conclusions: An novel automatic framework for gold fiducial marker detection in MRI is proposed and evaluated with detection accuracies comparable to manual detection. When radiation therapists are unable to determine the seed location in MRI, they refer back to the planning CT (only available in the existing clinical framework); similarly, an automatic quality control is built into the automatic software to ensure that all gold seeds are either correctly detected or a warning is raised for further manual intervention. [ABSTRACT FROM AUTHOR]

A new tool has been developed to verify the trajectory of dynamic multileaf collimators (MLCs) used in advanced radiotherapy techniques using only the information provided by the electronic portal imaging devices (EPID) measured image frames. The prescribed leaf positions are resampled to a higher resolution in a pre-processing stage to improve the verification precision. Measured MLC positions are extracted from the EPID frames using a template matching method. A cosine similarity metric is then applied to synchronise measured and planned leaf positions for comparison. Three additional comparison functions were incorporated to ensure robust synchronisation. The MLC leaf trajectory error detection was simulated for both intensity modulated radiation therapy (IMRT) (prostate) and volumetric modulated arc therapy (VMAT) (head-and-neck) deliveries with anthropomorphic phantoms in the beam. The overall accuracy for MLC positions automatically extracted from EPID image frames was approximately 0.5 mm. The MLC leaf trajectory verification system can detect leaf position errors during IMRT and VMAT with a tolerance of 3.5 mm within 1 s. [ABSTRACT FROM AUTHOR]

Purpose: Standard amorphous silicon electronic portal imaging devices (a-Si EPIDs) are x-ray imagers used frequently in radiotherapy that indirectly detect incident x-rays using a metal plate and phosphor screen. These detectors may also be used as two-dimensional dosimeters; however, they have a well-characterized nonwater-equivalent dosimetric response. Plastic scintillating (PS) fibers, on the other hand, have been shown to respond in a water-equivalent manner to x-rays in the energy range typically encountered during radiotherapy. In this study, the authors report on the first experimental measurements taken with a novel prototype PS a-Si EPID developed for the purpose of performing simultaneous imaging and dosimetry in radiotherapy. This prototype employs an array of PS fibers in place of the standard metal plate and phosphor screen. The imaging performance and dosimetric response of the prototype EPID were evaluated experimentally and compared to that of the standard EPID. Methods: Clinical 6 MV photon beams were used to first measure the detector sensitivity, linearity of dose response, and pixel noise characteristics of the prototype and standard EPIDs. Second, the dosimetric response of each EPID was evaluated relative to a reference water-equivalent dosimeter by measuring the off-axis and field size response in a nontransit configuration, along with the off-axis, field size, and transmission response in a transit configuration using solid water blocks. Finally, the imaging performance of the prototype and standard EPIDs was evaluated quantitatively by using an image quality phantom to measure the contrast to noise ratio (CNR) and spatial resolution of images acquired with each detector, and qualitatively by using an anthropomorphic phantom to acquire images representative of human anatomy. Results: The prototype EPID's sensitivity was 0.37 times that of the standard EPID. Both EPIDs exhibited responses that were linear with delivered dose over a range of 1-100 monitor units. Over this range, the prototype and standard EPID central axis responses agreed to within 1.6%. Images taken with the prototype EPID were noisier than those taken with the standard EPID, with fractional uncertainties of 0.2% and 0.05% within the central 1 cm2, respectively. For all dosimetry measurements, the prototype EPID exhibited a near water-equivalent response whereas the standard EPID did not. The CNR and spatial resolution of images taken with the standard EPID were greater than those taken with the prototype EPID. Conclusions: A prototype EPID employing an array of PS fibers has been developed and the first experimental measurements are reported. The prototype EPID demonstrated a much morewater-equivalent dose response than the standard EPID. While the imaging performance of the standard EPID was superior to that of the prototype, the prototype EPID has many design characteristics that may be optimized to improve imaging performance. This investigation demonstrates the feasibility of a new detector design for simultaneous imaging and dosimetry treatment verification in radiotherapy. [ABSTRACT FROM AUTHOR]

