PROLIFERATING PILOMATRICOMA - A HISTOPATHOLOGIC SIMULATOR OF MATRIC CARCINOMA

We report on 9 patients with pilomatricomas that showed unusual histop athologic features. Our patients were mainly elderly individuals (age range 42 to 88 years; mean age 70.1 years) who presented solitary cuta neous nodules situated on the head and neck (7 neoplasms), upper arm ( 1 neoplasm), and back (1 neoplasm).All the lesions were treated by sim ple excision. Follow-up data available in 7 of the 9 patients (mean fo llow-up, 17 months) revealed local recurrences in 1 patient whose lesi on recurred 3 times. No lymph node involvement or distant metastases w ere recorded in any of our cases. Histopathologically, most neoplasms were characterized by a relatively large lesion in the dermis that in some cases showed extension to the subcutis. Each lesion was predomina ntly composed of a lobular proliferation of basaloid cells in associat ion with adjacent focal areas containing eosinophilic, cornified mater ial with shadow cells. In some cases, relatively large areas of shadow cells were present, whereas, in others only small foci of shadows cel ls were observed. Cytomorphologically, the basaloid cells showed featu res of matrical and supramatrical cells of a normal hair follicle and exhibited variable nuclear atypia and mitotic figures. The overall arc hitectural pattern of the neoplasms was different from that of large f ully developed stereotypical pilomatricomas that maintain a cystic cha racter with basaloid cells predominantly aligned at the periphery. Bas ed on the histopathologic findings, namely the presence of a large, lo bular proliferation of basaloid cells in association with small to lar ge foci of shadow cells, we interpreted these neoplasms to be a distin ctive proliferative variant of pilomatricoma and propose the designati on ''proliferating pilomatricoma.'' Proliferating pilomatricomas shoul d be differentiated from the recently described matricoma, basal-cell carcinoma with matrical differentiation, and matrical carcinoma (pilom atrical carcinoma).