Is there a scenario in which you would consider observation for T4a SCC involving the mandible?

While PORT is well established, the scientific basis for it is relatively weak in the absence of a randomized trial, as all phase 3 trials have focused on adding something to radiation rather than the benefit of radiation alone.

Further, the rationale for PORT historically is if there is a belief of >15% recurrence, though this has changed in part due to using PORT for even lower risk sites, such as breast, or for HN, some of the HPV post TORs cases (like a 4 cm node w/o ECE).

Patients thus recommended for PORT have a huge range of recurrence risk from lower to nearly 100% in the case of a positive margin and ENE.

Thus, the question circles back to what is the risk of recurrence (assuming pN0) of a T4a SCC, further assuming the designation ...

My standard recommendation for a pT4a squamous cell carcinoma of the mandible would be adjuvant RT. As with every patient, I would have a risk/benefit discussion with the patient, and it is always the patient's decision, but my formal recommendation for pT4a disease is adjuvant RT. The only scenario where that might change is if this were a recurrent (previously radiated) disease with broadly negative margins.

Here's an NCDB paper that somewhat addresses the question by Harley et al., PMID 38300601.

This shows that the size of the T4a lesion matters. Perhaps there is a rationale for omitting PORT in patients with small (<2 cm) tumors.

I think the big picture is yes, I would recommend PORT. But in patients with small T4a tumors, reassuring pathological features, and depending on age and comorbidities, I wouldn't twist their arm if they refused.