D Pham*, D Breitkreutz, E Simiele, D Capaldi, L Vitzthum, M Gensheimer, H Bagshaw, B Han, M Surucu, L Xing, D Chang, N Kovalchuk, Stanford University School of Medicine, Stanford, CA

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  D Pham

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MO-IePD-TRACK 5-5 (Monday, 7/26/2021) 5:30 PM - 6:00 PM [Eastern Time (GMT-4)]

Purpose: The first clinical biology-guided radiotherapy (BgRT) system – RefleXion X1 - is installed and being commissioned for clinical use at our institution. RefleXion X1 is a 6MV-FFF linac mounted on a ring gantry rotating at 60 rpm while the couch advances in 2.1mm increments. The modulation is achieved via 50 firing positions with 64 binary MLCs (6.25mm at 85cm SAD) with either 1cm or 2cm jaws. This study was conducted to compare the treatment plan quality for IMRT cases without PET-guidance.

Methods: Nineteen patients previously treated with VMAT on C-arm linacs were selected for this retrospective study including 8 prostate cancer patients (80Gy/40fx), 6 lung cancer patients (66Gy/30fx) and 5 post-surgical parotid cancer patients (60Gy–66Gy/30fx). For each VMAT plan, a corresponding plan was generated on the RefleXion treatment planning system (TPS) using our institutional planning constraints. All clinically relevant metrics in this study, including plan PTV D95%, PTV D1%, Conformity Index (CI), and organs at risk constraints were analyzed and compared between the VMAT and RefleXion plans using paired t-tests.

Results: Clinically acceptable plans were obtained with both techniques. For the prostate, lung, and head and neck sites, no statistically significant difference was observed in PTV D95%, PTV D1%, or CI between the VMAT and RefleXion treatment plans. Due to inherent OAR dose penalties in the RefleXion optimizer, dose reduction to certain critical structures was observed in RefleXion plans: brainstem Dmax (-4.8Gy, p<0.008), contralateral cochlea Dmean (-4.8Gy, p<0.01), lips Dmax (-3.5Gy, p<0.007) for head and neck cases; esophagus Dmean (-2.3Gy, p<0.039), spinal cord Dmax (-3.6Gy, p<0.001) for lung cases; femurs left and right Dmax (-4.8Gy, p<0.003 and -5.0Gy, p<0.01) and rectum V75% (-1.6%, p<0.01) for prostate cases.

Conclusion: The RefleXion TPS provided comparable plan quality to VMAT plans with slightly superior OAR sparing.

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