Abstract Book

S938

ESTRO 37

Results

plans failing when adjustments to the MLC offset table in the TPS beam model were made. However, a decrease in the mean target dose and 90% dose coverage can been seen throughout all plans. Study 1(b) showed 10 plans failing at 2%/2mm when the leaf tip radius was increased by 6 cm. In an example case study, a prostate VMAT passed at 99.2% and decreased to 98.7% at 3%/3mm when the leaf tip radius increased from 10 cm to 16 cm. At 2%/2mm the pass rate finally failed at 89.4% for a 16 cm leaf tip radius (Figure 1). Study 2 showed minimal differences in passing rates when not accounting for the tongue-and-groove effects. However, an increase in the mean target dose and 90% dose coverage is seen for all but one plan. Differences in DVH are also seen, specifically in the example of a lung SBRT at 2%/2mm, where the pass rate was 97.7% for the original plan and 97.2% when the beam model was adjusted. The SDCS target dose was higher at 68.9 Gy as compared to the calculated target dose of 68.5 Gy for the original plan. The SDCS DVH curve shifted about 3.8% at D95 (Figure 2). Study 3 showed 2 plans failing at 2%/2mm and 10 plans with decreased gamma values when the profile data from a CC13 chamber replaced data taken with a CC01. Study 4 reviewed the original plans which revealed an underlying dose pattern corresponding to overestimation and underestimation of dose throughout. Inspection of the dose profiles showed clear systematic dose gradient errors. The study also revealed that as the passing criteria becomes tighter or as the dose grid becomes smaller, the pass rates decrease. Study 5 further attributed dose differences to narrow MLC segments, especially in highly-modulated cases. Individual dose profiles showed data consistent with reduction of dose in the high dose region and overestimation of dose for narrow segments.

The results show that over a period of 12 months, the outputs of the 6 and 10 MV beams as measured by the MPC tracked the outputs measured with the QA3 but with an average difference between the two of approximately 1%. The MPC consistently measures the output lower (- 1.08%) than the QA3 device. Farmer chamber output measurements aligned more closely with the QA measurements and the mean MPC-Farmer measurements was -1.26%. The MPC-EPID difference was -0.89%. There were 3 output adjustments during the 12 month period with the 6MV output adjusted from 102.62% to 100.02%, 101.36% to 99.94% and 101.24% to 99.54% respectively. The 10MV output showed a similar trend with a mean difference of -1.34% again with the MPC reading lower than QA3. Conclusion The small variation in MPC output indicates that a monthly intercomparison with a calibrated Farmer chamber is needed if the MPC is to replace the QA3 as a daily check. A weekly Farmer chamber output measurement should also be carried out until confidence in the MPC is achieved. EP-1750 Secondary Dose Calculation: Detecting Systematic Errors in the Treatment Planning System Beam Model J. Wong 1 , W. Warren 1 , K. Homann 1 1 Houston Methodist Hospital, Radiation Oncology, Houston, USA Purpose or Objective The purpose of this study was to evaluate a commercial secondary dose calculation software’s (SDCS) ability to detect systematic errors in a treatment planning system (TPS) beam model for model validation and treatment plan approval. Material and Methods Artificial changes to the TPS beam model introduced errors to patient plans. 12 cases were analyzed with the SDCS to determine effects of the following: 1. Incorrect TPS settings for leaf-end modeling regarding (a) MLC offset table, and (b) leaf tip radius, 2. Incorrect TPS settings to account for tongue-and-groove effects, 3. Dose gradient errors due to inaccurate volume-averaged dose profiles, 4. Inherent dose gradient errors in the TPS beam model/algorithm, and 5. TPS underestimation of dose for narrow MLC segments. Results Plans were evaluated with the SDCS for gamma dose criteria, mean target dose, dose coverage, dose volume histogram (DVH), and profiles. Study 1(a) resulted in no

Conclusion This study verified errors in commissioning measurement and beam modeling can be detected with the SDCS. However, large modeling errors or tighter tolerances are needed before clinical failures can be detected. EP-1751 Accuracy of Monte Carlo software PRIMO against a reference dosimetry dataset for 6 MV photon beams M. Hermida-López 1 , D. Sánchez-Artuñedo 1 , J.F. Calvo- Ortega 2 1 Hospital Universitario Vall d'Hebron, Servicio de Física y Protección Radiológica, Barcelona, Spain 2 Hospital Quirón, Servicio de Radioterapia, Barcelona, Spain