Purpose: In advanced radiotherapy treatments such as intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT), verification of the performance of the multileaf collimator (MLC) is an essential part of the linac QA program. The purpose of this study is to use the existing measurement methods for geometric QA of the MLCs and extend them to more comprehensive evaluation techniques, and to develop dedicated robust algorithms to quantitatively investigate the MLC performance in a fast, accurate, and efficient manner. Methods: The behavior of leaves was investigated in the step-and-shoot mode by the analysis of integrated electronic portal imaging device (EPID) images acquired during picket fence tests at fixed gantry angles and arc delivery. The MLC was also studied in dynamic mode by the analysis of cine EPID images of a sliding gap pattern delivered in a variety of conditions including different leaf speeds, deliveries at fixed gantry angles or in arc mode, and changing the direction of leaf motion. The accuracy of the method was tested by detection of the intentionally inserted errors in the delivery patterns. Results: The algorithm developed for the picket fence analysis was able to find each individual leaf position, gap width, and leaf bank skewness in addition to the deviations from expected leaf positions with respect to the beam central axis with sub-pixel accuracy. For the three tested linacs over a period of 5 months, the maximum change in the gap width was 0.5 mm, the maximum deviation from the expected leaf positions was 0.1 mm and the MLC skewness was up to 0.2°. The algorithm developed for the sliding gap analysis could determine the velocity and acceleration/deceleration of each individual leaf as well as the gap width. There was a slight decrease in the accuracy of leaf performance with increasing leaf speeds. The analysis results were presented through several graphs. The accuracy of the method was assessed as 0.01 mm for both the gap size and peak position determination. Conclusions: This study provides fast, easy, and accurate test methods for routine QA of the MLC performance and helps in faster troubleshooting of MLC problems in both IMRT and VMAT treatments. [ABSTRACT FROM AUTHOR]

In this study an amorphous silicon electronic portal imaging device (a-Si EPID) converted to direct detection configuration was investigated as a transit dosimeter for intensity modulated radiation therapy (IMRT). After calibration to dose and correction for a background offset signal, the EPID-measured absolute IMRT transit doses for 29 fields were compared to aMatriXX two-dimensional array of ionization chambers (as reference) using Gamma evaluation (3%, 3 mm). The MatriXX was first evaluated as reference for transit dosimetry. The accuracy of EPID measurements was also investigated by comparison of point dose measurements by an ionization chamber on the central axis with slab and anthropomorphic phantoms in a range of simple to complex fields. The uncertainty in ionization chamber measurements in IMRT fields was also investigated by its displacement from the central axis and comparison with the central axis measurements. Comparison of the absolute doses measured by the EPID and MatriXX with slab phantoms in IMRT fields showed that on average 96.4% and 97.5% of points had a Gamma index<1 in head and neck and prostate fields, respectively. For absolute dose comparisons with anthropomorphic phantoms, the values changed to an average of 93.6%, 93.7% and 94.4% of points with Gamma index<1 in head and neck, brain and prostate fields, respectively. Point doses measured by the EPID and ionization chamber were within 3% difference for all conditions. The deviations introduced in the response of the ionization chamber in IMRT fields were<1%. The direct EPID performance for transit dosimetry showed that it has the potential to perform accurate, efficient and comprehensive in vivo dosimetry for IMRT. [ABSTRACT FROM AUTHOR]

In modern radiotherapy, it is vitally important to monitor the performance of all linac components including the collimation system. In this study, a simple measurement method and accurate algorithm are introduced for investigation of the secondary and tertiary collimator sag during radiotherapy arc treatments. The method is based on cine EPID images of a ball bearing marker fixed to the gantry head and determines the jaw and MLC sag in all directions relative to the reference at zero gantry angle. Analysis was performed using different field sizes and collimator angles, different linacs and different gantry rotation directions. The accuracy of the method was tested and was less than 0.02 mm. The repeatability and reproducibility of the method was 0.005 and 0.09 mm, respectively. The setup is easy and quick and the algorithm is fast and fully automatic with sub-pixel accuracy. This method is suitable to be included in the routine quality assurance of linacs to monitor the collimator system performance. [ABSTRACT FROM AUTHOR]

Purpose: Electronic portal imaging devices (EPIDs) are high resolution systems that produce electronic dose maps with minimal time required for equipment setup, and therefore potentially present a time-saving alternative for intensity modulated radiation therapy (IMRT) pretreatment verification. A modified commercial EPID was investigated operated with an opaque sheet blocking the optical signal produced in the phosphor layer as a precursor to a switched mode dual dosimetry-imaging EPID system. The purpose of this study was to investigate the feasibility of using this system for direct dose to water dosimetry for pretreatment IMRT verification. Methods: A Varian amorphous silicon EPID was modified by placing an opaque sheet between the Gd2S2O:Tb phosphor layer and the photodiode array to block the optical photons. The EPID was thus converted to a direct-detecting system (dEPID), in which the high energy radiation deposits energy directly in the photodiode array. The copper build-up was replaced with dmax solid water. Sixty-one IMRT beams of varying complexity were delivered to the EPID, to EDR2 dosimetric film and to a 2D ion chamber array (MapCheck). EPID data was compared to film and MapCheck data using gamma analysis with 3%, 3mm pass criteria. Results: The fraction of points that passed the gamma test was on average 98.1% and 98.6%, for the EPID versus film and EPID versus MapCheck comparisons, respectively. In the case of comparison with film, the majority of observed discrepancies were associated with problems related to film sensitivity or processing. Conclusions: The very close agreement between EPID and both film and MapCheck data demonstrates that the modified EPID is suitable for direct dose to water measurement for pretreatment IMRT verification. These results suggest a reconfigured EPID could be an efficient and accurate dosimeter. Alternatively, optical switching methods could be developed to produce a dual-mode EPID with both dosimetry and imaging capabilities. [ABSTRACT FROM AUTHOR]

