Proliferating pilar tumors: a clinicopathologic study of 76 cases with a proposal for definition of benign and malignant variants.

We studied proliferating pilar tumors (PPTs) to establish histologic criteria that could predict behavior. We reviewed all cases in our consultation files (1989-2000) and evaluated 76 cases with meaningful follow-up information. Histologic examination involved attention to tumor silhouette, degree of nuclear atypia, mitotic activity, necrosis, and perineurial or angiolymphatic invasion. Tumors were stratified as follows: group 1 PPTs, circumscribed silhouettes with "pushing " margins, modest nuclear atypia, and an absence of pathologic mitoses, necrosis, and invasion of nerves or vessels; group 2 PPTs, similar to group 1 but manifested irregular, locally invasive silhouettes with involvement of the deep dermis and subcutis; group 3, invasive growth patterns, marked nuclear atypia, pathologic mitotic forms, and geographic necrosis, with or without involvement of nerves or vascular structures. Recurrence occurred in none of 48 group 1 PPTs; 3 (15%) of 20 group 2 PPTs had local regrowth; 4 (50%) of 8 of group 3 PPTs recurred and/or metastasized to regional lymph nodes. The differences between groups 1 and 3 and between 2 and 3 were statistically significant (P = .0002, P < .05, respectively). It seems justifiable to regard group 1 PPTs as benign, group 2 as having potential for locally aggressive growth, and group 3 also as having metastatic potential. The latter 2 categories might be equated with low and high grades of malignancy among PPTs of the skin.