The multi-leaf collimator (MLC) dosimetric leaf gap (DLG) offset and transmission are measured parameters which influence the dosimetric accuracy of radiation therapy treatment plans. An international consortium developed an efficient method of automatically measuring the twodimensional MLC DLG and transmission using the electronic portal imaging device (EPID) on multiple Varian TrueBeam linacs incorporating Millennium and high definition (HD) MLCs. Quality control was implemented as part of each test. Results were compiled including comparisons to baseline measurements and between machines. Validations were accomplished using ion chambers (IC) and a 2D IC array. Sensitivity was investigated by introducing deliberate leaf position offsets, repeatability was assessed, and performance was analyzed using statistical process control methods. The EPID measured DLG (EDLG) and transmission were consistently smaller than the IC measured DLG and transmission for all machines tested. EDLG variations across two dimensions averaged more than 0.24 mm for both Millennium and HD MLCs, demonstrating that leaf-to-leaf DLG variations should be assessed for both MLC types. The two-dimensional correlation coefficient between the IC array and EPID measurements was at least 0.937. A nearly linear relationship between changes in the EPID measured leaf gap and actual leaf positions was measured, with R 2 >0.999. EDLG results were analyzed as a difference relative to a baseline EDLG measurement for each machine, a metric we labeled the leaf offset constancy (LOC). The maximum change in LOC out of the 20 repeatability tests was −0.094 mm. The LOC QC process was found to be capable (C pk >1) for 4 machines using a ± 0.15 mm specification limit. LOC results showed that leaves may deviate from their reference positions at a level that approaches dosimetric significance for small stereotactic and highly modulated treatment fields, with average excursions measuring up to 0.21 mm for a Millennium MLC and 0.16 mm for a HD MLC. The effects of initialization, gantry angle, and common repairs are also reported.
Purpose: To investigate the effects of flatbed scanner red‐blue ratio and non‐uniformity corrections on the accuracy of EBT2 film dosimetry. Method and Materials: EBT2 film was exposed to a 40×40 6MV field with 10 cm buildup in a 30×30cm solid water phantom for a range of doses from 0.85 cGy to 400.16 cGy. The films were scanned on an Epson Expression 10000XL flatbed scanner. The resulting images were first corrected using the ratio of the scanner's red‐blue channel. The difference between the expected dose from a Varian Eclipse planning system and the measured dose distributions was then used to generate a correction in scanner space to compensate for the non‐uniform response of the scanner. The red‐blue ratio and the non‐uniformity corrections were applied to open‐field, MLC pattern, and patient RapidArc images. Uncorrected results were compared to the red‐blue ratio correction and to both red‐blue ratio and non‐uniformity corrections. Results: The effects of scanner non‐uniformity were found to vary with dose applied to the film. The maximum relative discrepancy for doses below 10 cGy was as high as 40% for areas close to the film edge. This non‐uniformity decreases rapidly with increasing dose and for scanning locations towards the center of the scanner. The effects of applying the red‐blue ratio correction reduced the high frequency noise observed on the raw images. The relative noise of the film was reduced from 0.96% to 0.65%. The red‐blue ratio correction was unable to compensate for low dose errors towards the film edges. Application of the non‐uniformity correction reduced the number of pixels failing 3%/3mm gamma by up to 75% depending on the dose level and complexity of the dose distribution in the image. Conclusions: The application of both a red‐blue density ratio correction and a non‐uniformity correction improves the accuracy of EBT2 film dosimetry.
