A Rare Case of Dermal Myofibroblastoma Emphasizing the Diagnostic Utility of Immunohistochemical Loss of Rb Expression

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 stains revealed CD34, desmin, and smooth muscle actin expression in the lesional cells. SOX10, S100, and Melan-A were negative. Retinoblastoma 1 (Rb) staining demonstrated loss of nuclear expression in the spindle-shaped cells. The findings support a diagnosis of 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 RB1 resides, with resultant loss of Rb expression. Although the differential diagnosis was vast, this case highlights the utility of Rb immunohistochemistry in the diagnosis of cutaneous myofibroblastoma.