The patient went on to have a resection. 

Histology

The sections show features of malignant epithelioid tumor. The tumor forms sheets and nests. The tumor cells have enlarged pleomorphic nuclei, prominent nucleoli and moderate amounts of eosinophilic cytoplasm. Binucleated cells are noted. Scattered tumor cells contain brown pigment in their cytoplasm. No evidence of lymphovascular invasion is seen. Most of the brown pigment is Masson Fontana positive, indicating melanin. Some Perl's stain positive hemosiderin deposits are also seen.

The tumor cells are focally S-100, HMB-45 and tyrosinase positive (melanocytic markers). CAM5.2, AE1/3, CK7, CK20, TTF-1, synaptophysin, melan-A, SOX-10, BRAF-V600E, GFAP, INI-1, p40, CD30, CD31, CD34, CD138, ERG, desmin and myogenin are negative.

Paraffin shavings have been sent for molecular testing. Result summary:

• BRAF Exon 15: No mutation detected. (predicts lack of response to BRAF tyrosine kinase inhibitors)
• NRAS Exon 3 mutation: DETECTED (may respond to MEK inhibitors)
• KIT: no mutation detected

FINAL DIAGNOSIS: metastatic melanoma.