Track: Case Reports

Abstract:

Myoepithelial carcinoma (MECA) is a rare malignant neoplasm derived from myoepithelial cells, typically in middle-aged adults. Histologically it is predominantly composed of myoepithelial cells, positive for EMA, S100, SOX10, GFAP, and patchy keratin positivity. Diagnosis is often challenging due to overlapping features with other tumors and variable immunoprofiles. Local recurrence and distant metastases are not uncommon in aggressive cases. We report a rare cutaneous pediatric case of possible MECA in a 10-year-old male with a 1.0 cm scalp lesion and lymphadenopathy. Histological analysis revealed a well-circumscribed nodule composed of severely atypical epithelioid cells with scattered mitoses and positive for cytokeratins (patchy), S100, SOX10, EMA, and BRAF V600E; while negative for MITF, Calponin, SALL4, BCOR, NUT and retained BRG1 and INI1/SMARCB1 expression by immunohistochemistry. Targeted gene sequencing showed a BRAF V600E mutation and a SMARCB1 splice site mutation. Copy number loss of SMARCB1 was not detected on FISH. No pathogenic fusions were identified. This case underscores the diagnostic ambiguity in extra-salivary sites of MECA, particularly in a rare cutaneous pediatric case. Furthermore, the atypical epithelioid cytology, polyphenotypic profile, and SMARCB1 mutation place the tumor in a diagnostic grey zone of S100/SOX10 positive neoplasms. The differential diagnosis includes epithelioid sarcoma (excluded by S100/SOX10 positivity), malignant rhabdoid tumor (SALL4 negative, atypical for age), and de-differentiated melanoma (negative MITF, BRAF-mutated). This case further highlights the importance of integrated histologic, immunohistochemical, and molecular assessment in a challenging case to achieve a definitive diagnosis and to inform the therapy and prognosis.

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Ashley Illingsworth