Margin and robustness settings for a library-of-plans IMPT strategy for locally advanced cervical cancer

Objective This study aims to determine a margin and robustness setting for treating locally advanced cervical cancer (LACC) with a library-of-plan based online-adaptive intensity-modulated proton therapy (IMPT). Approach We analyzed 13 LACC patients with delineated planning and weekly repeat CT scans. For each patient, 120 IMPT treatment of 25 fractions were simulated with a library-of-plans approach. Six different robustness settings (2 to 7 mm set-up robustness (SR) plus 3% range robustness (RR)) were used to create those 120 IMPT plan. Each fraction was simulated with a weekly repeat CT scan, combined with the sampling of inter- and intrafraction treatment uncertainties. The fraction doses were accumulated to obtain a treatment dose to the target volumes, distinguishing between the low-risk clinical target volume (CTV-T-LR) and the elective CTV (CTV-E). If one of the two targets obtained an adequate coverage for more than 90% of the treatments, different anisotropic margins were sampled on top of the robustness setting to the other target to obtain the Pareto-optimal margin in terms of adequate coverage versus increase in target volume. Main results The percentage of treatments that reach the dose criterion V42.75Gy>95% for the CTV-T-LR was 22.3%, 28.5%, 51.2%, 73.1%, 85.3%, and 90.0% for 2, 3, 4, 5, 6, and 7 mm SR plus 3% RR and for the CTV-E, this percentage was 60.4%, 73.8%, 86.5%, 92.3%, 96.9%, and 98.5%. The Pareto-optimal margin combined with a 5mm/3% robustness setting for the CTV-T-LR with an adequate coverage for >90% of the treatment was given by {0, 1, 0, 3, 3, 0} mm in the left, right, anterior, posterior, cranial, caudal direction. Significance Our study evaluated combinations of robustness and anisotropic margin settings for IMPT for LACC. With 5 mm SR and 3% RR for CTV-E and CTV-T-LR plus a margin to the CTV-T-LR of {0, 1, 0, 3, 3, 0} mm in LRAPCC ensured an adequate coverage for >90% of the simulated IMPT treatments.&#xD.