Would you consider proceeding with a sentinel lymph node biopsy after wide excision revealed 1.2 mm residual non-ulcerated T2a melanoma on the upper back?

Any melanoma with Breslow’s depth of more than 0.8 mm (more than T1a) needs a sentinel lymph node biopsy for complete staging, due to higher chance of lymph node metastasis. Ideally, it should be done at the time of wide local excision, as doing the sentinel lymph node biopsy afterwards may be more difficult due to scarring. If not able to proceed with sentinel lymph node biopsy, I would recommend getting a PET staging scan to see if there is any FDG-avid lymph node or other distant lesions.

Also, now with neoadjuvant looking super effective, we might need to change our work-up and proceed with a PET-CT before any surgery.

In any case, I agree that an SLNB is needed for melanoma > 0.8 mm (some say 0.75 mm) Breslow.

Yes. At this depth, we recommend conducting SLNB.