N. Nasser1,2, G. Q. Yang1, E. G. Moros1, J. J. Caudell1, and G. Redler1; 1H. Lee Moffitt Cancer Center and Research Institute, Department of Radiation Oncology, Tampa, FL, 2University of South Florida, Tampa, FL
Purpose/Objective(s): PTV margin reduction for Head and Neck (H&N) sequential (SEQ) boost plans, potentially made feasible by using a CBCT-guided ring-gantry online adaptive platform to reduce uncertainties and ensure precise treatment delivery daily, could provide dosimetric benefit. Materials/
Methods: Retrospective data from ten H&N patients treated clinically with SEQ boost plans, which also received offline plan adaptation clinically due to significant changes in anatomy, were used. Prescribed PTV dose levels were as follow: PTVHigh/PTVMed (Gy) 20/- (n=6) and 20/18 (n=4). In a previous study, these patients were re-planned within CBCT-guided ring-gantry online adaptive system using a 3mm PTV margin (expansion around CTV), followed by simulation of an online adaptive session (using data from the time point at which each patient was adapted offline clinically) in an emulator system. To develop a methodology for evaluating the effect of varying PTV margin as an isolated variable, first the similarity of dose calculation/optimization in the initial and online adaptive workspace of the system was investigated. All plans in both workspaces were imported, recalculated, and reoptimized in the initial planning workspace and the statistics for the change in dose (?D%) were calculated. Demonstrating negligible differences between workspaces allowed for using consistent data (system generated synthetic CT and online adapted structures) from a physician-driven online adaptive session to vary PTV margin (5,3,2,1,0 mm) and reoptimize plans for evaluation in the initial planning workspace. Results: Statistical data(|µ|±s) of ?D% of the initial original-recalculated, initial original reoptimized, adapted original-recalculated, and adapted original-reoptimized plans were: 0.00±0.02%, 0.26±0.21%, 0.38±0.3%, and 0.84±0.45%, respectively. Adapted plans reoptimized to varying PTV margins successfully maintained target coverage (all plans normalize to cover 95% of PTV with prescribed dose) while decreasing OAR doses by 6%, 10%, 13%, and 22% on average when going from 5 mm to 3 mm, 2 mm, 1 mm, and 0 mm PTV margins, respectively. Conclusion: The difference in dose between the original and the recalculated/reoptimized initial and adapted plans was less than 1%. Thus, the adjustment of adapted plan parameters in the initial workspace to simulate adaptive results is reasonable. Using this approach, the dosimetric effect of PTV margin reduction could be isolated and evaluated. Daily online plan adaptation can reduce target uncertainty, enabling reduced PTV margins, which is shown to provide dosimetric benefit and OAR sparing.
