O2161: Pelvic Location Predicts Worse Outcomes in Alveolar Rhabdomyosarcoma: Underuse of Radiotherapy and Missed Survival Benefit

Background: Alveolar rhabdomyosarcoma (ARMS) is a rare, aggressive soft-tissue tumor characterized by round cells and partial differentiation of skeletal muscles that has poor prognosis. Most primary tumor arise in the upper and lower extremities. However, pelvic presentation of ARMS has been reported, often posing diagnostic and therapeutic challenges due to its proximity to critical structures. Despite its clinical importance, the prognostic implications of pelvic ARMS relative to more common extremity-based tumors remain poorly understood, highlighting a critical gap in current oncologic knowledge and management strategies.

Methods: We conducted a retrospective cohort study using the Surveillance, Epidemiology, and End Results (SEER) database for patients diagnosed with ARMS between 2000 and 2021. Demographic variables included age, sex, income (above vs. below the 2022 national median), and rurality (urban vs. rural). Treatment modalities analyzed were surgery, chemotherapy, and radiation. Logistic regression was used to assess associations between tumor location and receipt of treatment modalities. Time of survival was analyzed using the Wilcoxon rank-sum test (Mann-Whitney U). Competing-risks survival outcomes were assessed using the Fine-Gray model.

Results: The study included 325 ARMS patients (232 [71.4%] extremity vs 93 [28.2%] pelvic) with the proportion of pelvic tumors increasing with age, rising from 20.0% in patients under 1 year to 44.0% in those aged 15-19 years (Figure 1). The proportion of extremity tumors decreased with age, from 80.0% in patients under 1 year to 56.0% in those aged 15-19 years. Patients with pelvic tumors were at significantly higher mortality risk (sub-hazard ratio [SHR]=1.53, 95%-CI: 1.06-2.19, p=0.021). Additionally, patients with pelvic tumors were less likely to undergo surgical resection (Odds ratio [OR]=0.36, 95%-CI: 0.21-0.62). Out of the 60 patients with pelvic tumors that eventually passed from their cancer specific diagnosis, only 36 (60%) received radiotherapy. Those that received radiotherapy for their pelvic tumors lived significantly longer, with a median increase in survival time of 9 months (25 months vs. 16 months, p=0.029). There was no significant difference in prognostic survival by sex, age, income, or rurality status.

Conclusion: In this cohort of ARMS patients, pelvic tumor location was associated with increased odds of mortality. Among patients with pelvic tumors who died from their cancer-specific diagnosis, only 60% received radiotherapy; however, those who did experienced a 9-month increase in median survival compared to those who did not. These findings highlight a potential survival benefit of radiotherapy in pelvic ARMS and underscore the need for consistent treatment strategies in high-risk anatomical sites.