Purpose:Verification of the mechanical isocenter position is required as part of comprehensive quality assurance programs for stereotactic radiosurgery/radiotherapy (SRS/SRT) treatments. Several techniques have been proposed for this purpose but each of them has certain drawbacks. In this paper, a new efficient and more comprehensive method using cine-EPID images has been introduced for automatic verification of the isocenter with sufficient accuracy for stereotactic applications.Methods: Using a circular collimator fixed to the gantry head to define the field, EPID images of a Winston-Lutz phantom were acquired in cine-imaging mode during 360° gantry rotations. A robust matlab code was developed to analyze the data by finding the center of the field and the center of the ball bearing shadow in each image with sub-pixel accuracy. The distance between these two centers was determined for every image. The method was evaluated by comparison to results of a mechanical pointer and also by detection of a manual shift applied to the phantom position. The repeatability and reproducibility of the method were tested and it was also applied to detect couch and collimator wobble during rotation.Results:The accuracy of the algorithm was 0.03 ± 0.02 mm. The repeatability was less than 3 μm and the reproducibility was less than 86 μm. The time elapsed for the analysis of more than 100 cine images of Varian aS1000 and aS500 EPIDs were ∼65 and 20 s, respectively. Processing of images taken in integrated mode took 0.1 s. The output of the analysis software is printable and shows the isocenter shifts as a function of angle in both in-plane and cross-plane directions. It gives warning messages where the shifts exceed the criteria for SRS/SRT and provides useful data for the necessary adjustments in the system including bearing system and/or room lasers.Conclusions: The comprehensive method introduced in this study uses cine-images, is highly accurate, fast, and independent of the observer. It tests all gantry angles and is suitable for pretreatment QA of the isocenter for stereotactic treatments. [ABSTRACT FROM AUTHOR]

The article describes the development of an alternative planning method using magnetic resonance imaging (MRI) for both organ delineation and dose calculation for prostate cancer radiation therapy. It describes the standard treatment planning for external-beam radiation therapy (EBRT). It compares computed tomography (CT) scans and MRI scans for radiation therapy planning. It concludes that atlas-based segmentation methods can automatically define anatomy on MRI scans of the pelvis, and map electron densities to MRI scans for radiotherapy dose planning.

Abstract: Purpose: To assess the accuracy of the initial CT plan dose–volume histograms (DVH’s) for prostate, rectum and bladder by comparison to delivered doses determined from cone beam CT (CBCT) scans acquired during image-guided treatment. Materials and methods: Twelve prostate patients were treated using daily implanted fiducial guidance and following local protocol for bladder and rectal preparation. CBCT scans were acquired twice weekly and contoured for prostate, rectum and bladder. The planned beams were applied to all CBCT scans to determine the delivered doses. Prostate dose coverage was assessed by the proportion of the CTV fully encompassed by the 95% and 98% isodose lines. Rectal and bladder volumes receiving 40Gy, 60Gy and 70Gy at treatment were compared to the initial plan, with significance determined using the one-sample t-test. Results: Four patients showed marginally compromised CTV coverage by the 95% isodose at all CBCT plans. For nine patients the initial plan rectal DVH was significantly outside the range of the treatment DVH’s. Conclusions: Dose coverage of the prostate was not achieved for all patients. Observed rectal and bladder doses were higher than predicted. The initial treatment plan cannot be assumed to represent accurate normal tissue doses. [Copyright &y& Elsevier]

