Significant Decrease of Low-Dose to Normal Tissue Through the Optimization of Collimator Angles and Dynamic Gantry Range in Cervical Cancer VMAT Plan
C Chen*, JL Shen, DZ Jiang, CH Xie, YH Zhong, XY Wang, H Liu, HJ Yu#, Department of Radiation and Medical Oncology, Hubei Key Laboratory of Tumor Biological Behaviors,Hubei Cancer Clinical Study Center, Zhongnan Hospital of Wuhan University, Wuhan, China, 430071
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Purpose: To calculate optimal collimator angles and dynamic gantry range in cervical cancer VMAT plan and evaluate the dosimetric quality of these VMAT plans comparing double full-arc VMAT plans with a fixed collimator angle.
Methods: Twenty patients with cervical cancer were retrospectively selected. Three plans were designed for each patiententire. Two were double entire arcs with a fixed collimator angle of 0˚ and 45˚, respectively (VMAT0, VMAT45). One was adaptive gantry range and variable collimator angle.The optimal collimator angles were calculated by minimizing the leakage area between the multi-leaf collimator apertures and target projection determined in beam’s-eye view (BEV). The dynamic gantry range segmentation of the entire arc was calculated according to the continuity of the optimal collimator angle. The dose-volumetric parameters, averaged JAW sizes, and total monitor units were evaluated.
Results: No differences were found in D2, D50, D98, D95 of PTVs between VMAT APT, VMAT0, and VMAT45. VMAT APT plans improved the HI(uniformity index) and CI(conformity index) for PTVs. VMAT APT reduced the Dmax (max dose) of the small bowel. The averaged JAW sizes for VMAT APT were lower than those for VMAT0 and VMAT45 (18.7 ± 0.89 cm2, 20.7 ± 1.11 cm2, 20.2 ± 0.99 cm2). The total MUs in VMAT APT were lower than those in VMAT0 and VMAT45 (402 ± 19.02 MUs, 450 ± 18.82 MUs, 432 ± 18.52 MUs). The averaged low dose to normal tissue is lowest in VMAT APT compared with VMAT0 and VMAT45 (1580±37.52 cGy, 1856±30.44 cGy, 1704 ±34.29 cGy ).
Conclusion: For cervical cancer, VMAT APT with the adaptive gantry range and variable collimator angle improved the HI, CI, and the sparing of low-dose to normal tissue with reduced the max dose of the small bowel and total MUs compared with VMAT with double entire arcs and fixed collimator.
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