Subcutaneous trichoblastoma.

We have recently observed three examples of solitary trichoblastomas (TB) with unusual histopathologic features characterized mainly by numerous aggregations of basaloid cells limited to the subcutis. The three trichoblastomas with unusual features were identified from a large series of 38 solitary TB cases collected over a period of 20 years. Clinically, all three neoplasms presented in men (49, 52, and 62 years old) as solitary, 1- to 1.5-cm skin-colored nodules situated on the scalp, face, and lower arm, respectively. Histopathologically, they showed numerous, smooth-bordered aggregations of basaloid cells limited to the subcutis and surrounded by a sclerotic and partly hyalinized stroma. Multiple sections revealed no connections of basaloid aggregations to the overlying epidermis or pre-existing follicular structures. All three cases displayed rather unusual morphologic growth patterns, including areas of variously sized, nodular aggregations of basaloid cells and extensive foci of elongated, thin columns and branching cords of basaloid cells. A striking feature in the stromal component in two cases was the presence of large, prominent areas of hyalinization and sclerosis. Characteristically, all three neoplasms showed numerous foci with rudimentary follicular germs and papillae. Cytomorphologically, the basaloid cells exhibited dark staining nuclei with large prominent nucleoli and scanty, pale or eosinophilic cytoplasm. Variable number of mitotic figures (2-4 mitoses per high-power field) and single necrotic cells were noted. In one case, small, foci of necrosis en masse were observed. Follow-up data after total excision in all three cases (80, 69, and 6 months) revealed no local recurrences. In light of our observations, we suggest that subcutaneous TB represents a rare variant of solitary TB. Besides the exclusive subcutaneous location, this neoplasm also displays a constellation of particular histopathologic features, namely, rather complex epithelial growth patterns and stroma with prominent foci of sclerosis and hyalinization.