Adherence to pretreatment and intratreatment imaging of head and neck squamous cell carcinoma patients undergoing (chemo) radiotherapy in a research setting.

•Patient drop-outs were due to patient-related or healthcare-related factors.•Physical treatment discomfort (pain, dysphagia, psychological anxiety) caused most drop-outs.•Drop-out reduction can be realised by early identification and addressing drop-out-factors, such as preemptive discussion or medication, and improving healthcare-related factors.•Early treatment phase mucosal toxicity was found predictive for patient drop-out.