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Purpose: It has been suggested that the Real-time Position Management (RPM) marker block should be placed directly on the breast or sternum to verify deep inspiration breath hold (DIBH) level for breast radiotherapy. We explore three potential issues with this practice: (a) surface dose effect of placing the marker block in the primary beam; (b) effect of marker block tilt on the accuracy of the RPM system; and (c) correlation between marker block positions on the patient surface and internal chest wall position., Methods: (a) The surface dose under the two-, four-, and six-dot marker blocks was measured at incident angles of 0° and 30°; (b) the motion amplitude detected when using the two- and six-dot marker blocks was recorded for block tilts from 0° to 60° about the RPM camera line of sight; (c) the correlation between median displacement of the chest wall and median displacement of the surface contour between breath holds was investigated for superior, middle, and inferior block positions using contours extracted from portal images of eight left-sided breast cancer patients., Results: (a) The marker blocks increased the surface dose for a 6 MV direct field by 48.2-52.2% of D max ; (b) at lateral tilts greater than 10°, the two-dot marker block overestimated the motion amplitude; however, the six-dot marker block amplitude remained accurate up to 60°; (c) the whole, superior, and middle surface positions were strongly correlated with chest wall displacement (R 2 = 0.83; R 2 = 0.90; R 2 = 0.83), whereas the inferior position was moderately correlated (R 2 = 0.36)., Conclusions: The RPM marker block can be placed on the breast for DIBH treatments; however, caution should be used regarding surface dose effects. The two-dot marker block should not be used for block tilts beyond 20°. Marker block placement at a middle or superior position on the breast results in the strongest correlation with chest wall position., (© 2016 American Association of Physicists in Medicine.)

Purpose: Recent knowledge on the effects of cardiac toxicity warrants greater precision for left-sided breast radiotherapy. Different breath-hold (BH) maneuvers (abdominal vs thoracic breathing) can lead to chest wall positional variations, even though the patient's tidal volume remains consistent. This study aims to investigate the feasibility of using optical tracking for real-time quality control of active breathing coordinator (ABC)-assisted deep inspiration BH (DIBH)., Methods: An in-house optical tracking system (OTS) was used to monitor ABC-assisted DIBH. The stability and localization accuracy of the OTS were assessed with a ball-bearing phantom. Seven patients with left-sided breast cancer were included. A free-breathing (FB) computed tomography (CT) scan and an ABC-assisted BH CT scan were acquired for each patient. The OTS tracked an infrared (IR) marker affixed over the patient's xiphoid process to measure the positional variation of each individual BH. Using the BH within which the CT scan was performed as the reference, the authors quantified intra- and interfraction BH variations for each patient. To estimate the dosimetric impact of BH variations, the authors studied the positional correlation between the marker and the left breast using the FB CT and BH CT scans. The positional variations of 860 BHs as measured by the OTS were retrospectively incorporated into the original treatment plans to evaluate their dosimetric impact on breast and cardiac organs [heart and left anterior descending (LAD) artery]., Results: The stability and localization accuracy of the OTS was within 0.2 mm along each direction. The mean intrafraction variation among treatment BHs was less than 2.8 mm in all directions. Up to 12.6 mm anteroposterior undershoot, where the patient's chest wall displacement of a BH is less than that of a reference BH, was observed with averages of 4.4, 3.6, and 0.1 mm in the anteroposterior, craniocaudal, and mediolateral directions, respectively. A high positional correlation between the marker and the breast was found in the anteroposterior and craniocaudal directions with respective Pearson correlation values of 0.95 and 0.93, but no mediolateral correlation was found. Dosimetric impact of BH variations on breast coverage was negligible. However, the mean heart dose, mean LAD dose, and max LAD dose were estimated to increase from 1.4/7.4/18.6 Gy (planned) to 2.1/15.7/31.0 Gy (delivered), respectively., Conclusions: In ABC-assisted DIBH, large positional variation can occur in some patients, due to their different BH maneuvers. The authors' study has shown that OTS can be a valuable tool for real-time quality control of ABC-assisted DIBH.

