This trial tests how well personalized ultra fractionated stereotactic adaptive radiotherapy (PULSAR) works together with HyperArc© radiation treatment planning technology for palliative (holistic pain and symptom control) tumor control in patients with primary or recurrent, localized or metastatic head and neck cancer (HNC) who are ineligible for or decline standard of care treatment. Researchers want to evaluate if using HyperArc and PULSAR together will deliver higher, possibly more effective doses, resulting in better tumor control with the same or fewer side effects than smaller routine doses. PULSAR is a radiation therapy regimen that uses a limited number of fairly large dose pulses while adjusting to specific anatomic and/or biological changes which may occur during the course of the treatment. HyperArc radiation treatment planning technology is a tool that allows for target dose escalation to tumor tissue while maintaining minimal head and neck organs-at-risk doses compared to other radiation treatment planning software. Undergoing PULSAR and HyperArc technology together may be a safe and effective palliative treatment option for patients with HNC.

A significant proportion of patients with de novo or recurrent head and neck cancer are not candidates for standard-of-care definitive treatment(s), including standard-of-care stereotactic body radiation therapy (SBRT). Current palliative radiation therapy regimens result in adequate symptom improvement though with suboptimal local control and/or toxicity. The durability of symptom and local control in these patients is becoming more important as advances in systemic therapy are improving these patients' survival.

After confirmation of eligibility, enrolled patients will undergo radiation simulation and planning per standard of care. HyperArc technology will be used for treatment planning given prior internal validation of its superiority. Dosimetric constraints will be adapted from the available literature as appropriate. Patients will receive an 11 Gray fraction of radiation to the head and neck tumor site every 14 +/-10 days.

It is possible that patients will experience anatomical and/or tumor changes during the radiation therapy course such that patient alignment for radiation therapy is no longer optimized to the original CT simulation and radiation plan. In such cases where it is determined that adaptive planning is needed, the Investigators will abort planned radiation therapy and immediately perform re-simulation and re-planning if the anatomic change is significant enough to put adjacent normal tissue at significant risk; otherwise, the Investigators will re-plan with the next fraction of radiation to minimize treatment delays. This will be determined by the treating physician through standard of care clinical criteria and procedures.

The frequency of re-simulation and adaptive planning as well as associated changes in dosimetry will be measured. Patients will receive 5 PULSAR fractions for a total radiation dose of 55 Gray. Treatment will be terminated early in cases of intolerable treatment-related toxicity, altered clinical context, or the patient declines further treatment.

Enrolled patients are permitted to receive systemic therapy at the discretion of their medical oncologist.

Patient follow-up will be measured from the time of receipt of first PULSAR fraction to 12 months after receipt of the last PULSAR fraction.

Inclusion Criteria:

• >= 18 years old
• Diagnosis of primary or recurrent, localized or metastatic (American Joint Committee on Cancer (AJCC) 8th Edition stages I-IV) head and neck cancer. In primary diagnosis cases, pathologic confirmation is required. In recurrent and/or metastatic diagnosis cases, pathologic confirmation is not required if not beneficial to the patient as standard of care, and diagnosis can be assumed based on clinical and/or radiographic evidence
• Ineligible for or declines standard of care definitive treatment(s), which will be documented in the patient's trial screening progress note in their electronic medical record by the treating physician
• Measurable disease within the head and/or neck clinically and/or on imaging studies (CT, PET, MRI) within 30 days from date of enrollment
• Patient maximum tumor(s) or tumor bed diameter must be less than 10cm
• In a woman of childbearing potential, a negative serum or urine pregnancy test within 1 week of treatment start must be documented. Women of childbearing potential must agree to use adequate contraception (hormonal or barrier method of birth control; or abstinence) for duration of study participation and for up to 4 weeks following the study treatment
• Patients with a tracheostomy and/or a percutaneous endoscopic gastrostomy tube are eligible for inclusion

Exclusion Criteria:

• Pregnant or breast-feeding
• More than 1 prior radiation treatment course directed to the treatment area over the patient's lifetime. In patients who have received 1 prior radiation treatment course directed to the treatment area, that prior radiation treatment course must have concluded at least 6 months prior to trial enrollment
• Any comorbidity or condition which would limit full compliance with the protocol