Characterization and clinical utility of different collimator shapes in accelerator-based BNCT systems for head and neck cancer

•Characterization of a new collimator with a convex shape on the patient side (extended collimator [EC]) in accelerator-based BNCT.•It was found that a sufficient thermal neutron flux for treatment was obtained using ECs.•In cases where the collimator-patient distance was difficult to decrease, the use of EC could reduce the dose to normal tissue.•The reduced distance between the collimator and the patient ensured by ECs significantly decreased the treatment time.