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Case Reports

Abstract

Desmoplastic melanoma (DM) is an uncommon variant of melanoma characterized by spindle cell morphology, prominent stromal collagen, and frequent diagnostic overlap with other spindle cell neoplasms. We present a case of a 77-year-old woman with a right medial scalp lesion, excised after a preoperative diagnosis of melanoma in situ and neuroma. Histologic examination revealed a large biphasic intradermal spindle cell neoplasm. One component consisted of a hypocellular spindle cell proliferation in a fibrillary collagen background with intercalation among mature adipocytes. The second component displayed a hypercellular spindle cells in fascicles and loose whorls within a myxoid stroma. Cytologic atypia was focal, with enlarged ovoid nuclei, vesicular chromatin, and nuclear pleomorphism. A single mitotic figure was identified in the deep dermis. Nodular lymphoid aggregates were present at the periphery. The tumor was narrowly excised at the deep margin (<1 cm) with no ulceration; Breslow thickness was 9 mm. Immunohistochemistry showed diffuse SOX10 and S-100 positivity, with negative staining for HMB-45, MART-1, PRAME, and CD34 (except in stromal elements). EMA highlighted perineurium of small nerves. Next generation sequencing revealed pathogenic variants in BRCA1, TP53, TERTpromoter, and NF1. The combination of biphasic histomorphology, immunoprofile, and molecular findings supported a diagnosis of desmoplastic melanoma, mixed phenotype. Desmoplastic melanoma with biphasic spindle cell morphology may mimic neural tumors. Any “neural” lesion in this setting should raise suspicion for desmoplastic melanoma. Integrating histology, immunohistochemistry, and molecular testing is essential for accurate diagnosis and optimal patient management.