Trichoblastic Carcinoma Arising Within a Trichoepithelioma: A Biphasic Follicular Neoplasm With Compartment-Specific CK15, CK20, and Ki-67 Findings

Trichoblastic carcinoma (TBC) is a rare malignant follicular neoplasm whose distinction from trichoblastoma/trichoepithelioma (TE) and basal cell carcinoma may be challenging. We report an 83-year-old woman with a nodule on the left superior parietal scalp suspected clinically to be a pilar cyst. Histopathologic analysis showed a large basaloid epithelial neoplasm in the dermis and subcutis composed of variably sized nests and clusters of basaloid cells with peripheral palisading and a hypercellular fibrocytic stroma. A biphasic pattern was evident, with one component composed of small, cytologically bland cells typical of TE and a second component composed of larger cells with atypical nuclei and nuclear crowding. CK15 staining diffusely labeled the benign component and showed minimal positivity in the atypical component. CK20-positive Merkel cells colonized tumor aggregations in the benign component but not in the atypical component. The Ki-67 proliferation index was low in the bland component and elevated in the atypical component. Taken together, the findings support TBC arising within a TE. This report highlights evolution from a TE to TBC and illustrates the value of immunohistochemistry in confirming a diagnosis of TBC.