A Case of Trichogerminoma With Pilomatrical Differentiation and a Unique Immunohistochemical Profile.

Trichogerminomas are rare adnexal neoplasms first described by Sau et al in 1992. Including the initial report, 20 cases have been reported, all with similar histological features, namely tumor nodules composed of basaloid cells that form densely packed, round nests or "cell balls" and which demonstrate variable degrees of pilosebaceous differentiation. In this study, the authors report a case of a trichogerminoma with pilomatrical differentiation and a unique immunohistochemical profile. The patient is a 71-year-old man with a well-delineated nodule on the top of the scalp. Histologically, the lesion measured 16 mm in greatest dimension and was composed of nodules of basaloid cells with central, compact, slightly eosinophilic cells nests. Immunohistochemically, the tumor nodules diffusely expressed cytokeratins 34βE12, AE1/3, and CK5/6. Diffuse expression of β-catenin and nuclear expression of p63 were also evident. The peripheral basaloid cells, but not the cell balls, expressed CD10, Ber-EP4, BCL-2, and CK7, the latter a previously unreported finding. The histological findings and immunohistochemical profile are compatible with a diagnosis of a trichogerminoma.