Track: Case Reports

Abstract:

Spindle cell melanoma (SCM) is a rare subtype of melanoma, accounting for <5% of primary cutaneous invasive melanomas. Significant histopathologic and immunophenotypic overlap with malignant peripheral nerve sheath tumor (MPNST) can complicate diagnosis. A 75-year-old patient presented with a 3.7 × 2.0 cm scalp mass. Excision of the mass revealed an atypical spindle cell proliferation arranged in a fibrosarcomatous herringbone pattern with numerous mitoses. Peripheral areas contained loosely arranged short spindle cells with undulant nuclei, neurofibroma-like myxoid zones, and intersecting to haphazard fascicular arrangements. Immunohistochemistry showed diffuse S100 and SOX10 positivity, focal CD34, and positive SMA but negative staining for Melan-A, HMB45, MART-1, keratin, desmin, PRAME, and BRAF V600E. A comprehensive NGS panel identified clinically relevant NF1:R440* (38.3%) and NF1:Q1822* (45%) truncating variants. The R440* variant was previously reported in several tumor types, including malignant melanoma (unspecified/amelanotic/ desmoplastic/nodular) and neurofibroma. The NF1:Q1822* variant was previously reported in several tumor types, including malignant melanoma (unspecified/desmoplastic). NF1 is inactivated by mutation or deletion in various solid tumors and has been reported in 18.2% of MPNSTs, and loss-of-function mutations in the NF1 gene were observed in a small subset of melanomas (~13%). The overlapping morphology, immunoprofile, and molecular alterations posed a diagnostic challenge between SCM and MPNST. Integrating clinical presentation, histology, and immunohistochemistry supported a final diagnosis of SCM. This case underscores the importance of a multidisciplinary approach, including molecular testing, in distinguishing between histologically similar spindle cell neoplasms of the skin.

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