Atypical pseudoangiomatous stromal hyperplasia showing rapid growth of the breast: Report of a case

Pseudoangiomatous stromal hyperplasia (PASH) of the breast is a benign lesion manifesting as myofibroblastic proliferation and anastomosing slit‐like spaces. Atypical PASH is an extremely rare lesion characterized by cytological alteration of myofibroblast, presenting as myofibroblastic sarcoma arising from PASH. To our knowledge, only one other case has been reported since the first report of Rosen. We present a case of atypical PASH. A 39‐year‐old female presented with a round, elastic hard, painless mass in the left breast. Mammography and ultrasonography revealed no definitive sign of malignancy. Core needle biopsy report was suggestive of atypical PASH. Five months later, the mass had grown rapidly with pain. Considering the clinicopathological features, excision was performed. Pathological examination revealed the spindle cells proliferation in collagenous stroma. The spindle cell involved the adipose tissue and lobules and lined peudoangiomatous spaces. These cells exhibited marked cytological atypia and mitotic activity. Immunohistochemically, these spindle cells were positive for SMA, CD10, and bcl‐2, and negative for podoplanin, p63, CD31, ERG and cytokeratins. The final diagnosis was atypical PASH. She is tumor‐free on 12 months follow‐up. The nature of atypical PASH remains unknown. Further studies are required for a clear definition, a new histological entity and diagnostic criteria.