Abstract
Pilomatrixoma is a benign skin adnexal tumor arising from the hair follicle matrix cells. It is relatively uncommon, mainly affecting children and adolescents. Clinically, pilomatrixoma presents as a solitary, slow-growing, firm, mobile mass, ranging in size from 0.5 to 3 cm. Morphologically, these tumors can pose a diagnostic challenge for pathologists, especially with limited or fine-needle aspiration (FNA) specimens. We report a case of a 67-year-old male who presented with a nodule in the left postauricular region. An ultrasound revealed a 0.5 cm hypoechoic, subcutaneous mass. Due to the history of prior prostate cancer and radiation, an ultrasound-guided FNA was performed. Cytologic evaluation revealed clusters of small cells with high nuclear to cytoplasmic ratio, scant cytoplasm, granular chromatin, prominent nucleoli, and scattered mitoses. These cells stained positive for p40 and p16 immunohistochemistry. The diagnosis of squamous cell carcinoma was rendered, and a wide local excision of the tumor was performed. The histopathologic findings were consistent with a pilomatrixoma, without any evidence of atypia or malignancy. The cytomorphologic pitfalls of pilomatrixoma in FNAs have been previously described. However, to our knowledge, this is the first case with reported immunoreactivity of tumor for p40 and p16, which may lead to misinterpretation as HPV-mediated or basaloid squamous cell carcinoma in head and neck lesions. Larger size studies are recommended to evaluate the immunoreactivity of p16 in these tumors and its potential implications in small biopsy and FNA specimens.
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