(P 250) OUTCOME OF INFANTS WITH RHABDOMYOSARCOMA: A REPORT FROM THE INTERNATIONAL SOFT TISSUE SARCOMA CONSORTIUM

Objective: Up to 11% of rhabdomyosarcoma (RMS) cases occur in infants ≤1 year. Due to its rarity, diverse treatment approaches, and limited sample sizes, published outcomes remain inconsistent. We conducted a pooled analysis of cooperative group trials (1991–2016) using the International Soft Tissue Sarcoma Consortium (INSTRuCT) database to evaluate survival outcomes and identify prognostic factors in infants with RMS.

Methods: Patients ≤1 year with newly diagnosed RMS included in the INSTRuCT database were eligible. Event-free survival (EFS) was defined from enrollment to progression/recurrence, second malignancy, or death; overall survival (OS) was defined from enrollment to death from any cause. Cox proportional hazards model was used to assess factors associated with survival.

Results: We included 389 infants treated on 15 trials from COG (n=95), CWS (n=83), MMT (n=74), ICG (n=24), and EpSSG (n=113). Genitourinary bladder/prostate was the common site (23%). Metastatic disease was present in 7% (n=26). In those with localized disease, 68% had Group III disease, 17% had Group II disease, and 8% had Group I disease. Histologically, 64% had embryonal/botryoid RMS, 26% alveolar RMS (ARMS) and 10% had spindle cell RMS. FOXO1 fusion status was known for 148; of these, 78% were fusion-negative, 17% had a PAX3-FOXO1 fusion, and 5% had a PAX7-FOXO1 fusion. The 5-year EFS and OS were 59.9% (95% CI, 54.3%–65.5%) and 76.1% (95% CI, 71.3%–80.9%), and 15.4% (95% CI, 0.0%–31.4%) and 37.6% (95% CI, 15.6%–59.6%) for those with localized and metastatic disease, respectively. On multivariable analysis, among patients with localized disease, younger age (≤3 months), ARMS histology, and Group III disease were associated with inferior EFS, while younger age (≤3 months), ARMS histology, T2 tumor, and larger tumor (>5 cm) were associated with worse OS. In those with metastatic RMS, only younger age (≤3 months) was associated with worse EFS and OS on univariable analysis.

Conclusion: In this largest pooled analysis of infants with RMS, survival outcomes remain suboptimal, particularly among those with metastatic disease. Younger age (≤3 months) predicts inferior survival among infants with both localized and metastatic disease.