Patterns of failure and toxicity profile following proton beam therapy for pediatric bladder and prostate rhabdomyosarcoma. Issue 11 (9th August 2019) - Buszek, Samantha M.
Abstract: Purpose/Objective(s): Bladder and prostate are unfavorable sites for rhabdomyosarcoma (B/P‐RMS), and represent a challenging location for radiotherapy. Materials/Methods: Nineteen patients with B/P‐RMS were enrolled on a prospective registry protocol (2008‐2017) and treated with chemotherapy, proton beam therapy (PBT), and surgical resection (n = 8; 42%). Emphasis was given to treatment technique, disease‐related outcomes, and toxicity associated with PBT. Results: The majority of patients had bladder RMS (74%) of embryonal histology (95%), Group III (68%), and intermediate‐risk disease by Children's Oncology Group (COG) risk stratification (89%). Seven patients (37%) had primary tumors >5 cm in size. All patients were treated according to COG protocols. With a median follow‐up of 66.2 months, 5‐year overall survival (OS) and progression‐free survival (PFS) were 76%. Four patients (21%) experienced disease relapse, all presenting with local failure. The 5‐year local control (LC) rate was 76%. Tumor size predicted LC, with 5‐year LC for patients with >5 cm tumors being 43% versus 100% for those with ≤5 cm tumors ( P = .006). Univariate analysis demonstrated an effect of tumor size on OS (tumor >5 cm, hazard ratio [HR] 17.7, P = .049) and PFS (HR 17.7, P = .049). Acute grade 2 toxicity was observed in two patients (11%, transient proctitis). Late grade 2+ toxicity was observed in three patients (16%; n = 1 grade 2 skeletal deformity; n = 3 transient grade 2 urinary incontinence; one patient experienced both). Conclusions: PBT for B/P‐RMS affords promising disease‐related outcomes with an acceptable toxicity profile. Higher local failure rates were observed for larger tumors, supporting dose‐escalation components of ongoing RMS clinical trials.