Immunohistochemical Study of CD34 Expression in Fibroadenomas, Phyllodes Tumors and Spindle Cell Carcinoma of the Breast

Aims: Strong expression of CD 34 has been described in many tumors. It has been proposed that these lesions arise from long-lived mesenchymal cells. We tested the hypothesis that spindle cell lesions of the breast arise from similar mesenchymal cells in mammary stroma, and to determine the potential diagnostic value of CD 34 immunostaining in these lesions. Methods and results: Sections of fibroadenomas (20), phyllodes tumors (7), spindle cell carcinoma (10) and normal breast tissues adjacent to these lesions (10) were stained immunohistochemically for CD34 in formalin fixed, paraffin embedded tissues. All the mammary stroma, particularly around lobules, stained for CD 34 . All cases of fibroadenoma showed strong CD 34 immunostaining (median 100%). The median percentage of CD 34 immunostaining in cases of phyllodes tumors were 65% (p=0.0002). In benign phyllodes tumors there was diffuse staining in the stroma and in half of the cases there was decreased staining adjacent to epithelium, However in the malignant phyllodes tumors the staining was patchy. All cases of spindle cell carcinoma were CD 34 negative. Conclusions: the expression of CD 34 in fibroadenomas and phyllodes tumors suggests that these lesions may arise from long-lived CD34 positive mesenchymal cells in the breast stroma, and the absence of CD 34 staining in spindle cell carcinoma is of potential diagnostic value in the distinction from malignant phyllodes tumors in difficult cases.