Proliferating tricholemmal cystic carcinoma: a case containing differentiated and dedifferentiated parts

A 46‐year‐old man had a cystic mass on the right side of his scalp. Histological examination revealed a cystic dermal nodule composed of relatively circumscribed lobules of proliferating squamous epithelium, with atypical mitoses and dyskeratotic cells of invasive structure, which was diagnosed as proliferating tricholemmal cystic carcinoma (PTCC). Most of the cyst was composed of thick layers of highly proliferating, atypical, dedifferentiated epithelium (dedifferentiated part), which was attached to a highly proliferative but mildly differentiated part. A completely differentiated, tricholemmal cyst (TC)‐like part was also attached to the main cyst, which supports the idea of PTCC beginning in a pre‐existing TC. The dedifferentiated and mildly differentiated parts exhibited a high frequency of proliferating cell nuclear antigen (PCNA)‐positive cells both in the basal and the suprabasal layers, while PCNA staining was almost negative in the TC‐like part. Expression of cytokeratin (CK)10 and CK16 suggested disturbed epidermal differentiation in dedifferentiated part, while TC‐like part showed well‐differentiated trichilemmal epithelium and the mildly differentiated part was in the middle of these two.