Purpose: AAPM Task Group (TG) 275 is charged with developing riskbased guidelines for plan and chart review clinical processes. As part of this work an AAPM‐wide survey was conducted to gauge current practices. Methods: The survey consisted of 103 multiple‐choice questions covering the following review processes for external beam including protons: 1) Initial Plan Check, 2) On‐Treatment and 3) End‐of‐Treatment Chart Check. The survey was designed and validated by TG members with the goal of providing an efficient and easy response process. The survey, developed and deployed with the support of AAPM headquarters, was released to all AAPM members who have self‐reported as working in the radiation oncology field and it was kept open for 7 weeks. Results: There are an estimated 4700 eligible participants. At the time of writing, 962 completed surveys have been collected with an average completion time of 24 minutes. Participants are mainly from community hospitals (40%), academicaffiliated hospitals (31%) and free‐standing clinics (18%). Among many other metrics covered on the survey, results so far indicate that manual review is an important component on the plan and chart review process (>90%) and that written procedures and checklists are widely used (>60%). However, the details of what is reviewed or checked are fairly heterogeneous among the sampled medical physics community. Conclusion: The data gathered from the survey gauging current practices will be used by TG 275 to develop benchmarks and recommendations for the type and extent of checks to perform effective physics plan and chart review processes.
Purpose: The MLC dosimetric leaf gap (DLG) and transmission are measured parameters which impact the dosimetric accuracy of IMRT and VMAT plans. This investigation aims to develop an efficient and accurate routine constancy check of the physical DLG in two dimensions. Methods: The manufacturer's recommended DLG measurement method was modified by using 5 fields instead of 11 and by utilizing the Electronic Portal Imaging Device (EPID). Validations were accomplished using an ion chamber (IC) in solid water and a 2D IC array. EPID data was collected for 6 months on multiple TrueBeam linacs using both Millennium and HD MLCs at 5 different clinics in an international consortium. Matlab code was written to automatically analyze the images and calculate the 2D results. Sensitivity was investigated by introducing deliberate leaf position errors. MLC calibration and initialization history was recorded to allow quantification of their impact. Results were analyzed using statistical process control (SPC). Results: The EPID method took approximately 5 minutes. Due to detector response, the EPID measured DLG and transmission differed from the IC values but were reproducible and consistent with changes measured using the ICs. For the Millennium MLC, the EPID measured DLG and transmission were both consistently lower than IC results. The EPID method was implemented as leaf offset and transmission constancy tests (LOC and TC). Based on 6 months of measurements, the initial leaf‐specific action thresholds for changes from baseline were set to 0.1 mm. Upper and lower control limits for variation were developed for each machine. Conclusion: Leaf offset and transmission constancy tests were implemented on Varian HD and Millennium MLCs using an EPID and found to be efficient and accurate. The test is effective for monitoring MLC performance using dynamic delivery and performing process control on the DLG in 2D, thus enhancing dosimetric accuracy. This work was supported by a grant from Varian Medical Systems.
Purpose: To estimate the delivered dose after each fraction of treatment as well as the delivered accumulative dose distribution. The goal of this project is to develop an automatic verification system using daily images and machine log files. Methods: First, we perform deformable registration from CT to CBCT and correct CBCT artifacts and CT numbers. Due to the limitation of the current scanning protocol, CBCT may not cover the whole treatment region needed for dose reconstruction. In that case, we use deformed CT for dose calculation which has correct CT number as well as daily patient geometry within whole treatment region. With machine log files, we generate actually delivered fluence maps at all delivery angles for that fraction. Then a GPU‐based Monte Carlo dose calculation package, gDPM, is employed to get delivered fractional dose on deformed CT or corrected CBCT. To compare the planned dose with the delivered dose, the delivered dose on new daily image is transferred back to the original CT geometry using the deformation vector fields obtained in the first step. This procedure is employed for each fraction to get the accumulative dose on original CT. Results: We demonstrate this treatment verification system on VMAT patients. We plot the DVHs for delivered and planned dose distributions and summarize the absolute errors in dose matrix report. Conclusions: We have proposed a procedure to estimate the delivered fractional and accumulative patient doses in IMRT and VMAT. If big errors are observed in the dose matrix report, then a warning should be sent out to responsible personnel, so the error can be indentified and corrected promptly.
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