Solitary fibrous tumor of the mammary gland: a potential pitfall in breast pathology.

We report three cases of solitary fibrous tumor of the breast. The patients were adult to elderly women and complained of a slowly but relentless growing lump. The tumors were fairly circumscribed and cured by means of lumpectomy or, in one case, simple mastectomy. Histologically, they featured the customary "patternless pattern" of short spindle cells haphazardly arranged in fascicles within a collagenized or myxoid ground substance. In two cases, a prominent hemangiopericytic arrangement of tumor cells around a rich vascular framework could be noticed. Cellular areas were often present and showed nuclear overlapping, clumping of chromatin, and a brisk mitotic activity. No atypical mitosis was recognized. Tumor cells were immunoreactive for CD34, bcl2, and vimentin only. On follow-up there was no evidence of either local recurrence or distant metastases. Solitary fibrous tumors of the breast may represent a significant diagnostic problem because of the close mimicry to numerous benign and malignant mammary lesions composed of spindle cells; diagnostic clues may be further obscured in cellular and actively proliferating tumors. A brief overview of mammary solitary fibrous tumor taxonomy along with the principal differential diagnoses within the breast is presented.