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Case Reports

Abstract

Trichogerminoma is a rare, benign cutaneous adnexal neoplasm with follicular differentiation, generally regarded as a subtype or variant of trichoblastoma. Clinically, the lesion presents as a slow growing nodular lesion. Histopathologic features of trichogerminoma include lobules of basaloid cells with peripheral palisading, sharp circumcision, no connection to the epidermis, minimal intervening stroma, and the formation of peculiar pale, rounded cell nests (“cell balls”) within tumor lobules. Immunohistochemistry typically shows expression of PHLDA1 and CK 5/6- with the latter diminished in the cell balls. Recently reported molecular characterization of trichogerminoma showed high expression of the somatostatin receptor 2A (SSTR2A); this was also demonstrated by immunohistochemistry, with strong expression seen in more than 50% of tumor cells. Histologic mimics generally showed less diffuse positivity, with only rare cases of basal cell carcinoma and trichoblastoma reportedly showing strong and diffuse expression.

We present a case of trichogerminoma in a 62-year-old male. Exam revealed a 2.2 cm soft tissue mass of the right forearm. Excision showed a large, lobular, and generally well-circumscribed proliferation of basaloid cells with numerous germinative cell balls and scattered small keratin cysts. Although mitotic figures were evident, infiltrative growth and necrosis were absent. Immunohistochemistry showed diffuse CK5/6 positivity, with reduced expression in the germinative cell balls. In addition, PHLDA1 stain was positive, P53 demonstrated wild-type staining pattern, and CK20 highlighted scattered intralesional Merkel cells. SSTR2A showed diffuse and strong membranous staining throughout the tumor.

Our case supports the utility of SSTR2A to aid in the diagnosis of trichogerminoma.