Trichoepithelioma – pathology

Classified as one of the tumours of the follicular epithelium, trichoepithelioma is thought to arise from the hair germ.

Histology of trichoepithelioma

Scanning power view reveals a tumour comprised of multiple nodules situated within the dermis (Figure 1). Small horn cysts, abortive hair follicles and calcification are frequently seen (Figure 2). The basaloid epithelial tumour demonstrates follicular differentiation in the form of papillary mesenchymal bodies. The basaloid epithelium forms small hair papillae and is associated with a focal increased stromal density most obviously seen as increased numbers of fibroblast nuclei (Figure 3).

Trichoepithelioma – pathology

Histological variants of trichoepithelioma

Desmoplastic trichoepithelioma: Scanning power view demonstrates an infiltrating tumour which appears to be arising from the epidermis (Figures 4 and 5). The tumour is comprised of small islands of basaloid epithelium superficially which form cords and strands towards the base and periphery (Figures 6 and 7). Horn cysts, frequently many, calcification and foreign body granulomas can be seen (Figures 6 and 7). Occasionally papillary mesenchymal bodies may be seen (Figure 8). Duct like structures may be evident (Figure 7) and can create difficultly in discriminating this lesion from syringoma or sometimes microcystic adnexal carcinoma.

Desmoplastic trichoepithelioma – pathology

Differential diagnosis of trichoepithelioma

Basal cell carcinoma: Trichoepithelioma may demonstrate prominent basal layer palisading making differentiation difficult in many cases. Trichoepithelioma is differentiated by the presence of follicular differentiation, horn cysts, papillary mesenchymal bodies and a loose stromal reaction. Immunohistiochemistry may be helpful on occasion though the H&E findings are typically more reliable. Ber-EP4 stains basal cell carcinoma diffusely while this may be negative in trichoepithelioma (Figure 9). BCL-2 typically stains basal cell carcinoma diffusely while frequently staining the periphery of the epithelium in trichoepithelioma (Figure 10). CD34 may stain the stroma which tightly abuts the epithelium of trichoepitheliomas, while negative in basal cell carcinomas. Due to significant variability in the staining here cautious interpretation is recommended.

Trichoepithelioma – special stains

Syringoma: Desmoplastic trichoepithelioma can be differentiated by the presence of horn cysts, follicular differentiation, calcification and the presence of long epithelial strands, all of which are uncommonly seen in syringoma.

Draft 14 February 2010