Purpose: With the increasing use of external 3D optical tracking cameras to guide modern radiation therapy procedures, it has become vitally important to have an accurate camera to linear accelerator (LINAC) reference frame calibration. To eliminate errors present in current calibration procedures based on the manual hand alignment of a device using the light field crosshairs and in room guidance lasers, a semiautomated quantitative calibration approach requiring only use of an electronic portal imaging device (EPID) was developed., Methods: A phantom comprised of seven highly IR reflective plastic BBs was placed on the LINAC treatment couch and imaged with both a 3D stereoscopic IR imager and the on board megavoltage (MV) EPID imager. Having knowledge of the optically determined 3D positions and projected EPID images of the BBs, simulated annealing was used to optimize the location of the BBs in the LINAC frame using four different optimization functions. Singular value decomposition was then used to calculate the transformation matrix between the camera and LINAC reference frames. Results were then compared to a traditional camera calibration method for overall accuracy., Results: Using modeled data, the simulated annealing process was able to determine the actual locations of the BBs with a RMSE of 0.23 mm. Using projection images acquired with an MV imager, the process was able to determine locations of BBs within .26 mm. The results depend on the choice of optimization function., Conclusions: Results show that the method can be used to provide highly accurate spatial registration between an external 3D imaging reference frame and the LINAC frame. The experimental MV imager results, while not as precise as the simulated results, exceed 1 mm accuracy and the current accepted AAPM TG-142 standard of ≤2 mm positioning accuracy.

Purpose: Optical image-guided systems (e.g., AlignRT, frameless SonArray, ExacTrac) have been used with advantages of avoiding excessive radiation exposure and real-time patient monitoring. Although these systems showed proven accuracy, they need to modify a full facemask for patients with H&N cancer and brain tumor. We developed an optical-based guidance system to manage interfractional and intrafractional setup errors by tracking external markers behind a full facemask., Methods: Infra-red (IR) reflecting markers were attached on the face of a head phantom and then the phantom was immobilized by a full face thermoplastic mask. A stereo camera system consisting of two CCD cameras was mounted on the inferior wall of treatment room. The stereo camera system was calibrated to reconstruct 3D coordinates of multiple markers with respect to the isocenter using the direct linear transform (DLT) algorithm. The real-time position of the phantom was acquired, through the stereo camera system, by detecting the IR markers behind the full facemask. The detection errors with respect to the reference positions of planning CT images were calculated in six degrees of freedom (6-DOF) by a rigid-body registration technique., Results: The calibration accuracy of the system was in submillimeter (0.33 mm +/- 0.27 mm), which was comparable to others. The mean distance between each of marker positions of optical images and planning CT images was 0.50 mm +/- 0.67 mm. The maximum deviations of 6-DOF registration were less than 1 mm and 1 degrees for the couch translation and rotation, respectively., Conclusions: The developed system showed the accuracy and consistency comparable to the commercial optical guided systems, while allowing us to simultaneously immobilize patients with a full face thermoplastic mask.

Purpose: In-treatment fiducial tracking has recently received attention as a method for improving treatment accuracy, dose conformity, and sparing of healthy tissue. 3-D fiducial localization in arc-radiotherapy remains challenging due to the motion of targets and the complexity of arc deliveries. We propose a novel statistical method for estimating 3-D fiducial motion using limited 2-D megavoltage (MV) projections., Methods: 3-D fiducial motion was estimated by a maximum a posteriori (MAP) approach to integrating information of fiducial projections with prior knowledge of target motion. To obtain the imaging geometries, short sequences of MV projections were selected in which fiducials were continuously visible. The MAP algorithm estimated the 3-D motion by maximizing the probability of displacement of fiducials in the sequences. Prior knowledge of target motion from a large statistical sample was built into the model to enhance the accuracy of estimation. In the case that a motion prior was unavailable, the algorithm can be simplified to the maximum likelihood (ML) approach. To compare tracking performance, a multiprojection geometric method was also presented by extending the typical two-project ion geometric estimation approach. The algorithms were evaluated using clinical prostate motion traces, and the performance was measured in quality indices and statistical evaluation., Results: The results showed that the MAP method significantly outperforms the geometric method in all measures. In our simulations, the MAP method achieved an accuracy of less than 1 mm RMS error using only five continuous projections, whereas the geometric method required 15 projections to achieve a similar result., Conclusions: The approach presented can accurately estimate tumor motion using a limited number of continuous projections. The MAP motion estimation is superior to both the ML and geometric estimation methods.

Purpose: Helical tomotherapy is a complex delivery technique, integrating CT image guidance and intensity modulated radiotherapy in a single system. The integration of the CT detector ring on the gantry not only allows patient position verification but is also often used to perform various QA procedures. This convenience lacks the rigor of a machine-independent QA process., Methods: In this article, a Si strip detector, known as the Dose Magnifying Glass (DMG), was used to perform machine-independent QA measurements of the multileaf collimator alignment, leaf open time threshold, and leaf fluence output factor (LFOF)., Results: The DMG measurements showed good agreements with EDR2 film for the MLC alignment test while the CT detector agrees well with DMG measurements for leaf open time threshold and LFOF measurements. The leaf open time threshold was found to be approximately 20 ms. The LFOF measured with the DMG agreed within error with the CT detector measured LFOF., Conclusions: The DMG with its 0.2 mm spatial resolution coupled to TERA ASIC allowed real-time high temporal resolution measurements of the tomotherapy leaf movement. In conclusion, DMG was shown to be a suitable tool for machine-independent QA of a tomotherapy unit.

