Oral mucosal malignant melanoma (OMM) is extremely rare and has a poor prognosis. Owing to the rarity, it has not yet been possible to establish an optimum treatment modality. The objective of this study was to evaluate the long-term efficacy of carbon-ion radiotherapy (C-ion RT) for OMM. Between 1997 and 2013, 19 patients with OMM were treated with C-ion RT alone. The patients ranged in age from 44 to 84 years (median, 69 years). Nine men and 10 women were included. OMMs were restaged in accordance with the seventh edition of the tumor-node-metastasis (TNM) staging system of the International Union Against Cancer. Before treatment, 14 patients had T3 disease and 5 had T4a disease. Three patients were classified as having N1 disease. All patients were classified as M0. The hard palate was the most frequently involved oral subsite. All patients were treated with 57.6 Gy (relative biological effectiveness) in 16 fractions. The median follow-up period was 61 months (range, 8-190 months). The 5-year local control, overall survival, and progression-free survival rates were 89.5%, 57.4%, and 51.6%, respectively. For local control and overall survival, T classification was found to be a significant prognostic factor. Grade 2 and 3 osteoradionecrosis was observed in three patients and four patients, respectively. The presence of teeth within the planned target volume was a significant risk factor for developing osteoradionecrosis. C-ion RT was a safe and effective treatment option for OMM.
