Spindle cell lesions of the breast—the pathologic differential diagnosis

Spindle cell lesions of the breast represent an interesting diagnostic problem, as the differential diagnoses are wide. Diagnosing this is particularly problematic but important when encountered in a needle core biopsy, as treatments of different entities are different. In the histologic assessment of spindle cell lesions, the simplified approach is to evaluate the spindle cells and the accompanying epithelial cells. In the biphasic lesions with predominance of spindle cells with benign epithelial component, fibroepithelial lesions including fibroadenomas and phyllodes tumors are the most common, followed by pseudoangiomatous stromal hyperplasia, hamartoma and adenomyoepithelioma. For biphasic lesions with predominance of spindle cells with malignant epithelial component, the biphasic metaplastic carcinoma is likely. For monophasic lesions with pure pleomorphic spindle cell only, the monophasic metaplastic carcinoma is more common than the rare primary sarcomas like malignant fibrous histiocytoma, angiosarcoma, and other high grade sarcomas. In monophasic lesions with pure bland spindle cells only, the possible lesions include fibromatosis, fibromatosis like metaplastic carcinoma and other unusual conditions like dermatofibrosarcoma protuberance. By careful searching for the accompanying epithelial element, and with the aid of appropriate clinical input and judicious use of immunohistochemistry, many of these lesions can be confidently diagnosed in the needle core biopsy, thus facilitating appropriate treatments.