How do you manage a supraclavicular only recurrence in NSCLC previously treated with chemoradiation for Stage III disease?

If the patient is otherwise fit and wishes to pursue aggressive therapy, I would consider definitive dose radiation therapy either as consolidation after systemic therapy, or concurrent with systemic therapy.

While any recurrence of lung cancer is often a harbinger of systemic progression, a minority of patients with apparently limited recurrence are curable by salvage therapy. For example, besides patients treated definitively for oligometastatic disease, patients treated with salvage therapy for isolated regional recurrence after SABR for early-stage lung cancer can have survival similar to de novo stage III lung cancer (Brooks et al., PMID 30646121).

If the disease-free interval before recurrence is short, starting with systemic therapy (appropriate to the molecular characteristics of the tumor) as a biological test would be reasonable, followed by complete restaging and definitive salvage RT if still limited. I would consider proceeding directly to definitive salvage chemoRT if the disease-free interval is longer (already passing the test of time that wider systemic dissemination is not imminent). This is similar to the approaches taken with oligometastatic disease (and shown to provide a survival benefit in that context) in fit patients desiring maximal therapy.

A supraclavicular only recurrence after previous chemoRT for Stage III NSCLC is rare, occurring in approximately 7% of cases. I would treat it with palliative radiation alone, to be followed by systemic therapy (chemotherapy, chemo-IO, or IO alone, depending on circumstances). The dose would depend on any overlap with previously irradiated fields, but I would avoid prolonged 6 week courses of standard fractionation. Instead, 30-36 Gy in 3 Gy fractions may be appropriate, respecting brachial plexus tolerance. Such supraclavicular recurrence is likely a harbinger of future systemic relapse. 

If this is truly the only site of disease, and sufficiently far away from prior radiation field with a decent disease-free interval, one could certainly consider an oligometastatic treatment paradigm.

I have treated situations like this with 25Gy to the PTV with simultaneously integrated boost to 35-45Gy (5 fractions) while respecting brachial plexus constraints (taking into consideration size, time from prior RT, overlap with prior fields, etc). 

Also, as previously posted, systemic therapy of some sort to follow.