Question:
I recent came across the result of a subcutaneous biopsy I sent. The
pathologist told me that it was either a poorly differentiated metatstaic
carcinoma or a melanoma, but no way to tell for sure. I couldn't find any
thing relevant in my med school path books. Anyone familiar with the
subject want to comment on how melanoma is diagnosed histologically, or
any current special stains to make the diagnosis of melanoma in difficult
situations?
Answer:
This used to be true in the past, but with modern immunohistochemical
stains, one can distinguish melanoma from carcinoma almost every time. If
the pathologist did not do an immunohistochemical stain battery consisting
of S-100, HMB-45, and cytokeratins at the very least, then he or she did
not do a sufficiently thorough work-up to make the diagnosis.
If those and other stains were all negative, then you may have to go look
for a primary. To look for a primary melanoma, look over every square
centimeter of skin and mucous membrane, including between the toes. In
terms of metastatic carcinoma, the typical offenders are renal cell
carcinoma, lung cancer, and breast cancer. With modern imaging techniques,
primaries in those organs should be easy to rule out.
Of course, a lot of this depends on what the slides look like. If
everything is negative, you may also have to consider that the skin lesion
is neither carcinoma nor melanoma. Atypical fibroxanthomas and intradermal
Spitz nevi can look like carcinomas or melanomas, as can large cell
lymphomas. Immunohistochemical stains can help you out with those
possibilities, too.
