Prognostic factors of adenoid cystic carcinoma of the head and neck in carbon-ion radiotherapy: The impact of histological subtypes

Purpose: The aim of this study was to evaluate the effect of histological subtypes of head and neck adenoid cystic carcinoma (ACC) on the results of carbon-ion radiotherapy (CIRT).Material and methods: Of the 113 patients with ACC who were treated with CIRT between December 2006 and July 2013, 100 patients with identified histological subtypes were enrolled in this study. CIRT at a total dose of 57.6 or 64.0 Gy (RBE) was administered in 16 fractions. Histological grading was defined as the presence or absence of a solid growth pattern.Results: Median follow-up was 60 months. 5-Year local control (LC), overall survival (OS) and distant metastasis free survival (DMFS) of all patients were 68.6%, 74.8% and 65.7%, respectively. On multivariate analysis, the prescribed dose (p = 0.001) and gross tumor volume (p = 0.002) were significant independent risk factors for LC. No significant difference for local control of solid/non-solid growth patterns was found (p = 0.093). Solid growth pattern was an independent risk factor for both OS (p = 0.033) and DMFS (p = 0.024).Conclusions: CIRT appears able to locally control solid growth pattern ACC in the head and neck. Improved intervention is needed to extend DMFS and OS