Characterization of Photon Intensity Modulated Radiation Therapy Robustness in Patients With Prostate Cancer as a Proposed Benchmark for Proton Therapy Robustness Evaluation

Purpose: Robustness evaluation is increasingly used in particle therapy planning to assess clinical target volume (CTV) coverage in the setting of setup and range uncertainty. However, no clear standard exists as to an acceptable degree of plan robustness. The aim of this study is to quantify x-ray robustness parameters, as this could inform proton planning when held to a similar standard. Methods and Materials: Consecutive patients with prostate adenocarcinoma treated with definitive x -irradiation to the prostate alone at a single institution in 2019 were retrospectively reviewed. CTV to planned target volume (PTV) margins of 7 mm in all directions, except 4 mm posteriorly, were used in the main cohort. Plans were normalized to PTV V100% >= 95%. Patient setup errors were simulated by shifting the isocenter relative to the patient in each of the cardinal directions. The magnitude of each shift equaled the magnitude of the CTV to PTV expansion in that direction. Range uncertainty was set to 0%. Results: A total of 27 patients were evaluated. The mean (SD) nominal plan CTV V100% was 99.6% (1.1%). The mean (SD) worst -case shift CTV V100% was 97.2% (2.8%). The mean (SD) nominal and worst -case CTV V95% were 100% (0%) and 99.7% (0.5%), respectively. A worst -case CTV V100% > 90% and a worst -case CTV V95% > 99% were achieved in over 95% of plans. The mean (SD) nominal and worst -case rectal V70 Gy were 2.37 cc (1.00 cc) and 11.60 cc (3.16 cc), respectively. The mean (SD) nominal and worst -case bladder V60 Gy were 7.8% (4.8%) and 14.5% (9.3%), respectively. Paired 2 -tailed t tests comparing the nominal to worst -case dosevolume histograms were significant for each dosimetric parameter (P < .01). Conclusions: X-ray planning uses PTV margins to inherently provide robustness to patient setup errors. Although the prostate remains well covered in various setup uncertainty scenarios, organs at risk routinely exceeded nominal treatment plan institutional constraints in the worst -case scenarios. Robustness metrics obtained from x-ray plans could serve as a benchmark for proton therapy robust optimization and evaluation. (c) 2023 American Society for Radiation Oncology. Published by Elsevier Inc. All rights reserved.