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Case Reports

Abstract

A 74-year-old female presented with a progressively enlarging and intermittently tender hyperpigmented lesion beneath her left rib cage. Physical examination revealed a firm, telangiectatic linear plaque clinically suspected to represent a hypertrophic scar. A shave biopsy was performed. Histologic sections demonstrated a dermal proliferation of plump spindled cells, without cytologic atypia, organized in short fascicles interspersed with hyalinized collagen bundles. Immunohistochemical reactions revealed CD34, smooth muscle actin, and desmin expression in the lesional cells. SOX10, S100, and Melan-A were negative. Retinoblastoma 1 (Rb1) stain demonstrated loss of nuclear expression in the spindle shaped cells. The findings support a diagnosis of cutaneous myofibroblastoma, which rarely occurs in the skin. Myofibroblastoma is an uncommon benign mesenchymal neoplasm composed of fibroblasts and myofibroblasts with recurrent monoallelic loss of the 13q14 region, where RB resides, with resultant loss of Rb1 expression. Although the differential diagnosis included dermatofibroma with myofibroblastic differentiation, the presence of desmin expression and lack of nuclear Rb1 expression did not support this possibility. This case highlights the utility of Rb1 immunohistochemistry in the diagnosis of cutaneous myofibroblastoma.