The authors have developed an automatic quality assurance method based on computerized tomography (CT) data for physically manufactured proton range compensators. Eight range compensators used for proton therapy at their institution were scanned using CT and their depth distributions were compared with data provided by a proton treatment planning system (TPS). The depth difference (DD), distance to agreement (DTA), and a composite analysis (CA) indicating regions that passed either of the tests were used for depth verification. When the tolerance limits were set at 3 mm for DD and 1 mm for DTA, the average percentages of points exceeding the acceptance criteria for DD, DTA, and CA for the eight compensators were 9.0%, 5.3% and 3.2%, respectively. In general, the percentages of points exceeding the acceptance criteria were much higher in DD than in DTA analysis and mismatch of depth was greater in regions with high depth gradients. This is considered to arise mainly because of systematic errors such as the physical size of the drill, slight bending or slipping of the drill bit near high gradient regions, the limit of CT resolution, and the uncertainty in CT number. When the tolerance limit of DTA was increased from 1 to 2 mm, the average percentage of points exceeding the acceptance criteria for DTA was reduced from 5.3% to 0.9%. Their results show that depth comparison of proton range compensators based on CT images can provide more systematic data than does the current method, in which a representative group of drill points is measured manually.

Helical tomotherapy is a technique for delivering intensity modulated radiation therapy treatments using a continuously rotating linac. In this approach, fan beams exiting the linac are dynamically modulated in synchrony with the motion of the gantry and couch. Helical IMRT deliveries have been applied to treating surface lesions, and the purpose of this study was to evaluate the accuracy of dose calculated by the TomoTherapy HiArt treatment planning system for superficial planning target volumes (PTVs). TomoTherapy treatment plans were developed for three superficial PTVs (2-, 4-, and 6-cm deep radially by 90 degrees azimuthally by 4-cm longitudinally) contoured on a 27-cm diameter cylindrical white opaque, high-impact polystyrene phantom. The phantom included removable transverse and sagittal film cassettes that contained bare Kodak EDR2 films cut such that their edges matched the phantom surface (+/-0.05 cm). The phantom was aligned to the machine's isocenter (+/-0.05 cm) and was irradiated according to the treatment plans. Films were scanned with a Vidar film digitizer, and optical densities were converted to dose using a calibration determined from a 6 MV perpendicular film exposure. This perpendicular calibration required that axial film doses (parallel irradiation) be scaled by 1.02 so that mid-arc depth doses matched those measured in the sagittal plane (perpendicular irradiation). All film readings were scaled by 0.935 to correct for over-response due to phantom Cerenkov light. Measured dose distributions were registered to calculated ones and compared. Calculated doses overpredicted measured doses by as much as 9.5% of the prescribed dose at depths less than 1 cm. At depths greater than 1 cm, calculated dose distributions showed agreement to measurement within 5% in the high-dose region and within 0.2 cm distance-to-agreement in the dose falloff regions. In the low-dose region posterior to the PTVs (<10% of the prescribed dose), the dose algorithm underpredicted the dose by 1%-2% of the prescribed dose. Clinically, it is recommended that 1 cm of bolus be used on the surface to ensure that cancerous tissues less than 1 cm depth are not underdosed.

Image-guided radiotherapy using a three-dimensional (3D) camera as the on-board surface imaging system requires precise and accurate registration of the 3D surface images in the treatment machine coordinate system. Two simple calibration methods, an analytical solution as three-point matching and a least-squares estimation method as multipoint registration, were introduced to correlate the stereo-vision surface imaging frame with the machine coordinate system. Both types of calibrations utilized 3D surface images of a calibration template placed on the top of the treatment couch. Image transformational parameters were derived from corresponding 3D marked points on the surface images to their given coordinates in the treatment room coordinate system. Our experimental results demonstrated that both methods had provided the desired calibration accuracy of 0.5 mm. The multipoint registration method is more robust particularly for noisy 3D surface images. Both calibration methods have been used as our weekly QA tools for a 3D image-guided radiotherapy system.

