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.