The aim of this study is to present an efficient method to generate imager-specific Monte Carlo (MC)-based dose kernels for amorphous silicon-based electronic portal image device dose prediction and determine the effective backscattering thicknesses for such imagers. EPID field size-dependent responses were measured for five matched Varian accelerators from three institutions with 6 MV beams at the source to detector distance (SDD) of 105 cm. For two imagers, measurements were made with and without the imager mounted on the robotic supporting arm. Monoenergetic energy deposition kernels with 0–2.5 cm of water backscattering thicknesses were simultaneously computed by MC to a high precision. For each imager, the backscattering thickness required to match measured field size responses was determined. The monoenergetic kernel method was validated by comparing measured and predicted field size responses at 150 cm SDD, 10×10 cm2 multileaf collimator (MLC) sliding window fields created with 5, 10, 20, and 50 mm gaps, and a head-and-neck (H&N) intensity modulated radiation therapy (IMRT) patient field. Field size responses for the five different imagers deviated by up to 1.3%. When imagers were removed from the robotic arms, response deviations were reduced to 0.2%. All imager field size responses were captured by using between 1.0 and 1.6 cm backscatter. The predicted field size responses by the imager-specific kernels matched measurements for all involved imagers with the maximal deviation of 0.34%. The maximal deviation between the predicted and measured field size responses at 150 cm SDD is 0.39%. The maximal deviation between the predicted and measured MLC sliding window fields is 0.39%. For the patient field, gamma analysis yielded that 99.0% of the pixels have γ<1 by the 2%, 2 mm criteria with a 3% dose threshold. Tunable imager-specific kernels can be generated rapidly and accurately in a single MC simulation. The resultant kernels are imager position independent and are able to predict fields with varied incident energy spectra and a H&N IMRT patient field. The proposed adaptive EPID dose kernel method provides the necessary infrastructure to build reliable and accurate portal dosimetry systems. [ABSTRACT FROM AUTHOR]

A simplified method of verifying intensity modulated radiation therapy (IMRT) fields using a Varian aS500 amorphous silicon electronic portal imaging device (EPID) is demonstrated. Unlike previous approaches, it does not involve time consuming or complicated analytical processing of the data. The central axis pixel response of the EPID, as well as the profile characteristics obtained from images acquired with a 6 MV photon beam, was examined as a function of field size. Ion chamber measurements at various depths in a water phantom were then collected and it was found that at a specific depth dref, the dose response and profile characteristics closely matched the results of the EPID analysis. The only manipulation required to be performed on the EPID images was the multiplication of a matrix of off axis ratio values to remove the effect of the flood field calibration. Similarly, dref was found for 18 MV. Planar dose maps at dref in a water phantom for a bar pattern, a strip pattern, and 14 clinical IMRT fields from two patient cases each being from a separate anatomical region, i.e., head and neck as well as the pelvis, for both energies were generated by the Pinnacle planning system (V7.4). EPID images of these fields were acquired and converted to planar dose maps and compared directly with the Pinnacle planar dose maps. Radiographic film dosimetry and a MapCHECK dosimetry device (Sun Nuclear Corporation, Melbourne, FL) were used as an independent verification of the dose distribution. Gamma analysis of the EPID, film, and Pinnacle planar dose maps generated for the clinical IMRT fields showed that approximately 97% of all points passed using a 3% dose/3mm DTA tolerance test. Based on the range of fields studied, the author’s results appear to justify using this approach as a method to verify dose distributions calculated on a treatment planning system, including complex intensity modulated fields. [ABSTRACT FROM AUTHOR]

In this work, an amorphous silicon electronic portal imaging device (a-Si EPID) dose prediction model based on the energy fluence model of the Pinnacle treatment planning system Version 7 (Philips Medical Systems, Madison, WI) is developed. An energy fluence matrix at very high resolution (<1 mm) is used to incorporate multileaf collimator (MLC) leaf effects in the predicted EPID images. The primary dose deposited in the EPID is calculated from the energy fluence using experimentally derived radially dependent EPID interaction coefficients. Separate coefficients are used for the open beam energy fluence component and the component of the energy fluence transmitted through closed MLC leaves to each EPID pixel. A spatially invariant EPID dose deposition kernel that describes both radiative dose deposition, central axis EPID backscatter, and optical glare is convolved with the primary dose. The kernel is further optimized to give accurate EPID penumbra prediction and EPID scatter factor with changing MLC field size. An EPID calibration method was developed to reduce the effect of nonuniform backscatter from the support arm (E-arm) in a calibrated EPID image. This method removes the backscatter component from the pixel sensitivity (flood field) correction matrix retaining only field-specific backscatter in the images. The model was compared to EPID images for jaw and MLC defined open fields and eight head and neck intensity modulated radiotherapy (IMRT) fields. For the head and neck IMRT fields with 2%, 2 mm criteria 97.6±0.6% (mean ±1 standard deviation) of points passed with a gamma index less than 1, and for 3%, 3 mm 99.4±0.4% of points were within the criteria. For these fields, the 2%, 2 mm pass score reduced to 96.0±1.5% when backscatter was present in the pixel sensitivity correction matrix. The model incorporates the effect of MLC leaf transmission, EPID response to open and MLC leakage dose components, and accurately predicts EPID images of IMRT fields. Removing the backscatter component of the pixel sensitivity matrix correction reduces the effect of nonuniform E-arm backscatter. [ABSTRACT FROM AUTHOR]

