For patients with advanced NSCLC who have radiographic disease progression on their follow-up scan after starting a PD-1 inhibitor, what characteristics (either clinical or radiographic) do you use to determine whether to continue with therapy or switch to something else?

I agree with @Dr. First Last - there are two situations in which I would consider continuing therapy for another 4 or 6 weeks, 1) if as he has noted, there is some inconsistency (ie some areas worse and some better) AND the patient is feeling well, 2) if there is modest radiographic progression BUT the patient feels well. In the third situation when the patient is symptomatically worse and the radiographs are worse - this is true progression, and I look to next options.

True pseudoprogression is very rare in non-small cell lung cancer. There are instances in which the scan results are inconsistent (e.g. some areas improved and some worsened), and I think that in such cases, it is reasonable to individualize treatment decisions. If scans at or after 8 weeks are clearly worse and the patient's symptoms are worse, I do not see any data indicating that such a patient should continue a PD-1 inhibitor.