Pilomatricoma mimicking small round cell tumor on fine needle aspiration cytology: a case report

BACKGROUND: Pilomatricoma, a benign skin adnexal tumor, frequently leads to false positive diagnosis cytologically. We report a rapidly growing nodular swelling misdiagnosed as round cell tumor cytologically and found to be pilomatricoma histopathologically. CASE: A 32-year-old man presented with a rapidly growing, mobile nodule on his left arm for 4 months with fixed, shiny overlying skin. Fine needle aspiration cytology (FNAC) sample was cellular, showing round to ovoid cells dispersed or in clusters with occasionally rosette-like appearance. Cells displayed round, granular nuclei, single to multiple small nucleoli, absent to scant cytoplasm, a moderate amount of granular cytoplasm and apoptotic and mitotic figures in places. Nuclear moldings were encountered occasionally. Cytologically the diagnosis of blue round cell tumor was made. Histopathologic examination showed islands of basaloid cells with scant cytoplasm and shadow cells and occasional giant cells. The diagnosis was pilomatricoma bistopathologically. Rapidly growing, early lesions of pilomatricoma are predominantly composed of basaloid cells and mostly devoid of other diagnostic clues, leading to a false impression of malignancy. To avoid misdiagnosis, all skin-based nodules should undergo extensive cytologic sampling from diferent sites. Pilomatricoma should be considered in diferential diagnosis when primitive-appearing cells are aspirated, especially in rapidly growing early lesions.