Clinical features

•Should be distinguished from trichoepithelioma since the latter may be multiple and associated with Brooke-Spiegler syndrome & multiple familial trichoepitheliomas

•Exceptional association with Birt-Hogg-Dubé syndrome

•M=F

•5^th-7^th decades

•Presents as a solitary, slowly growing nodule particularly affecting the scalp, head & neck, trunk, extremities & genitalia in decreasing order of frequency

Plaque-like variant (F>>M)

Histological features

•Biphasic tumors comprising germinative epithelial and mesenchymal components

•Sharply circumscribed nodule, sometimes with a pseudocapsule

•Uniform basaloid cells with peripheral palisading and often marked mitotic activity

•No pleomorphism

•Absence of  retraction artifact and mucin deposition

•Variable keratin cysts

•Clear cell change

•Sebaceous & glandular differentiation

•Cribriform and palisaded (rippled) pattern

•Pigmented variant

•Stromal component associated with indentation of epithelial component- follicular mesenchymal bodies (uncommon, more often seen in trichoepithelioma)

•Stromal amyloid often present

•Merkel cells are commonly present

•Trichogerminoma (see images)

•Most common tumor arising in nevus

•CK20, PHLDA1 +ve Merkel cells+

•Androgen receptor –ve

•CD10+ (stromal)

•CD34 + (stromal)Bcl2+ve (peripheral)