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Benchmarking Flattening-Filter Free Photons for IMRT/VMAT

S Ashmeg

S Ashmeg*, J O'Daniel, Q Wu, F Yin, Duke University Medical Center, Durham, NC

TH-E-BRB-11 Thursday 1:00:00 PM - 2:50:00 PM Room: Ballroom B

Purpose: To create a set of benchmark data, using the AAPM TG119 test suite, for 6MV flattening-filter free IMRT/VMAT plans to compare with 6MV IMRT/VMAT plans and with TG119 multi-institutional results.

Methods: TG119 test cases were created: AP/PA, multiple dose bands, and 5 patient cases (c-shape tumor with easy and hard dosimetric constraints, head&neck, prostate, and multi-target plans). The 6MV/6MVFFF IMRT/VMAT plans were designed in Eclipse for a total of 28 plans. Measurements were conducted using a CC01 ionization chamber and a 30x30x15 cm³ solid water phantom in both high dose regions (targets) and low dose regions (organs at risk) at locations specified by the TG119 protocol. Statistical analyses used the student's paired t-test.

Results: All individual measurements agreed well with multi-institutional results (high dose average+/-stdev: -1.0% +/- 1.2%; low dose: -0.4% +/- 1.0%). There was no significant difference between IMRT and VMAT results (p=0.9696). However, results were significantly different (p=0.0003) for 6MV (-0.3% +/- 1.0%) and 6MVFFF (-1.1% +/- 1.2%). This could be due to beam modeling differences from commissioning, to increased dose gradients in the 6MVFFF beam which may increase the measurement uncertainty, or to other measurement issues (Pion variability). There was no statistically significant difference between the results in the high and low dose regions (p=>0.1000). Prostate and H&N plans all measured lower than planned while the C-shape and Multi-target plans didn't show any consistent higher/lower trends compared to planned doses.

Conclusions: While 6MVFFF and 6MV results were acceptable, there is room for improvement of the 6MVFFF accuracy. In future work we will investigate the effects of changing the 6MVFFF beam model and increasing the accuracy of the ion chamber measurements. Additionally, we will extend our analysis to 2D (film) and 3D (radiochromic dosimeter PRESAGE(TM)).

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