For infants with localized rhabdomyosarcoma who were enrolled on Children's Oncology Group ARST0331 and ARST0531, local therapy guidelines were provided, but adherence was not mandated because of toxicity concerns. We examined adherence to protocol for local therapy guidelines, treatment variations, and outcomes for these patients.

Children aged ≤24 months who were enrolled on ARST0331 and ARST0531 were evaluated. Data were verified through radiologic, surgical, pathologic, and clinical records. Local therapy was assessed for adherence to protocol guidelines, with variations termed "individualized local therapy." The subdistribution hazards model assessed local failure, the Kaplan-Meier product limit method assessed event-free survival (EFS) and overall survival, and the log-rank test was used to evaluate prognostic impact.

The median age of the patients was 14 months, and 124 patients were eligible. Common primary sites were genitourinary (40%), parameningeal (14%), and the extremities (11%). Most patients had unresected disease (group 3, 64%) and embryonal histology (73%). Fifty-eight percent of patients received radiation therapy at a median of week 12 (weeks 1-45). The median radiation dose was 48.6 Gy (30.6-54 Gy). Forty-three percent of patients received individualized local therapy (outside protocol guidelines), typically omission or delay of radiation therapy. Delayed primary excision was performed in 28% at a median of week 14 (weeks 7-34). With a median follow-up of 5.6 years, the 5-year local failure, EFS, and overall survival rates were 24%, 68%, and 82%, respectively. Local failure was significantly higher (35%) in patients receiving individualized local therapy than in patients who received protocol-specified local therapy (16%; P = .02). The site of failure was local in 64% of patients, local and distant in 5%, and distant only in 23%. EFS was significantly higher among patients who were aged 12 to 24 months, had tumors ≤5 cm, had group 1/2 disease, and underwent protocol-specified therapy.

Local recurrence was the predominant pattern of failure and was more common in those receiving individualized local therapy. De-escalation of effective therapies because of concerns about treatment toxicity should be considered cautiously.