Electronic portal imaging devices (EPIDs) integrated with medical linear accelerators utilize an indirect-detection EPID configuration (ID-EPID). Amorphous silicon ID-EPIDs provide high quality low dose images for verification of radiotherapy treatments but they have limitations as dosimeters. The standard ID-EPID configuration includes a high atomic number phosphor scintillator screen, a 1 mm copper layer, and other nonwater equivalent materials covering the detector. This configuration leads to marked differences in the response of an ID-EPID compared to standard radiotherapy dosimeters such as ion chambers in water. In this study the phosphor and copper were removed from a standard commercial EPID to modify the configuration to a direct-detection EPID (DD-EPID). Using solid water as the buildup and backscatter for the detector, dosimetric measurements were performed on the DD-EPID and compared to standard dose-in-water data for 6 and 18 MV photons. The sensitivity of the DD-EPID was approximately eight times less than the ID-EPID but the signal was sufficient to produce accurate and reproducible beam profile measurements for open beams and an intensity-modulated beam. Due to the lower signal levels it was found necessary to ensure that the dark field correction (no radiation) DD-EPID signal was stable or updated frequently. The linearity of dose response was comparable to the ID-EPID but with a greater under-response at low doses. DD-EPID measurements of field size output factors and beam profiles at the depth of maximum dose (dmax), and tissue-maximum ratios between the depths of 0.5 and 10 cm, were in close agreement with dose in water measurements. At depths beyond dmax the DD-EPID showed a greater change in response to field size than ionisation chamber measurements and the beam penumbrae were broader compared to diode scans. The modified DD-EPID configuration studied here has the potential to improve the performance of EPIDs for dose verification of radiotherapy treatments. [ABSTRACT FROM AUTHOR]

The aim of this work was to experimentally determine the difference in response of an amorphous silicon (a-Si) electronic portal imaging device (EPID) to the open and multileaf collimator (MLC) transmitted beam components of intensity modulated radiation therapy (IMRT) beams. EPID dose response curves were measured for open and MLC transmitted (MLCtr) 10×10 cm2 beams at central axis and with off axis distance using a shifting field technique. The EPID signal was obtained by replacing the flood-field correction with a pixel sensitivity variation matrix correction. This signal, which includes energy-dependent response, was then compared to ion-chamber measurements. An EPID calibration method to remove the effect of beam energy variations on EPID response was developed for IMRT beams. This method uses the component of open and MLCtr fluence to an EPID pixel calculated from the MLC delivery file and applies separate radially dependent calibration factors for each component. The calibration procedure does not correct for scatter differences between ion chamber in water measurements and EPID response; these must be accounted for separately with a kernel-based approach or similar method. The EPID response at central axis for the open beam was found to be 1.28±0.03 of the response for the MLCtr beam, with the ratio increasing to 1.39 at 12.5 cm off axis. The EPID response to MLCtr radiation did not change with off-axis distance. Filtering the beam with copper plates to reduce the beam energy difference between open and MLCtr beams was investigated; however, these were not effective at reducing EPID response differences. The change in EPID response for uniform sliding window IMRT beams with MLCtr dose components from 0.3% to 69% was predicted to within 2.3% using the separate EPID response calibration factors for each dose component. A clinical IMRT image calibrated with this method differed by nearly 30% in high MLCtr regions from an image calibrated with an open beam calibration factor only. Accounting for the difference in EPID response to open and MLCtr radiation should improve IMRT dosimetry with a-Si EPIDs. [ABSTRACT FROM AUTHOR]

Amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) have typically been calibrated to dose at central axis (CAX). Division of acquired images by the flood-field (FF) image that corrects for pixel sensitivity variation as well as open field energy-dependent off-axis response variation should result in a flat EPID response over the entire matrix for the same field size. While the beam profile can be reintroduced to the image by an additional correction matrix, the CAX EPID response to dose calibration factor is assumed to apply to all pixels in the detector. The aim of this work was to investigate the dose response of the Varian aS500 amorphous silicon detector across the entire detector area. First it was established that the EPID response across the panel became stable (within ∼0.2%) for MU settings greater than ∼200 MU. The EPID was then FF calibrated with a high MU setting of ∼400 for all subsequent experiments. Whole detector images with varying MU settings from 2–500 were then acquired for two dose rates (300 and 600 MU/min) for 6 MV photons for two EPIDs. The FF corrected EPID response was approximately flat or uniform across the detector for greater than 100 MU delivered (within 0.5%). However, the off-axis EPID response was greater than the CAX response for small MU irradiations, giving a raised EPID profile. Up to 5% increase in response at 20 cm off-axis compared to CAX was found for very small MU settings for one EPID, while it was within 2% for the second (newer) EPID. Off-axis response nonuniformities attributed to detector damage were also found for the older EPID. Similar results were obtained with the EPID at 18 MV energy and operating in asynchronous mode (acquisition not synchronized with beam pulses), however the profiles were flatter and more irregular for the small MU irradiations. By moving the detector laterally and repeating the experiments, the increase in response off-axis was found to depend on the pixel position relative to the beam CAX. When the beam was heavily filtered by a phantom the off-axis response variation was reduced markedly to within 0.5% for all MU settings. Independent measurements of off-axis point doses with ion chamber did not show any change in off-axis factor with MUs. Measurements of beam quality (TMR20-10) for MU settings of 2, 5, and 100 at central axis and at 15 cm off-axis could not explain the effect. The response change is unlikely to be significant for clinical IMRT verification with this imaging/acclerator system where MUs are of the order of 100–300, provided the detector does not exhibit radiation damage artifacts. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this study was to develop an evidence-based off-line setup correction protocol for systematic errors in prostate radiation therapy. Daily orthogonal electronic portal images were acquired from 30 patients. Field displacements were measured in the medial-lateral (ML), superior-inferior (SI), and anterior-posterior (AP) directions for each treatment fraction. The off-line protocol corrects the mean field displacement found from n consecutive images, starting at a particular fraction of treatment, with a fixed tolerance level. Simulations were performed with the measured data to determine (1) how many images (n) should be averaged to determine the systematic error; (2) on which treatment fraction should the protocol be initiated; and (3) what tolerance level should be applied to determine whether the patient position should be corrected. Uncorrected systematic errors in the ML, SI, and AP directions were (mean position ± 1 standard deviation [SD]): −0.7 ± 2.2 mm, −1.5 ± 1.3 mm, and 1.4 ± 2.6 mm, respectively. Random errors (1 SD and range) were 1.9 mm (1.3–3.3), 1.5 mm (0.–4.1), and 1.8 mm (1.0–2.6), respectively. A correction based on a single image taken on the first fraction actually increased the systematic errors in the ML and SI directions compared with no correction. More accurate correction of systematic errors was achieved with increasing number of images averaged, with only small benefit after 5 images. With fewer images averaged, delaying the start of the protocol resulted in more accurate correction because of the influence of unrepresentative positions at early fractions. The number of corrections made on patients with small (&#60;2 mm) systematic errors was minimized for tolerance values of 2 mm and n ≥ 5 images averaged. The optimal off-line setup correction protocol would be to shift the patient by the mean displacement of the first 5 portal images of a radical course of radiation therapy. A small tolerance level should be utilized with 2 mm giving good accuracy with minimal unnecessary shifts. [Copyright &y& Elsevier]

• Patient specific quality assurance can miss clinically significant errors in SABR plan deliveries. • Remote auditing of the sensitivity of patient specific quality assurance is feasible. • Assessing the sensitivity of clinically used patient specific quality assurance procedures is a new concept for dosimetry audits. The quality of radiotherapy delivery has been shown to significantly impact clinical outcomes including patient survival. To identify errors, institutions perform Patient Specific Quality Assurance (PSQA) assessing each individual radiotherapy plan prior to starting patient treatments. Externally administered Dosimetry Audits have found problems despite institutions passing their own PSQA. Hence a new audit concept which assesses the institution's ability to detect errors with their routine PSQA is needed. Purposefully introduced edits which simulated treatment delivery errors were embedded into radiation treatment plans of participating institutions. These were designed to produce clinically significant changes yet were mostly within treatment delivery specifications. Actual impact was centrally assessed for each plan. Institutions performed PSQA on each plan, without knowing which contained errors. Seventeen institutions using six radiation treatment planning systems and two delivery systems performed PSQA on twelve plans each. Seventeen erroneous plans (across seven institutions) passed PSQA despite causing >5% increase in spinal cord dose relative to the original plans. Six plans (from four institutions) passed despite a >10% increase. This novel audit concept evolves beyond testing an institution's ability to deliver a single test case, to increasing the number of errors caught by institutions themselves, thus increasing quality of radiation therapy and impacting every patient treated. Administered remotely this audit also provides advantages in cost, environmental impact, and logistics. [ABSTRACT FROM AUTHOR]

