Pilomatrixoma: a diagnostic pitfall in fine-needle aspiration biopsies. A review from a small county hospital

A 10-year-old Hispanic girl presented with a 3-cm mass over her right parotid region. A fine-needle aspiration (FNA) was performed and the preliminary and final diagnoses were "suspicious for carcinoma, possible mucoepidermoid carcinoma." A subsequent computed tomography demonstrated an extra-parotid lesion attached to the skin. The FNA smears were reviewed and the diagnosis of a pilomatrixoma (PMT) was reached. The lesion was excised and the diagnosis of benign PMT confirmed. Pilomatrixomas are uncommon skin adnexal tumors most commonly found in the head and neck area of young adults and children. In 10 years there were 16 PMTs among 60,280 surgical pathology specimens in the pathology files of our 300-bed general county hospital. Pilomatrixoma is recognized as a diagnostic pitfall not only clinically but also on FNA cytology. On the physical examination, the bluish skin discoloration over the lesion so typical of PMT was overlooked. On FNA, the basaloid cells of PMT were mistaken for intermediate cells of mucoepidermoid carcinoma. The presence of anucleated squames was not properly recognized. These are usually absent in mucoepidermoid carcinoma and represent a hallmark of PMT. This case illustrates well the risk of misdiagnosing a pre-auricular PMT as a malignant tumor of the parotid gland.