Clinical Implementation of Preoperative Short-Course Pencil Beam Scanning Proton Therapy for Patients With Rectal Cancer

Purpose: For treatment of rectal cancer, pencil beam scanning proton therapy (PBS-PT) may reduce radiation exposure to normal tissues compared with 3-dimensional conformal photon radiation therapy (3DCRT) or volumetric modulated arc photon radiation therapy (VMAT). The purpose of this study was to report the clinical implementation and dosimetric analysis of preoperative short-course PBS-PT for rectal cancer. Methods and Materials: Eleven patients with stage IIA-IVB rectal cancer received preoperative short-course (25 Gy in 5 fx) PBS-PT between 2018 and 2019 preceding curative-intent total mesorectal excision. PBS-PT plans were generated using single-field optimization with 2 posterior-oblique fields. Verification computed tomography scans were performed on the first 3 days of treatment. Each patient had a backup 3DCRT and VMAT plan. Results: Clinical target volume coverage was similar between PBS-PT, 3DCRT, and VMAT. PBS-PT had statistically significant reductions in dose to the small bowel, large bowel, bladder, and femoral heads across multiple dosimetric parameters. All patients completed PBS-PT as planned without need for replanning. All computed tomography verification scans demonstrated good target coverage with clinical target volume V100 > 95%. Conclusions: Preoperative short-course PBS-PT has been successfully implemented and offers a significant reduction of dose to normal tissues. Prospective studies are warranted to evaluate if dosimetric advantages translate into clinical benefit.