Patterns of Failure in Pediatric Rhabdomyosarcoma After Proton Therapy

Abstract

To report on the patterns of failure in children with rhabdomyosarcoma treated with proton therapy. Between February 2007 and November 2013, 66 children with a median age of 4.1years (range, 0.6-15.3years) diagnosed with nonmetastatic rhabdomyosarcoma were treated with proton therapy. Clinical target volume 1 was defined as the prechemotherapy tumor plus a 1-cm anatomically constrained margin. Clinical target volume 2 was defined as the postchemotherapy tumor (or tumor bed) plus a 0.5-cm anatomically constrained margin, further expanded to encompass potential pathways of spread, including soft tissue infiltrated with tumor at diagnosis. Of the 66 children, 11 developed locally progressive disease at a median of 16months (range, 14-32months), for an actuarial 2-year local control rate of 88%. Among the children who progressed, median age and tumor size at diagnosis were 6.7years (range, 0.6-16years) and 6cm (range, 2-8cm), respectively. Of the recurrences, 64% and 36% were embryonal and alveolar, respectively. Disease progression was observed in 7 (64%) parameningeal, 2 (18%) head and neck (other), and 2 (18%) bladder/prostate subsites. At diagnosis, 8 of 11 patients who developed a recurrence were Intergroup Rhabdomyosarcoma Study stage 3, and all 11 were group III. Of the relapses, 100% (11 of 11) were confirmed as in-field within the composite 95% isodose line. One of the 11 patients (9%) developed a new simultaneous regional nodal recurrence outside of the previously treated radiation field. Early data suggest that the sharp dosimetric gradient associated with proton therapy is not associated with an increased risk of marginal failure. Routine use of a 0.5- to 1-cm clinical target volume 1/2 margin with highly conformal proton therapy does not compromise local control in children diagnosed with rhabdomyosarcoma with unfavorable risk features.