Secondary Rhabdomyosarcoma in Children, Adolescents, and Young Adults

BACKGROUND: The clinical features and outcomes of rhabdomyosarcoma (RMS) occurring as a secondary neoplasm are unknown. We aimed to determine the prevalence of secondary RMS (sRMS) and compare patient characteristics, treatment, and overall survival (OS) between primary RMS (pRMS) and sRMS in children, adolescents, and young adults. METHODS: We queried Surveillance, Epidemiology, and End Results data for patients 0-39 years old diagnosed with RMS from 2000 to 2021. Prevalence of sRMS was reported with 95% confidence intervals (CIs). Comparisons between pRMS and sRMS were evaluated using Chi-square or Fisher's exact test for categorical variables and Mann-Whitney U tests for continuous variables. Five-year OS from time of RMS diagnosis was calculated using Kaplan-Meier analysis. RESULTS: We identified 2676 patients with pRMS and 71 patients with sRMS (prevalence: 2.6%; 95% CI: 2.0%-3.3%). Patients with sRMS were older (p < 0.001) and more likely to have non-alveolar, non-embryonal histology (p < 0.001). Radiotherapy (63.6% vs. 32.4%, p < 0.001) and chemotherapy (93% vs. 77.5%, p < 0.001) were less frequently administered to patients with sRMS. Primary malignancies preceding sRMS included germ cell tumors (28.2%) and leukemia/lymphoma (23.1%). The 5-year OS in sRMS was inferior to pRMS (37% vs. 60%, p < 0.001). OS in sRMS was associated with age (p = 0.028) and site (p = 0.004). CONCLUSION: sRMS is rare and has distinct clinical features compared to pRMS. Treatment characteristics differed, including decreased use of chemotherapy or radiotherapy, and patients with sRMS had inferior 5-year OS. While this inferior OS is likely multifactorial, decreased use of chemotherapy and radiotherapy may be contributory.