Abstract
Adenoid cystic carcinomas (ACC) and, to a lesser extent, cutaneous squamous cell carcinomas and basal cell carcinomas may exhibit perineural invasion (PNI). A subset of patients have tumors with extensive PNI tracking to the skull base that are incompletely resectable and are treated with definitive radiotherapy (RT). RT may be administered with intensity-modulated RT or proton RT. Patients with ACC may also be considered for neutron RT, although the number of available neutron facilities is limited. A substantial proportion of patients with incompletely resectable ACCs and cutaneous carcinomas may be cured with definitive RT. Proton RT provides a more conformal dose distribution compared with other modalities and is likely associated with a lower risk of complications.