Purpose: Methods have been recently developed to generate pseudo-computed tomography (pCT) for dose calculation in magnetic resonance imaging (MRI)-only radiation therapy. This study aimed to propose an original nonlocal mean patch-based method (PBM) and to compare this PBM to an atlas-based method (ABM) and to a bulk density method (BDM) for prostate MRI-only radiation therapy.Materials and Methods: Thirty-nine patients received a volumetric modulated arc therapy for prostate cancer. In addition to the planning computed tomography (CT) scans, T2-weighted MRI scans were acquired. pCTs were generated from MRIs using 3 methods: an original nonlocal mean PBM, ABM, and BDM. The PBM was performed using feature extraction and approximate nearest neighbor search in a training cohort. The PBM accuracy was evaluated in a validation cohort by using imaging and dosimetric endpoints. Imaging endpoints included mean absolute error and mean error between Hounsfield units of the pCT and the reference CT (CTref). Dosimetric endpoints were based on dose-volume histograms calculated from the CTref and the pCTs for various volumes of interest and on 3-dimensional gamma analyses. The PBM uncertainties were compared with those of the ABM and BDM.Results: The mean absolute error and mean error obtained from the PBM were 41.1 and -1.1 Hounsfield units. The PBM dose-volume histogram differences were 0.7% for prostate planning target volume V95%, 0.5% for rectum V70Gy, and 0.2% for bladder V50Gy. Compared with ABM and BDM, PBM provided significantly lower dose uncertainties for the prostate planning target volume (70-78 Gy), the rectum (8.5-29 Gy, 40-48 Gy, and 61-73 Gy), and the bladder (12-78 Gy). The PBM mean gamma pass rate (99.5%) was significantly higher than that of ABM (94.9%) or BDM (96.1%).Conclusions: The proposed PBM provides low uncertainties with dose planned on CTref. These uncertainties were smaller than those of ABM and BDM and are unlikely to be clinically significant. [ABSTRACT FROM AUTHOR]

Purpose: To report on the primary and secondary outcomes of a prospective clinical trial of electromagnetic-guided multileaf collimator (MLC) tracking radiation therapy for prostate cancer.Methods and Materials: Twenty-eight men with prostate cancer were treated with electromagnetic-guided MLC tracking with volumetric modulated arc therapy. A total of 858 fractions were delivered, with the dose per fraction ranging from 2 to 13.75 Gy. The primary outcome was feasibility, with success determined if >95% of fractions were successfully delivered. The secondary outcomes were (1) the improvement in beam-target geometric alignment, (2) the improvement in dosimetric coverage of the prostate and avoidance of critical structures, and (3) no acute grade ≥3 genitourinary or gastrointestinal toxicity.Results: All 858 planned fractions were successfully delivered with MLC tracking, demonstrating the primary outcome of feasibility (P < .001). MLC tracking improved the beam-target geometric alignment from 1.4 to 0.90 mm (root-mean-square error). MLC tracking improved the dosimetric coverage of the prostate and reduced the daily variation in dose to critical structures. No acute grade ≥3 genitourinary or gastrointestinal toxicity was observed.Conclusions: Electromagnetic-guided MLC tracking radiation therapy for prostate cancer is feasible. The patients received improved geometric targeting and delivered dose distributions that were closer to those planned than they would have received without electromagnetic-guided MLC tracking. No significant acute toxicity was observed. [ABSTRACT FROM AUTHOR]

