How long after surgery would you no longer offer adjuvant postoperative treatment for head and neck cancer when a patient (with multiple adverse features that needs adjuvant treatment) keeps postponing the start of radiation?

Ultimately, I feel this depends on why the patient keeps postponing treatment. If there are legitimate medical reasons for delay, I would not have a cutoff for starting adjuvant RT at any time - even several months after surgery - as long as the patient can tolerate curative-intent therapy and I have diagnostic re-staging imaging to document any gross disease recurrence that may need re-resection or definitive (chemo)RT. If re-staging imaging is negative, I would likely choose to give 66 Gy in 33 fx to any areas at higher risk for recurrence, ie primary site and ECE areas, with consideration for adding concurrent chemotherapy depending on the patient's health and original pathologic risk factors.

If the patient is delaying for psychosocial r...

I agree that there’s not line in the sand. I try to start postop RT within 6 weeks and certainly within 8 weeks. My enthusiasm begins to wane after 8 weeks and by 3 months it’s pretty much gone.