A multi-institutional retrospective study of carbon-ion radiotherapy for non-squamous cell malignant tumors of the nasopharynx: Subanalysis of Japan Carbon-Ion Radiation Oncology Study Group study 1402 HN

Background: This multi-institutional retrospective study focused on the clinical outcome of carbon-ion radiotherapy (C-ion RT) for non-squamous cell malignant tumors of the nasopharynx. Methods: The Japan Carbon-ion Radiation Oncology Study Group collected and analyzed data for 43 patients with non-squamous cell malignant tumors of the nasopharynx treated with C-ion RT at four institutions in Japan. Results: Twenty-nine patients had adenoid cystic carcinomas, seven had malignant melanomas, three had adenocarcinomas, two had mucoepidermoid carcinomas, and two had other pathologies. Twenty-six of the 43 patients (61%) had T4 tumors. The most common dose-fractionation schedule was 64 Gy (relative biological effectiveness) in 16 fractions. The median follow-up period was 30 months. The 2-year local control (LC) and overall survival (OS) rates were 88% and 84%, respectively. For late toxicity, one patient developed grade 4 optic nerve disorder and two developed grade 5 pharyngeal hemorrhage. Actual incidence of grade 3 or higher late adverse events was 19%, and included cranial nerve dysfunction, jaw bone necrosis, central nervous system necrosis, and ear inflammation. Conclusions: C-ion RT provided good LC and OS rates with acceptable toxicity for treatment of non-squamous cell malignant tumors of the nasopharynx.