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Case Reports

Abstract

Leiomyosarcoma (LMS) is a soft tissue sarcoma with smooth muscle differentiation. Histopathology and immunohistochemistry findings are key to differentiating it from similar lesions like desmoplastic and spindle cell melanoma. Herein, we present a case of cutaneous LMS with a diagnostically challenging immunophenotypic profile. An 81-year-old patient presented to clinic with a non-ulcerated, flesh-colored nodule on the right upper extremity. A shave biopsy was performed which showed a dermal proliferation of mitotically active spindle cells (~5 mitoses/10 HPF) containing blunt-ended nuclei and abundant eosinophilic cytoplasm without evident melanin pigmentation, arranged in vague fascicles. The tumor expressed S100 and smooth muscle actin without significant expression of pancytokeratin, Mart-1, PRAME, P40, or CD10. Given the immunohistochemical profile, a differential diagnosis was posited which included a spindle cell amelanotic malignant melanoma. Targeted cytogenetic for RREB1, MYC, and CCND1 amplification and deletions involving CDKN2A revealed amplifications in CMYC and CCND1. Upon review at our institution, additional stains were performed, which yielded positivity for desmin and caldesmon and negativity for SOX10. Given the results of the expanded immunohistochemical panel, the diagnosis was revised to cutaneous leiomyosarcoma. This case report highlights the diagnostic challenge posed by overlapping features among spindle cell neoplasms and emphasizes the importance of broad immunohistochemical panels. While rare, S100 positivity can occur in LMS and should be interpreted in conjunction with the histologic appearance when included in the differential. Recognition of the histologic and immunophenotypic variability of leiomyosarcomas is critical for appropriate management and surveillance due to its risk of recurrence and metastasis.