Periductal Stromal Tumor of the Breast: One Institution's Review of 6 Tumors Over a 22 Year Period With Immunohistochemical Analysis.

Introduction. Periductal stromal tumor (PST) of the breast is a rare fibroepithelial neoplasm with controversial pathogenesis. Methods. A retrospective search of our Pathology database from 2000 to 2021 identified 6 PST, all evaluated according to the Armed Forces Institute of Pathology (AFIP) criteria. Immunohistochemistry for CD10, CD34, CD117, GATA3, p63, SOX10, ER, PR, HER2, smooth muscle actin (SMA), beta-catenin, and myogenin was performed as well. Results. All 6 patients were female and age ranged from 29 to 55 years (mean 40 years). Tumor size ranged from 2.9 to 5.9 cm (mean 3.0 cm). Data showed absence of leaf-like architecture (0/6), at least moderate hypercellularity (6/6), lack of a circumscribed border (5/6), coalescing nodules with intermixed adipose tissue (4/6), at least moderate stromal atypia (4/6), and an elevated mitotic activity ≥3mitotic figures/10 HPF (6/6). The stromal cells were positive for CD10 (4/4), CD34 (4/4), CD117 (3/4), and SMA (3/4), and negative for GATA3 (0/6), p63 (0/6), SOX10 (0/6), ER (0/4), PR (0/4), HER2 (0/4), nuclear beta-catenin (0/5), and myogenin (0/4). No patient had a PST recurrence or metastasis (average follow-up of 91 months). Conclusion. We confirm that PST shares morphologic and immunophenotypic similarities with phyllodes tumor (PT). However, PST can be reliably differentiated from PT using the AFIP criteria. Additionally, PST's immunoprofile of positive CD117 and CD34 stromal expression alongside the negative GATA3, p63, and SOX10 reactivity can aid the pathologist in excluding metaplastic carcinoma. All 6 of our PST behaved as benign neoplasms akin to benign PT.