This pilot study is designed to evaluate the feasibility of delivering ultra-high dose-rate conformal proton therapy (ConformalFLASH) in participants requiring re-irradiation for head and neck cancer who are not amenable to surgical resection. ConformalFLASH treatment planning and delivery workflows are comparable to those of conventional Stereotactic Body Radiation Therapy (SBRT), a standard approach for re-irradiation in this population.

C-FLASH-01 is a pilot study evaluating the feasibility of delivering ultra-high dose-rate conformal proton therapy (ConformalFLASH) to participants requiring re-irradiation for head and neck cancer who are not amenable to surgical resection. Stereotactic Body Radiation Therapy (SBRT) re-irradiation is a standard approach that enables highly conformal dose delivery over a limited number of fractions. ConformalFLASH is an investigational approach that applies SBRT principles using proton therapy delivered at ultra-high dose rates (≥ 40 Gray Relative Biological Effectiveness (GyRBE)/second), with the potential to reduce normal tissue toxicity while maintaining tumor control. Participants will undergo treatment planning and delivery consistent with standard SBRT workflows. Treatment consists of five fractions (8 GyRBE per fraction) delivered over approximately 1.5 to 2 weeks. The primary objective is to assess the feasibility of delivering conformal proton SBRT at dose rates ≥ 40 GyRBE/second, along with evaluation of acute toxicity. Secondary objectives include assessment of treatment delivery accuracy, late toxicity, clinical outcomes, and patient-reported quality of life.

Inclusion Criteria:

1. Patients must sign an informed consent document that indicates they are aware of the investigational nature of the treatment in this protocol as well as the potential risks and benefits
2. Patients ≥ 18 years old
3. Patients ineligible for or decline upfront surgical resection
4. Histologically confirmed diagnosis of carcinoma of the head and neck
5. Prior receipt of at least 45 GyRBE radiotherapy to the head and neck
6. Eastern Cooperative Oncology Group (ECOG) Performance Status 0-2 or evidence of similar in the electronic medical record
7. Life expectancy of more than 12 weeks
8. Ability to understand and the willingness to provide written informed consent and to follow the study procedures

Exclusion Criteria:

1. Receipt of prior head and neck radiotherapy within the prior 6 months
2. Surgical resection of the current disease in the head and neck
3. More than 3 areas of disease requiring reirradiation
4. Disease focus exceeding 6.5 cm in greatest dimension
5. Presence of distant metastatic disease
6. Involvement of any of the following structures: skin, carotid artery, or mandible
7. Receipt of concurrent chemotherapy during radiotherapy
8. Uncontrolled intercurrent illness including medical and psychiatric, and social situations that would limit compliance with study requirements
9. Enrolled in any other investigational studies that involve administrations of another cancer therapeutic (such as surgery, chemotherapy, immunotherapy) during RT
10. Pregnant and/or breastfeeding