Mammary pseudoangiomatous stromal hyperplasia. A reappraisal of the fine needle aspiration cytology findings.

Objective: To describe and reevaluate the fine needle aspiration cytology findings of pseudoangiomatous stromal hyperplasia (PASH) of the breast, with histologic, immunohistochemical and ultrastructural correlation.

Study Design: The authors reviewed the clinical features, fine needle aspiration cytology, histology and immunohistochemical results in all cases of mammary PASH encountered at Pamela Youde Nethersole Eastern Hospital, Hong Kong, during the 4-year period from January 1998 to May 2002. Ultrastructural examination was carried out in a selected example. The findings were compared with those in the literature.

Results: Four cases of PASH of the breast were encountered during the study period. The ages of the patients ranged from 34 to 56 years. One of them was a male presenting with gynecomastia. Only 2 cases had fine needle aspiration biopsies with a satisfactory cellular yield available for review. The cytospin preparations were of moderate cellularity and showed cohesive clusters of bland-looking ductal cells in a background of single, naked nuclei and some spindle cells containing fine chromatin and a discernible amount of cytoplasm. Occasional ductal cell clusters assuming a "staghorn" pattern, a feature commonly seen in fibroadenoma, were noted. Besides, there were scantly, loose and hypocellular stromal tissue fragments that contained spindle cells and occasional paired, elongated nuclei embedded in a fibrillary matrix. Histologic examination of the excisional biopsies confirmed the presence of PASH. It was characterized by many slitlike spaces rimmed by CD34-positive myofibroblasts/fibroblasts in a focally hyalinized stroma. Sometimes, ill-formed, fusiform aggregates of fibroblasts were also observed. Their fibroblastic nature was confirmed by electron microscopy.

Conclusion: Fine needle aspiration cytology of PASH closely resembles that of fibroadenoma. Though subtle differences do exist, a definitive diagnosis is unlikely on the basis of the cytologic examination alone. PASH needs to be distinguished from borderline lesions, such as phyllodes tumor, and more sinister conditions, which sometimes have a similar cytologic appearance.