Program Information
An Analysis of the Accuracy of TPS Extrapolation of Commissioning Data
H Alkhatib1*, S Oves2 , W Gebreamlak3 , D Mihailidis4 , (1) Richland Memorial Hospital, Columbia, SC, (2) South Carolina Oncology Associates, Columbia, SC, (3) South Carolina Oncology Associates, Columbia, SC, (4) CAMC Cancer Center, Charleston, WV
Presentations
SU-E-T-99 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To investigate discrepancies between measured percent depth dose curves of a linear accelerator at depths beyond the commissioning data and those generated by the treatment planning system (TPS) via extrapolation.
Methods: Relative depth doses were measured on an Elekta Synergy™ linac for photon beams of 6-MV and 10-MV. SSDs for all curves were 100-cm and field sizes ranged from 4x4 to 35x35-cm². As most scanning tanks cannot provide depths greater than about 30-cm, percent depth dose measurements, extending 45-cm depths, were performed in Solid Water™ using a 0.125-cc ionization chamber (PTW model TN31012). The buildup regions of the curves were acquired with a parallel plate chamber (PTW model TN34001). Extrapolated curves were generated by the TPS (Phillips Pinnacle³ v. 9.6) by applying beams to CT images of 50-cm of Solid Water™ with density override set to 1.0-g/cc.
Results: Percent difference between the two sets of curves (measured and TPS) was investigated. There is significant discrepancy in the buildup region to a depth of 7-mm. Beyond this depth, the two sets show good agreement. When analyzing the tail end of the curves, we saw percent difference of between 1.2% and 3.2%. The highest disagreement for the 6-MV curves was 10x10-cm² (3%) and for the 10-MV curves it was the 35x35-cm² (3.2%).
Conclusion: A qualitative analysis of the measured data versus PDD curves generated by the TPS shows generally good agreement beyond 1-cm. However, a measurable percent difference was observed when comparing curves at depths beyond that provided by the commissioning data and at depths in the buildup region. Possible explanations for this include inaccuracies in modeling of the Solid Water™ or drift in beam energy since commissioning. Additionally, closer attention must be paid for measurements in the buildup region.
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