(P 316) SECONDARY RHABDOMYOSARCOMA IN CHILDREN, ADOLESCENTS, AND YOUNG ADULTS

Objective: The primary aim of the study was to estimate the prevalence of secondary RMS compared to primary RMS. Secondary aims included determining differences between secondary and primary RMS in patient demographics, disease characteristics, general treatment approaches, primary malignancy diagnosis, and overall survival (OS) since primary or secondary RMS diagnosis.

Methods: We performed a retrospective cohort study using the Incidence – Surveillance, Epidemiology, and End Results (SEER) Research Plus Data, 17 Registries, Nov 2023 Sub [2000-2021]. We included patients 0-39 years old who were diagnosed with RMS from January 1, 2000 to December 31, 2021. Cases were defined as primary if RMS was the only or first of multiple neoplasms and as secondary if RMS was a second or later neoplasm. RMS histologies were categorized as alveolar, embryonal, or other. For the primary aim, we reported the prevalence and 95% confidence intervals (CI). For the secondary aims, patient demographics, disease characteristics, and treatment characteristics between those with primary and secondary rhabdomyosarcoma were evaluated using Chi-square/Fisher’s exact test for categorical variables and Mann-Whitney U tests for continuous variables as appropriate. The 5-year OS from the time of RMS diagnosis (primary or secondary) was calculated using Kaplan-Meier analysis.

Results: We identified 2747 patients with RMS in the SEER dataset; 2676 with primary RMS and 71 with secondary RMS (prevalence of secondary RMS 2.6%, 95% CI 2.0-3.3%). For secondary RMS, 60 cases were a second neoplasm and 11 cases were a third neoplasm. There were no differences in sex, race, cancer site, and summary stage at diagnosis for primary and secondary RMS (Table 1). Secondary RMS occurred in older patients (p<.001) and were more likely to have “other” (non-alveolar, non-embryonal) histology (p<.001). In regards to therapy, there was no difference in the rate of cancer-directed surgery between primary and secondary RMS, however radiation (63.6% vs 32.4%, p<.001) and chemotherapy (93% vs. 77.5%, p<.001) were given less frequently in secondary RMS (Table 1). Common primary malignancies for patients with secondary RMS were germ cell tumors (28.2% of cases) and leukemia/lymphoma (23.1% of cases). The 5-year OS of patients with secondary RMS (37%, CI 27-50%) was inferior to patients with primary RMS (60%, CI 58-62%, p<.001, Figure 1). The median OS for secondary RMS was 24 months and was not reached for primary RMS. OS in secondary RMS was associated with age (p=.028), and site (p=.004, Figure 2).

Conclusion: Secondary RMS is a rare event and has different presenting characteristics from primary RMS, including age and histology. Patients with secondary RMS are less likely to receive radiation or chemotherapy as part of their treatment and have inferior 5-year OS compared with primary RMS. Based on the primary malignancies noted in secondary RMS, we hypothesize that prior cancer therapy, underlying cancer predisposition syndromes, and somatic transformation of germ cell tumors are responsible for most secondary RMS. The inferior survival seen of secondary RMS is likely multifactorial, however the decreased use of chemotherapy and radiation compared to primary RMS may contribute to this finding.