A new integrated CT/LINAC combination, in which the CT scanner is inside the radiation therapy treatment room and the same patient couch is used for CT scanning and treatment (after a 180-degree couch rotation), should allow for accurate correction of interfractional setup errors. The purpose of this study was to evaluate the sources of uncertainties, and to measure the overall precision of this system. The following sources of uncertainty were identified: (1) the patient couch position on the LINAC side after a rotation, (2) the patient couch position on the CT side after a rotation, (3) the patient couch position as indicated by its digital readout, (4) the difference in couch sag between the CT and LINAC positions, (5) the precision of the CT coordinates, (6) the identification of fiducial markers from CT images, (7) the alignment of contours with structures in the CT images, and (8) the alignment with setup lasers. The largest single uncertainties (one standard deviation or 1 SD) were found in couch position on the CT side after a rotation (0.5 mm in the RL direction) and the alignment of contours with the CT images (0.4 mm in the SI direction). All other sources of uncertainty are less than 0.3 mm (1 SD). The overall precision of two setup protocols was investigated in a controlled phantom study. A protocol that relies heavily on the mechanical integrity of the system, and assumes a fixed relationship between the LINAC isocenter and the CT images, gave a predicted precision (1 SD) of 0.6, 0.7, and 0.6 mm in the SI, RL and AP directions, respectively. The second protocol reduces reliance on the mechanical precision of the total system, particularly the patient couch, by using radio-opaque fiducial markers to transfer the isocenter information from the LINAC side to the CT images. This protocol gave a slightly improved predicted precision of 0.5, 0.4, and 0.4 mm in the SI, RL and AP directions, respectively. The distribution of phantom position after CT-based correction confirmed these results. Knowledge of the individual sources of uncertainty will allow alternative setup protocols to be evaluated in the future without the need for significant additional measurements.

A clinical helical tomotherapy treatment machine has been installed at the University of Wisconsin Comprehensive Cancer Center. Beam alignment has been finalized and accepted by UW staff. Helical tomotherapy will soon be clinically available to other sites. Clinical physicists who expect to work with this machine will need to be familiar with its unique dosimetric characteristics, and those related to the geometrical beam configuration and its verification are described here. A series of alignment tests and the results are presented. Helical tomotherapy utilizes an array of post-patient xenon-filled megavoltage radiation detectors. These detectors have proved capable of performing some alignment verification tests. That is particularly advantageous because those tests can then be automated and easily performed on an ongoing basis.

A quality assurance procedure was developed for x-ray beam intensity modulated conformal radiotherapy (IMCRT) using dynamic multileaf collimators (MLC). The procedure verifies a prescribed intensity modulated x-ray beam pattern in the beam eye's view (BEV) before the treatment procedure is applied to a patient. It verifies that (a) the leaf sequencing computer files were transferred correctly to the linac control computer; (b) the treatment can be correctly executed without machine faults. A fast beam imaging system (BIS) consisting of a Gd2O2S scintillation screen, a charge-coupled device (CCD) camera, and a portable personal computer (Wellhöfer Dosimetrie, Schwarzenbruck, Germany) was commissioned for this purpose. Measurements for the BIS performance are presented in this work. Reference images were derived from MLC leaf sequencing files that were used to drive a dynamic MLC system (Varian Oncology Systems, Palo Alto, CA). A correlation method was developed to compare the BIS measurements with the calculated reference images. A correlation coefficient calculated using 26 correct intensity modulated fields was shown to be a reliable threshold to identify inaccurate treatment delivery files. The study has demonstrated the feasibility of using the BIS and the correlation method to carry out on-line quality assurance tasks for IMCRT treatment fields in the BEV.

Segmented conformal radiation therapy is a new computer-controlled treatment technique under investigation in which the target volume is subdivided into thick transverse segments each of which is then treated individually by rectangular transverse abutting fields. In order to obtain uniform dose at abutments, the machine isocenter remains fixed in the patient and field edges are defined by independently moving focused collimator jaws to give matching geometric divergence. Mechanical variation in jaw and gantry positioning will create some dose variation at field abutments. Film dosimetry was used to study the radiation field positioning accuracy and precision of a commercial linear accelerator. A method of field position calibration was developed using multiple nonabutting fields exposed on the same radiograph. Verification of collimator jaw calibration measurements was performed using multiple abutting fields exposed on a single radiograph. Measurements taken over 5 months of clinical accelerator operation studied the effects of simple jaw motion, simple gantry motion, and combined jaw/gantry motion on jaw position precision and accuracy. The inherent precision and accuracy of radiation field positioning was found to be better than +/- 0.3 mm for both jaws with all types of motions except for the Y2 jaw under combined jaw/gantry motion. When the ability to deliver abutting beams was verified in clinical mode, the average dose variation at abutments was less than 6% at all gantry angles except for one. However, due to accelerator software limitations in clinical mode, the settings for collimator positions could not take advantage of the maximum accuracy of which the hardware is capable.(ABSTRACT TRUNCATED AT 250 WORDS)