Pseudoangiomatous Stromal Hyperplasia: A Rare Case of Bilateral Axillary Ectopic Breast Tissue

Pseudoangiomatous stromal hyperplasia (PASH) is a benign but rare mesenchymal proliferation of the mammary stroma, characterized by pseudovascular spaces within a hyperplastic matrix. PASH can be classified as either non-tumor-forming or tumor-forming. The non-tumor-forming type is an infiltrative and clinically undetectable mass, incidentally found in approximately a quarter of breast biopsies. Tumor-forming PASH, however, is a rare neoplasm of the breast parenchyma, with its occurrence in extra-mammary sites, such as the axilla, reported in less than a dozen cases. We report the case of a 38-year-old female who presented with bilateral, progressively enlarging axillary growths. Following mammography, which classified the masses as breast imaging-reporting and data system (BI-RADS) 2, the patient underwent bilateral subcutaneous mastectomy for the excision of the axillary ectopic breast tissue. Final pathology revealed PASH with complete excision. This report highlights the importance of including PASH as a differential diagnosis for any mass presenting along the mammary ridge. Diagnosing and properly managing this rare neoplasm require a nuanced examination integrating clinical, radiological, and histopathological perspectives.