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Case Reports

Abstract

Pilomatrix carcinoma (PC) is a rare entity arising from hair follicle matrix with propensity for locally aggressive behavior and distant metastases. PCs may evolve from preexisting pilomatricoma or arise de novo, both associated with beta-catenin mutations. PC usually occurs in head and neck, but has been reported in the extremities, trunk, genitalia and spine. Rapid growth of lesions and histologic features including nuclear atypia, atypical mitoses, necrosis, infiltrating growth, and lymphovascular invasion support malignancy. PC of the trunk has been reported in 11 of 76 cases over the last 12 years, ranging in size from 0.8 to 28 cm. We report an additional case of PC arising from a preexisting pilomatricoma in a 56-year-old man who presented with a large mass of his left mid-back, stable for about 10 years. Recent growth causing discomfort was reported. Core biopsy showed pale eosinophilic cells lacking nuclei and giant cells suggesting pilomatricoma. Wide local excision revealed a 16.5 cm, heavily calcified, multinodular mass. Histologic examination showed multinodular proliferation of basaloid cells with pleomorphic nuclei and prominent nucleoli, abruptly transitioning to pink ghost cells. Areas of smaller basaloid cells lacking similar atypia were noted. Solid nests of basaloid cells infiltrated surrounding stroma and lymphovascular channels.  Beta-catenin immunostain showed diffuse nuclear positivity. The overall features were diagnostic of PC arising from a longstanding pilomatricoma. Further re-excision of the site with negative margins was completed. To date, no local recurrence or metastases have been reported.