Though trichogenic tumors were classified as trichoblastoma, trichoblastic fibroma, trichogenic trichoblastoma and trichogenic myxoma by Headington(1970), their true classification depends upon the epithelial and mesodermal component as well as evidence of their induction. Because of the rarity of hair germ cell tumors their classification is still controversial. In this report, we describe a case of trichogerminoma which is not included in the above classification. The trichogerminoma was first described by Sau et al. in 1992 and characterized by its morphologic pattern of germinal centers and lymphoid follicle-like structures in the nests of trichoblasts. Herein we reporte a tumor which arose on the skin on the back of a 51-year-old man and presented as a sharply circumscribed mass(4.5x2.0x1.5 cm) involving both the dermal and subcutaneous tissues without any epidermal connection. The tumor had many germinal center-like structures in the basaloid trichoblasts. Lobular cell nests were separated by variable amounts of stroma, but no horn cyst were noted. The germinal center-like cells showed early differentiation of hairs, resembling early hair bulbs. Trichogerminoma is considered to be a type of tumor located between trichoblastoma and trichoblastic fibroma.
Trichoblastic fibroma is a benign trichogenic tumor that has both epithelial and mesenchymal components and exhibits partial follicular induction. We studied 4 cases of trichoblastic fibroma and reviewed their clinical and histologic features. Two tumors were present in the face. The remaining two were in the vulva and perianal area, respectively. The age of the patients ranged from 53 to 68 years, with an average age of 62. All were female. Histologically, the lesions showed a well circumscribed mass, located at dermo-subcutaneous junction in three patients and subcutaneous in one. They demonstrated mesenchymal induction evidenced by hair germ-like structure and perifollicular sheath. There was no connection between the tumor and epidermis.
Differentiation toward hair structure led to the formation of the infundibulum through inner root sheath. Trichoblastic fibroma may be confused clinically and/or histologically with basal cell carcinoma. Identification of the mixed epithelial and mesenchymal components, and the absence of epidermal connection and cleft within the stroma are important in differentiating this benign neoplasm from basal cell carcinoma.