Intermediate Risk Rhabdomyosarcoma in Very Young Children (Less Than or Equal to 24 Months) Treated on the Prior Children's Oncology Group Protocol ARST1431

BACKGROUND: Rhabdomyosarcoma is the most common soft tissue sarcoma in pediatrics and infants are reported to have inferior outcomes. ARST1431 recommended delayed primary excision (DPE) while discouraging radiation (RT) deviations. This analysis compared outcomes among very young and older patients in this study. PROCEDURES: Demographic and tumor characteristics were summarized using frequencies and percentages. Local failure (LF) frequency was summarized by age group, RT, and DPE status. Event-free survival (EFS) and overall survival (OS) were estimated using the Kaplan-Meier method and compared between age groups using the log-rank test. RESULTS: Of 296 patients, 48 (16.2%) were <24 months: 14 (4.7%) < 12 months, 34 (11.5%) 12-24 months-compared with 248 (83.8%) ≥24 months. DPE was performed in: 7 (50%), <12 months; 9 (26%), 12-24 months; 56 (22%), ≥24 months. RT was given to 9 (64%), 27 (79%), and 195 (79%) of <12, 12-24, and >24 months, respectively. LF in DPE+RT was 8.3% and 6.1%, < 24 months and ≥ 24 months, respectively, compared to 29.3% and 20.5% for RT without DPE. The 4-year EFS did not vary: 56% (95% CI = 26.5, 86.0), 68% (95% CI = 51.2, 84.1), and 61% (95% CI = 54.2, 68.3) <12, 12-24, and >24 months old, respectively. CONCLUSIONS: Outcomes in younger children are comparable to older children with local failure more common in those without DPE. Optimization of local control will be important for future studies. TRIAL REGISTRATION NUMBER: NCT02567435.