Purpose: To validate automatic substitute computed tomography CT (sCT) scans generated from standard T2-weighted (T2w) magnetic resonance (MR) pelvic scans for MR-Sim prostate treatment planning.Patients and Methods: A Siemens Skyra 3T MR imaging (MRI) scanner with laser bridge, flat couch, and pelvic coil mounts was used to scan 39 patients scheduled for external beam radiation therapy for localized prostate cancer. For sCT generation a whole-pelvis MRI scan (1.6 mm 3-dimensional isotropic T2w SPACE [Sampling Perfection with Application optimized Contrasts using different flip angle Evolution] sequence) was acquired. Three additional small field of view scans were acquired: T2w, T2*w, and T1w flip angle 80° for gold fiducials. Patients received a routine planning CT scan. Manual contouring of the prostate, rectum, bladder, and bones was performed independently on the CT and MR scans. Three experienced observers contoured each organ on MRI, allowing interobserver quantification. To generate a training database, each patient CT scan was coregistered to their whole-pelvis T2w using symmetric rigid registration and structure-guided deformable registration. A new multi-atlas local weighted voting method was used to generate automatic contours and sCT results.Results: The mean error in Hounsfield units between the sCT and corresponding patient CT (within the body contour) was 0.6 ± 14.7 (mean ± 1 SD), with a mean absolute error of 40.5 ± 8.2 Hounsfield units. Automatic contouring results were very close to the expert interobserver level (Dice similarity coefficient): prostate 0.80 ± 0.08, bladder 0.86 ± 0.12, rectum 0.84 ± 0.06, bones 0.91 ± 0.03, and body 1.00 ± 0.003. The change in monitor units between the sCT-based plans relative to the gold standard CT plan for the same dose prescription was found to be 0.3% ± 0.8%. The 3-dimensional γ pass rate was 1.00 ± 0.00 (2 mm/2%).Conclusions: The MR-Sim setup and automatic sCT generation methods using standard MR sequences generates realistic contours and electron densities for prostate cancer radiation therapy dose planning and digitally reconstructed radiograph generation. [ABSTRACT FROM AUTHOR]

Purpose: Gantry-mounted megavoltage electronic portal imaging devices (EPIDs) have become ubiquitous on linear accelerators. WatchDog is a novel application of EPIDs, in which the image frames acquired during treatment are used to monitor treatment delivery in real time. We report on the preliminary use of WatchDog in a prospective study of cancer patients undergoing intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) and identify the challenges of clinical adoption.Methods and Materials: At the time of submission, 28 cancer patients (head and neck, pelvis, and prostate) undergoing fractionated external beam radiation therapy (24 IMRT, 4 VMAT) had ≥1 treatment fraction verified in real time (131 fractions or 881 fields). EPID images acquired continuously during treatment were synchronized and compared with model-generated transit EPID images within a frame time (∼0.1 s). A χ comparison was performed to cumulative frames to gauge the overall delivery quality, and the resulting pass rates were reported graphically during treatment delivery. Every frame acquired (500-1500 per fraction) was saved for postprocessing and analysis.Results: The system reported the mean ± standard deviation in real time χ 91.1% ± 11.5% (83.6% ± 13.2%) for cumulative frame χ analysis with 4%, 4 mm (3%, 3 mm) criteria, global over the integrated image.Conclusions: A real-time EPID-based radiation delivery verification system for IMRT and VMAT has been demonstrated that aims to prevent major mistreatments in radiation therapy. [ABSTRACT FROM AUTHOR]

Purpose: Prostate radiation therapy dose planning directly on magnetic resonance imaging (MRI) scans would reduce costs and uncertainties due to multimodality image registration. Adaptive planning using a combined MRI-linear accelerator approach will also require dose calculations to be performed using MRI data. The aim of this work was to develop an atlas-based method to map realistic electron densities to MRI scans for dose calculations and digitally reconstructed radiograph (DRR) generation. Methods and Materials: Whole-pelvis MRI and CT scan data were collected from 39 prostate patients. Scans from 2 patients showed significantly different anatomy from that of the remaining patient population, and these patients were excluded. A whole-pelvis MRI atlas was generated based on the manually delineated MRI scans. In addition, a conjugate electron-density atlas was generated from the coregistered computed tomography (CT)-MRI scans. Pseudo-CT scans for each patient were automatically generated by global and nonrigid registration of the MRI atlas to the patient MRI scan, followed by application of the same transformations to the electron-density atlas. Comparisons were made between organ segmentations by using the Dice similarity coefficient (DSC) and point dose calculations for 26 patients on planning CT and pseudo-CT scans. Results: The agreement between pseudo-CT and planning CT was quantified by differences in the point dose at isocenter and distance to agreement in corresponding voxels. Dose differences were found to be less than 2%. Chi-squared values indicated that the planning CT and pseudo-CT dose distributions were equivalent. No significant differences (p > 0.9) were found between CT and pseudo-CT Hounsfield units for organs of interest. Mean ± standard deviation DSC scores for the atlas-based segmentation of the pelvic bones were 0.79 ± 0.12, 0.70 ± 0.14 for the prostate, 0.64 ± 0.16 for the bladder, and 0.63 ± 0.16 for the rectum. Conclusions: The electron-density atlas method provides the ability to automatically define organs and map realistic electron densities to MRI scans for radiotherapy dose planning and DRR generation. This method provides the necessary tools for MRI-alone treatment planning and adaptive MRI-based prostate radiation therapy. [Copyright &y& Elsevier]