Patients with head and neck cancer treated with radiotherapy (RT) have a substantial change of recurrence of the tumor in the pharynx or lymph nodes in the neck. Once tumor and/or lymph nodes have recurred, the prognosis is poor. To increase the efficacy of RT, usually chemotherapy is added to the treatment. However, due to age or co-morbidity chemotherapy is not always feasible to give in all patients. In head and neck patients unfit for chemotherapy, there is a clinical need to increase the effectiveness of RT, without adding substantial toxicity. To this end, the use of thermotherapy in this disease site is investigated. The goal of this clinical trial is to learn about the recommended dose of thermotherapy in addition to radiotherapy for patients with head and neck cancer. This recommended dose is the dose that is tolerable and does not give additional side effects. The main question our study aims to answer is: What is the recommended dose of thermotherapy for patients with primary head and neck cancer treated with radiotherapy? Participants will receive thermotherapy once a week in addition to the standard radiotherapy. Researchers will investigate if side effects occur during the treatment and until 6 months after the last treatment has been given. The thermotherapy will be applied using a device that was made in Erasmus MC and allows for precise heating of the tumor and lymph nodes.