Verification Studies Confirm Reproducibility of Post-Hysterectomy Pelvic Proton Radiotherapy

Abstract

<h3>Purpose/Objective(s)</h3> Post-hysterectomy radiotherapy (RT) is associated with urinary, gastrointestinal, and hematologic toxicity. Concern for organ and target motion has limited exploration of proton RT as a tool to reduce toxicity. Attention to planning and treatment delivery is paramount given sensitivity of protons to small changes in soft tissue in the beam path. We sought to evaluate the consistency of target coverage and organ at risk (OAR) sparing in post-hysterectomy proton RT. <h3>Materials/Methods</h3> The first 10 patients treated with proton RT on an institutional prospective non-randomized trial of proton post-hysterectomy RT for endometrial cancer were included. CT simulation was performed with pelvic immobilization and a rectal balloon, which was also used for treatment. Full and empty bladder and IV contrast scans were obtained. Clinical target volumes (CTV) included the vaginal cuff, proximal 3 cm of vagina, and pelvic lymph nodes to the level of L4/L5. Prescription was 45 Gy (relative biological effectiveness (RBE) 1.1) in 25 fractions. Planning was multi-field optimization with two posterior oblique beams and a posterior beam. Plans were evaluated for robustness to ensure CTV coverage in worst case scenarios of ±5 mm isocenter shifts in x, y, and z directions and ±5% range uncertainty. A variable RBE calculation assuming a linear relationship between RBE and linear energy transfer was used to evaluate for target coverage and increased RBE within OARs. If needed, the plan was modified to limit hot spots without compromising coverage in the calculation with uniform 1.1 RBE. Patients underwent weekly CT-on-rails verification scans. Target coverage and dose to OARs were assessed at each verification and replanning was done if target coverage or OAR constraints did not meet original planning objectives. <h3>Results</h3> The first patient on the study required replan after first verification due to excess air in the rectum at simulation and verification, resulting in rectum V30Gy 90% and V40Gy 81%. After replan, subsequent verifications met objectives. Subsequent patients were planned with air in the rectum or bowel accounted for by evaluation of plans with air overridden to HU = -300 to -450 and HU = 0. Target coverage met acceptable planning parameters in all cases (Table 1). There was no statistical difference in bladder mean, V30Gy, and V45Gy; sigmoid V45Gy; rectum mean, V30Gy, and V40Gy; and small bowel V40% and V45Gy. All patients were successfully treated with protons for all planned fractions. <h3>Conclusion</h3> Proton post-hysterectomy pelvic RT with robust optimization is feasible, reproducible, and associated with appropriate target coverage and acceptable dose to organs at risk as assessed with weekly verification scans.