PSEUDOANGIOMATOUS STROMAL HYPERPLASIA (PASH) - A MAMMARY STROMAL TUMOR WITH MYOFIBROBLASTIC DIFFERENTIATION

Pseudoangiomatous stromal hyperplasia (PASH) is frequently a microscop ic incidental finding in breast biopsies performed for benign or malig nant disease. However, it may also produce a mass lesion. We reviewed PASH seen first as a tumor in 40 women aged 14 to 67 years (mean, 37 y ears). All but one lesion were clinically palpable. The exceptional tu mor was found by mammography. The mass, typically unilateral, was usua lly diagnosed clinically as a fibroadenoma. Most specimens contained a well-circumscribed tumor with a firm white-gray cut surface. In six c ases, there was no discrete gross lesion in the surgical specimen. Mic roscopically, there was a spectrum of pathological stromal changes ran ging from classical PASH with anastomosing slit-shaped spaces outlined by flat, bland spindle cells to more proliferative lesions composed o f bundles of plump spindle cells that obscured the underlying pseudoan giomatous architecture in the most florid lesions. The spindle cells w ere vimentin and CD34 positive and factor VIII negative. In more cellu lar fascicular lesions, the stromal cells acquired desmin and actin po sitivity. These immunohistochemical features were consistent with myof ibroblastic histogenesis of PASH. Reactivity for progesterone receptor (PR) typically exceeded estrogen receptor (ER) in the nuclei of strom al and glandular cells. In most lesions, the nuclei of stromal spindle cells were ER negative. The majority of the patients were treated by excisional biopsy. One lesion, incompletely excised, spontaneously reg ressed. One patient had bilateral mastectomies. Follow-up was 0.6-11 y ears (mean, 4.5 years). Five patients had ipsilateral recurrences, and two had subsequent contralateral PASH. The morphological spectrum of cellular proliferation and staining qualities indicates that the myofi broblast plays a major role in the histogenesis of PASH. The pathogene sis of PASH remains uncertain, but aberrant reactivity of myofibroblas ts to endogenous or exogenous hormones is likely to be an important fa ctor. Simple excision is adequate treatment initially and for infreque nt recurrences. Diffuse PASH occasionally presents a difficult managem ent problem that may necessitate mastectomy.