Treatment of Large-Size Prostate Cancer Patients On An O-Ring Linac with 6MV FFF: Plan Quality Comparison with a C-Arm Linac Using 10MV Or 15MV Photons
B Liu1*, C Cheng1, X Wang1, J Zhu1, R Parikh1, K Nie1, N Yue1, Y Zhang1, (1) Rutgers Cancer Institute of New Jersey, Rutgers Robert Wood Johnson Medical School, New Brunswick, NJ
Presentations
PO-GePV-T-253 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: To evaluate the dosimetric performance of the Halcyon Linac equipped with 6MV FFF photon beam in planning for large-size prostate cancer patients, compared to TrueBeam with Millennium MLC using 10MV or 15MV photons.
Methods: Eight large-size prostate cancer patients (BMI = 27.7 ± 2.2 kg/m² and pelvis lateral separation = 36.3 ± 1.1 cm) were retrospectively included in the treatment planning study. The dose prescription (79.2Gy to prostate and proximal seminal vesicles in 44 fractions) and clinical planning objectives were defined as in RTOG 0126. Halcyon and TrueBeam plans were optimized using 3 full arcs. For each patient, three plans were developed: Halcyon plan with 6MV FFF (Hal), TrueBeam plan with 10MV (TB-10X), and TrueBeam plan with 15MV (TB-15X). The plans were normalized so that prescription isodose surface covers 98% of the PTV volume.
Results: The dosimetric parameters of all plans are meeting RTOG 0126 objectives. The Dmax of all plans are below 107% of prescription dose, and no difference of Dmax was observed among Hal, TB-10X and TB-15X. The V75Gy, V70Gy, V65Gy, and V60Gy of rectum on Hal are: 11.5%±3.6%, 17.8%±5.5%, 23.9%±7.0%, and 30.8%±8.4%, respectively; on TB-10X are: 11.1%±3.7%, 17.1%±5.6%, 23.1%±7.3%, and 29.9%±8.9%, respectively; on TB-15X are: 11.2%±3.7%, 17.3%±5.6%, 23.5%±7.1%, and 30.6%±8.6%, respectively. The V80Gy, V75Gy, V70Gy, V65Gy of bladder on Hal are: 4.3%±3.6%, 8.4%±6.7%, 11.5%±8.9%, and 14.6%±11.0%, respectively; on TB-10X are: 4.2%±3.6%, 8.2%±6.7%, 11.2%±9.0%, and 14.2%±11.1%, respectively; and on TB-15X are: 4.2%±3.5%, 8.1%±6.8%, 11.2%±9.1%, and 14.2%±11.3%, respectively. The mean penile bulb doses are: 19.9Gy±8.5Gy, 16.2Gy±6.7Gy, and 16.3Gy±7.0Gy, respectively for Hal, TB-10X, and TB-15X.
Conclusion: This retrospective treatment planning study, based on the dose constraints derived from RTOG 0126 protocol, suggested that the essential equivalence between Halcyon plan using 6MV FFF and TrueBeam plans using 10MV/15MV for the arc treatment of large-size prostate cancer patients.
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