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Dosimetric Accuracy for Lung and Brain SBRT VMAT Plans Delivered by Two Beam-Matched Linacs

Z Xu

Z Xu1*, G Warrell1 , S Lee2 , V Colussi1,2 , Y Zheng1,2 , T Podder1,2 , (1) Case Western Reserve University, Cleveland, OH, (2) University Hospital Cleveland Medical Center, Cleveland, OH


SU-I-GPD-T-576 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Dose uncertainty due to small field sizes and high beam modulation is one of the major concerns in swapping SBRT VMAT patients between beam-matched linacs. This study is to evaluate the dose delivery accuracy of SBRT VMAT plans for two beam-matched linacs.

Methods: Photon beam parameters of the matched 6MV beams were compared. Fifteen lung and ten brain SBRT VMAT plans were generated for matched linacs (Elekta-Agility™ and Versa-HD™) in Philips Pinnacle TPS (ver9.10) using the same 6MV beam model. For each plan, the modulation complexity score of VMAT (MCSv) was determined to account for the plan complexity considering variability in MLC leaf position, MU/segments, and degree of irregularity in field shape (on a scale of 0 to 1; 1 indicating no modulation). The plans were delivered by both the linacs and measured using MapCHECK2 and MapPhan. Dosimetric evaluation of measurements against TPS calculated doses was performed through absolute dose gamma comparison. Differences in passing rates of measurements between matched linacs were computed and their correlation with plan MCSv was evaluated.

Results: Differences in beam parameters (PDD₁₀, output factors, flatness, and symmetry) were within ±1.0%. Mean MCSv of the plans was 0.52±0.11 (range:0.35-0.68), indicating medium to high plan complexity. The gamma passing rates of measurements from both linacs, difference in passing rates of measurements, correlation coefficient between pass rate difference, and MCSv were 97.8%±1.5%, 0.3%±0.2% (p=0.45), 0.15, respectively, using 3%/2mm gamma criteria; and 95.2%±2.5%, 0.9%±0.6% (p=0.37), and 0.19, respectively, using 2%/2mm gamma criteria.

Conclusion: The matched linacs demonstrated good agreement between measurements and TPS calculations for SBRT VMAT plans. Agility linac demonstrated marginally better pass rate. The effect of plan complexity on pass rate difference is insignificant. Minimal differences in measurement passing rates between two linacs proved the availability of swapping SBRT VMAT patients between matched linacs.

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