3595 - Evaluating the Potential of Proton Therapy for Whole Abdominal Radiotherapy

C. Y. Zhao¹, K. M. Frechette², R. O. Kowalchuk³, S. K. Ahmed⁴, W. Allen-Rhoades⁵, P. F. Gargollo⁶, C. F. Granberg⁶, S. Polites⁷, P. J. Schoettler⁵, N. N. Laack II⁸, and A. Mahajan⁸; ¹Mayo Clinic, Rochester, MN, ²Mayo Clinic Department of Radiation Oncology, Rochester, MN, ³University of Virginia / Riverside Radiosurgery Center, Newport News, VA, ⁴Department of Radiation Oncology, Mayo Clinic AZ, Phoenix, AZ, ⁵Department of Pediatric Hematology/Oncology, Mayo Clinic, Rochester, MN, ⁶Mayo Clinic Department of Urology, Rochester, MN, ⁷Department of Pediatric Surgery, Mayo Clinic, Rochester, MN, ⁸Department of Radiation Oncology, Mayo Clinic, Rochester, MN

Purpose/Objective(s): Whole abdomen radiotherapy (WART) is used to treat pediatric malignancies with a high risk of peritoneal spread. WART with X-Ray (XRT) based intensity-modulated radiation therapy (IMRT) has been reported to reduce acute toxicities compared to conventional radiotherapy (CRT). Proton beam therapy (PBRT) may further reduce toxicities. We compare treatment logistics, dosimetry, and outcomes for WART with PBRT and XRT. Materials/

Methods: A retrospective chart of pediatric patients receiving WART at a single center from 2014 to 2023 was performed. Dosimetric parameters, treatment delays, replanning needs and outcomes were compared between patients receiving XRT and PBRT. T-tests were used for statistical analysis where appropriate.
Results: 13 patients (7 male) with Wilms tumor (WT) (n=11) or embryonal rhabdomyosarcoma (ERMS) (n=2) were included. Median age at RT was 5.8 years (range 1.4-11.0). Nine WT patients received 10-17 Gy in 7 fractions and two received 18-20 Gy in 12-13 fractions. The ERMS patients received 30-36 Gy in 20 fractions. Median time from simulation to RT start was 5.5 days (range 1-8) for XRT and 8.4 days (range 5-13) for PBRT (p=0.09). No significant differences were observed for mean dose to kidneys, femoral growth plates, vertebrae, bladder, liver, heart or V8.5 Gy and V15Gy for the liver between XRT or PBRT. The 6Gy volume in the kidneys approached significance, PBRT plans indicated significant bilateral femoral head sparing (p=0.027). Median RT duration was 9 days (range 8-30). Of the seven PBRT patients, two required mid-treatment replanning, and two switched mid-PBRT to IMRT due to significant daily bowel content variability. Two PBRT patients had unanticipated interruptions during RT: one for replanning and a 2nd for management of increased ascites. All 6 XRT (5 IMRT, 1 3D XRT) patients completed their treatment as planned with no interruptions. Median follow-up duration post-treatment was 29.4 months (range 2.7-101.2). At last follow-up, 1 patient died of disease, 1 was undergoing treatment, and 11 had no evidence of disease recurrence or progression.
Conclusion: IMRT and PBRT WART achieved at least comparable abdominal organ-at-risk sparing in pediatric patients. PBRT may offer some dosimetric advantages, such as better sparing of femoral heads. PBRT requires more planning time, verification scans, and replans. We note that careful evaluation of treatment set up and bowel content surveillance is necessary to insure appropriate planning and delivery of PBRT for WART. Individualized modality selection balancing efficacy, availability, and efficiency is warranted. Further study of long-term outcomes and strategies to reduce PBRT delivery complexity is needed.