Stereotactic Ablative Radiotherapy for Oligo-Progressive Disease REfractory to Systemic Therapy in Metastatic Cancer

Inclusion Criteria: * Age ≥18 years * Biopsy proven HNSCC (oropharynx, oral cavity, nasopharynx, sinonasal, larynx or hypopharynx) * Metastatic HNSCC, with pathological or radiological proof of metastasis * Ability to provide written informed consent * Eastern Cooperative Oncology Group (ECOG) performance status 0-2 * Progressive disease while on systemic treatment (any line), defined as per RECIST criteria 1.1 on CT metrics as a greater than 20% increase in the sum measurement of lesions, non-target unequivocal progressive disease or a new lesion on CT. * Oligoprogression to 1-5 extracranial lesions ≤ 5cm and involving ≤ 3 organs. Progression at the primary tumor site should be counted within the total of 5 lesions. For patients with lymph node metastases, each node is counted as one site of metastasis. * All sites of disease can, in the opinion of the investigator, be safely treated and targetable with SABR (taking into account prior local therapy, organ function and underlying medical condition such as inflammatory bowel disease, pulmonary fibrosis, etc.) * Patients with prior metastases that have been treated with ablative therapies (e.g. radiotherapy, surgery or radiofrequency ablation) before their current line of systemic therapy, are eligible. Exclusion Criteria: * Pregnancy or breastfeeding * Any medical condition that could, in the opinion of the investigator, preclude radiotherapy or prevent follow-up after radiotherapy. * Presence of spinal cord compression * Metastatic disease that invades the GI tract (including esophagus, stomach, small or large bowel