Abstract

Background: Spitz tumors are known to harbor fusions in receptor tyrosine kinases (TRKs): ROS1, NTRK1, ALK, and RET, while Spitzoid tumors are known to harbor gene alterations in serine/threonine kinases, such as BRAF and MAP3K8. These fusions are associated with specific morphologic features. The lineage of some Spitz and Spitzoid lesion can be determined using fusion-related morphologic clues, in conjunction with molecular confirmation. Case: A healthy 22-year-old male with a family history of melanoma presented with a 3mm red papule on his right posterior helix, present for several months before bleeding. A shave biopsy demonstrated a dermal melanocytic population of relatively small nests of Spitzoid melanocytes with occasional rosette-like structures. Several mitotic figures were identified in the superficial and deep dermis with an absence of maturation. Immunohistochemistry showed MART-1, PRAME, and pan-TRK positivity. p16 was lost. FISH analysis demonstrated loss of PTEN with NTRK1 fusion, supporting a diagnosis of an NTRK1 Spitz melanoma with a Breslow depth of at least 2.1mm. The lesion was removed with wide local excision and sentinel lymph node biopsy was negative. Discussion: NTRK1 fusion Spitz lesions have a distinct morphology of smaller nests, more spindled cells, and rosette-like structures. In our case, the clinical appearance overall favored a benign angioma or pyogenic granuloma. Thus, a high index of suspicion by the clinician, in partnership with a dermatopathologist looking for histologic fusion clues and obtaining the proper molecular testing, was helpful in making this diagnosis.

Financial Disclosure:
No current or relevant financial relationships exist.