Nodular pseudoangiomatous stromal hyperplasia of mammary stroma in a case showing rapid tumor growth.

Pseudoangiomatous stromal hyperplasia (PASH) is an uncommon benign breast disease that presents as a localized breast mass. Breast tissue affected by PASH is characterized by a dense, collagenous proliferation of mammary stroma, forming interanastomosing capillary-like spaces. The importance of this benign lesion lies in distinguishing it from low grade angiosarcoma. We report a case of a 38-year-old woman who presented with a rapidly growing breast tumor. She visited our hospital with a complaint of a painless right breast mass. Physical examination revealed a 3.6 x 2.2 cm, oval, elastic-firm, well-defined and easily movable mass. Mammograms revealed no discrete mass or calcifications. Sonographic examination revealed a 3.5 x 2.5 x 2.2 cm, oval, well-defined and homogenous hypoechoic mass without a cyst. A fine-needle aspiration sample of the breast mass showed some clusters of epithelial cells with small papillary structures and many scattered stromal cells with naked nuclei. Based on these findings, a provisional diagnosis of fibroadenoma was made and the patient was followed up. One year after the first visit, the mass enlarged rapidly and a follow-up mammogram revealed an 8.2 x 5.5 cm circumscribed mass without calcifications. Given the history of rapid growth of the mass, tumor excision was performed. The excised tumor was well demarcated and had a smooth external surface. Histologic examination revealed normal breast ducts and lobules, and specific proliferative epithelial changes were not seen. The lobular and duct structure of the breast parenchyma were separated by an increased amount of stroma. The fibrous stroma contained numerous anastomosing slit-like spaces. Isolated spindle cells appeared intermittently at the margins of the spaces resembled endothelial cells. Immunohistochemical staining showed that endothelial cells lining true blood vessels were positive for Factor VIII-related antigen, but the spindle cells were negative for Factor VIII. Pseudoangiomatous stromal hyperplasia was diagnosed.