Distinct patterns of osteoradionecrosis after photon-based radiotherapy and carbon ion radiotherapy for unresectable adenoid cystic carcinoma of the head and neck: case series from two institutions.

Background and purpose: To present the clinical outcomes of two series of patients treated with carbon-ion radiotherapy (CIRT) and definitive photon radiotherapy (RT) for adenoid cystic carcinoma of the head and neck (HN-ACC).

Material and methods: The first cohort of six patients was referred from Oslo University Hospital (OUS) to Centro Nazionale di Adroterapia Oncologica (CNAO, Pavia, Italy) for CIRT in 2014-2017. The second cohort included 18 patients treated with definitive photon RT at OUS in 2005-2017. The primary endpoint was an evaluation of osteoradionecrosis (ORN) in the two cohorts. The secondary endpoints were treatment efficacy by local control (LC), progression-free survival (PFS), and overall survival (OS).

Results: The tumor stage was T4 for all the patients in the CIRT group and 15 (84%) in the photon group. There were three (50%) patients with grade 3 ORN in the CIRT group compared to one (6%) with grade 3 ORN in the photon group (p = 0.05). The 5-year LC (95% CI), PFS, and OS rates in the CIRT group and the photon group were 33% (11-100) and 39% (19-76), 17% (9-100) and 23% (2-59), and 80% (52-100) and 50% (31-82), respectively.

Interpretation: Half of the patients in the CIRT cohort experienced ORN requiring surgical management during the follow-up. Patients with ACC referred for CIRT often have a worse prognosis and more advanced disease than patients treated with photons. When returning from the referring center, these patients need close follow-up often in collaboration with treating centers to manage toxicity that impacts quality of life.