A case of malignant trichilemmoma occured on the lesion of poroepithelioma folliculare was reported. A 81-year-old man was admitted to our clinic with a red nodule measuring 12×10mm in size on the right side of lumbar region in July 1991. This oval shaped nodule with the erosive surface was located in the central area of brown colored pigmentation mesuring 21×26mm in size, which had been present for about ten years. The clinical impression of this nodule was that of squamous cell carcinoma. In July 1991, this lesion was completely excised. No recurrence or metastasis were noted after a year of follow up. Histologically, the nodule on the pigmentation was the tumor to consist of irregular masses of epidermal cells that proliferated downward into the dermis. This tumor had some clear cells and bizzare keratinocytes with trichilemmal keratinization in the surface of the nodule, mitotic figures, many dyskeratotic cells and some squamous eddies. Partially, tumor masses were poorly demarcated from the surrounding stroma, because some atypical cells had no recognizable intercellular bridges and invaded into the dermis. The dermis around the tumor showed a marked inflammatory infiltration composed of lymphoid cells and plasma cells. This feature was confirmed to be a typical malignant trichilemmoma. In the other hand, the pigmented lesion had many rounded nests of epithelial cells, well defined from the surrounding epidermal keratinocytes. These nests were composed of chiefly small cells with round basophilic nuclei, which sometimes showed mild cytological atypicality and mitotic figures. In the nest, there were some pigment blockade melanocytes, clear cells and squamous eddies but no tubular differentiation like sweat gland tumors. This lesion were considered as a case of so-cared intraepthelial epithelioma and these findings were consistent with those of poroepithelioma folliculare.
Poroepithelioma folliculare is one of precancerous tumors first recognized by Morioka in 1976. This tumor has been interpreted as a type of follicular intraepidermal epithelioma, which is divided into five histopathological types. Five histological patterns were a trichilemmoma-like, inverted follicular keratosis-like, tumor of follicular infundibulum-like type and the clonal type of seborrhoeic keratosis in addition
