Particle beam radiation therapy for head and neck rhabdomyosarcoma in adults

BACKGROUND: Rhabdomyosarcoma (RMS) is rare in adults, with a significantly worse prognosis than its pediatric counterpart. Radiotherapy (RT) plays a significant role in treating head and neck RMS (HNRMS), but the outcomes of conventional RT are limited by the complex anatomy and unfavorable pathology subtypes of the adult H&N RMS. Here, we aim to report the effectiveness and safety of carbon-ion beam RT (CIRT), either alone or in combination with proton radiotherapy (PRT) in the management of adult HNRMS. METHODS: Fifteen adult patients with HNRMS were enrolled on a prospective registry protocol between 06/2015 and 12/2019. Eight patients presented with parameningeal tumors, and eight had unfavorable pathology subtypes [alveolar =7, not otherwise specified (NOS) =1]. Eleven patients had gross tumors before the start of RT (volume range, 46.1–137.6 cm(3)). Two patients failed the earlier RT. All except for one patient received multi-drug chemotherapy. The median absolute dose of particle beam RT was 70.0 Gy [relative biological effectiveness (RBE)]. RESULTS: With a median follow-up of 21 months, local or distant recurrence occurred in three and four patients, respectively, and two added patients had both local and distant failure. One patient died of distant metastasis (DM), and another died of an unrelated condition. The 1- and 2-year overall survival (OS), local relapse-free survival (LRFS), distant metastasis-free survival (DMFS), and progression-free survival (PFS) rates were 87.5% and 70.0%, 92.3% and 67.1%, 72.2% and 54.2%, and 65.0% and 24.4%, respectively, for the entire cohort. Both patients who failed earlier RT and received salvage CIRT developed DM but were alive at last follow-up. No acute toxicity of ≥ grade 3 or late toxicity of ≥ grade 2 was observed. CONCLUSIONS: CIRT, either used alone or in combination with PRT, is not only feasible and safe but also useful in local disease control for HNRMS. DM is the most important cause of treatment failure; thus, more effective systemic treatment is needed to improve the prognosis of HNRMS further.