Fine needle aspiration diagnosis of pilomatrixoma. A monomorphic population of basaloid cells with squamous differentiation not to be mistaken for carcinoma.

In a case of pilomatrixoma, an initial fine needle aspiration (FNA) biopsy of a skin lesion yielded only a monomorphic population of mitotically active basaloid cells with squamous differentiation, without anucleated ghost cells, chronic inflammatory cells or foreign-body giant cells. These findings suggested a metastatic squamous-cell carcinoma or a malignant tumor of adnexal skin. The correct diagnosis was made by repeat FNA, which yielded the basaloid cells and ghost cells characteristics of pilomatrixoma; the FNA diagnosis was confirmed by subsequent excisional biopsy. This case demonstrates that pilomatrixoma should be considered in the differential diagnosis of skin aspirates that contain a dominant population of basaloid cells. The differentiation of pilomatrixoma from malignant lesions of the head and neck region is discussed.