Polypoid Atypical Spitz Tumor With a Fibrosclerotic Stroma, CLIP2-BRAF Fusion, and Homozygous Loss of 9p21.

We report a case of a polypoid atypical Spitz tumor with a prominent fibrosclerotic stromal component, harboring a CLIP2-BRAF fusion, which has hitherto been not reported in melanocytic lesions. The neoplasm occurred in a 78-year-old male patient and appeared microscopically as a predominantly dermal, barely symmetrical, polypoid lesion composed mainly of epithelioid cells showing moderate degree of nuclear pleomorphism with ample amphophilic cytoplasm arranged in nests, fascicles, or single units. The mitotic rate was 2/mm, and the mitoses were confined to the upper portion of the lesion. The Breslow thickness was 2.3 mm. The stroma contained conspicuous plumped fibroblasts and thickened collagen bundles associated with dilated medium-sized vessels. Focally, sclerotic areas were found. A moderately dense, lymphocyte-predominant inflammatory infiltrate scattered through the whole lesion was seen. Despite strong nuclear and cytoplasmic positivity of p16, FISH revealed homozygous loss in locus 9p21. A CLIP2-BRAF fusion was found by next-generation sequencing. No other genetic alterations including a TERT-promoter mutation was found. The patient is disease-free without recurrence or evidence of metastatic disease after 5 years and 2 months of follow-up.