Radiation therapy for rhabdomyosarcoma: local failure risk for Clinical Group III patients on Intergroup Rhabdomyosarcoma Study II

A subset of 362 pediatric patients with rhabdomyosarcoma was selected from a total of 532 eligible IRS-II patients in Clinical Group III to assess the local and regional failure rates following radiotherapy and to determine patient, tumor, and treatment factors contributing to the risk for local and regional failure.

The study population was selected from all eligible IRS-II Clinical Group III patients. Excluded patients were those with "special pelvic" primary sites whose protocol management restricted radiotherapy (n = 123), and those who were removed from the study before radiotherapy was to begin, or because it was omitted (n = 47). A binary recursive partitioning model was used to identify subgroups of the remaining 362 patients at risk of local or regional failure.

The local (only) failure rate was 17% (95% confidence interval, 13-21%), and the local (all) failure rate was 20% (95% confidence interval, 16-24%). The 5-year actuarial risk of local (all) failure was 22% (95% confidence interval, 18-27%). The risk of regional (nodal) failure was between 2% and 23%. Increasing tumor size predicted an increased local failure risk. Primary tumors located above the clavicle had a reduced risk of local failure. The binary recursive partitioning model identified a subset of patients at high risk of local failure. Those patients had primary tumors in the chest, pelvic region, extremity, or trunk, or tumors > 10 cm in diameter. Their local failure rate was 35% (compared to 15% for the remaining patients). The subset of patients at high risk for regional (nodal) failure had node involvement at diagnosis and a primary tumor originating at a site other than orbit, parameningeal, or trunk. Compliance with radiation treatment guidelines approached but did not achieve statistical significance as a predictive factor for local failure. By univariate analysis, factors not influencing local failure risk were age, race, gender, adenopathy, and histology.

Radiation therapy and chemotherapy administered to Clinical Group III patients entered into the IRS-II protocol produced sustained local control in most cases. Knowledge of the factors which predict an increased risk of local or regional failure will facilitate the design of new treatment strategies.