Tricholemmal Carcinoma/Squamous Cell Carcinoma With Tricholemmal Differentiation

Abstract

Abstract: Tricholemmal carcinoma is a low-grade cutaneous malignancy, which typically arises on the sun-exposed, hair-bearing areas of elderly male patients. Histologically, tricholemmal carcinoma is composed of a lobular proliferation of pale to clear squamoid cells with peripheral palisading. The tumors are typically centered on a pilosebaceous unit and demonstrate follicular and epidermal connections. The presence of cytologic atypia, frequent mitoses, and dermal invasion distinguishes tricholemmal carcinoma from tricholemmomas. Other clear cell tumors that may be considered in the differential diagnosis include clear cell Bowen's disease and squamous cell carcinoma (SCC), clear cell basal cell carcinoma, sebaceous carcinoma, clear cell hidradenoma, hidradenocarcinoma, and metastatic renal cell carcinoma. Tricholemmal carcinoma is typically considered a cutaneous adnexal tumor with differentiation towards the external root sheath epithelium. Although some authors have questioned the validity of this concept and asserted that tricholemmal carcinoma is merely a clear cell SCC, there are distinct histologic differences between tricholemmal carcinoma and clear cell SCC. These include the former's lobular growth pattern, centering on pilosebaceous units, peripheral palisading, and presence of PAS-positive glycogen accumulations, all of which are lacking in cases of clear cell squamous cell carcinomas. However, it may be argued that tricholemmal carcinoma is simply a unique type of clear cell SCC, which either arises from or recapitulates outer root sheath epithelium, explaining its distinct features. In either case, the excellent clinical prognosis of tricholemmal carcinoma argues that this entity should continue to be separated diagnostically from other clear cell tumors of the skin.