Purpose: This study aimed to report on the institutional outcomes after proton therapy for pelvic rhabdomyosarcoma (RMS).

Methods and materials: Thirty-one children (≤21 years old) with group III pelvic RMS were enrolled on a prospective outcome study and treated between 2007 and 2018. Patients with vaginal/cervical RMS were excluded. The median age was 2.6 years. Twenty-four patients had embryonal RMS. At diagnosis, the median tumor volume was 185 cm³ and the median maximum diameter was 9.4 cm. Seven patients had N1 disease. Nineteen and 12 patients received European Pediatric Soft Tissue Sarcoma Study Group- and Children's Oncology Group-based chemotherapy, respectively. Fourteen patients underwent resection of the primary tumor after induction chemotherapy, including 6 patients who had a total cystectomy. The median radiation dose was 50.4 Gy relative biological effectiveness.

Results: With a median follow-up of 4.2 years, the 5-year local control, progression-free survival, and overall survival rates were 83%, 80%, and 84%, respectively. Patients <3 years old had better local control (100% vs 68%; P = .02), and patients with embryonal histology had better survival (96% vs 54%; P = .02). No other factors were significantly associated with disease control or survival. Specifically, no statistically significant difference was observed in local control, progression-free survival, or overall survival when comparing patients who underwent biopsy versus gross total resection (75% vs 93%, 68% vs 93%, 75% vs 93%, respectively). Excluding patients who underwent cystectomy, urinary toxicity was limited to 2 patients with nocturnal enuresis. Exploratory surgery to address a persistent mass or thickened bladder wall after radiation was the most common source of serious toxicity.

Conclusions: This cohort of young children with large pelvic tumors treated with proton therapy demonstrates similar local control with less toxicity than historic reports. Functional bladder preservation is possible in most patients. Exploratory biopsy in the 18 months after radiation should be approached with caution.