Collagen type III α1 as a useful diagnostic immunohistochemical marker for fibroepithelial lesions of the breast.

Phyllodes tumors (PTs) of the breast constitute an uncommon group of fibroepithelial neoplasms that are classified into benign, borderline, and malignant categories based on a constellation of histologic characteristics including cytologic atypia, mitotic count, degree of stromal cellularity, stromal overgrowth, and microscopic margins. Accurately and reproducibly differentiating these tumors is a long-standing diagnostic challenge. In addition, the distinction between benign PT from cellular fibroadenoma (FA) is especially difficult because of overlapping microscopic features. We have previously shown differential expression of various collagens, including collagen type III α1 (Col3A) in breast carcinomas. In this study, we evaluated clinicopathological characteristics of 95 cases of fibroepithelial lesions including 56 PTs and 39 FAs (25 cellular FA, 14 typical FA) and correlated them with the immunohistochemical staining pattern for Col3A. We found that stromal Col3A expression was significantly increased in PTs when compared with FAs (P < .0001). Among the PT groups, there was significantly increased expression from benign tumors through borderline to malignant tumors. High Col3A expression was associated with PT type, irregular margin status, and high mitotic activity. A distinct periductal cuffing pattern of Col3A staining was unique to PTs and absent in FAs. These findings suggest that Col3A can be a potential adjunct marker for both differentiating FA from PT and assessing malignant potential in PTs.