Solid papillary carcinoma of the breast: A clinicopathological study with emphasis on unusual immunohistochemical and genetic features.

PURPOSE: To characterize the clinicopathological spectrum of solid papillary carcinoma (SPC) in our institutional cohort and to attempt to interpret unusual findings observed during routine diagnostic workup. METHODS: We retrospectively analyzed the clinicopathological features of 20 cases of SPC from our institution. RESULTS: All 20 patients were women, aged 45-79 years (mean, 66.4 years; median, 69.5 years). Among these, 18 cases were detected by the patients themselves, one was detected during routine examination, and the remaining patient presented with bloody nipple discharge. Imaging showed that most tumors were well‑defined solid or complex cystic and solid nodules. On microscopic examination, the tumors exhibited solid nests with delicate fibrovascular cores. The tumor cells were morphologically diverse, generally showing mild atypia and rare mitotic figures. Necrosis, hemorrhage, mucin deposition, and lymphocytic infiltration were observed in the stroma. Some cases showed unusual immunohistochemical patterns, including negativity for GATA binding protein 3 (GATA3) and synaptophysin (Syn). Transcriptional repressor GATA binding protein 1 (TRPS1) and insulinoma‑associated protein 1 (INSM1) may aid in the diagnosis of such cases. Furthermore, 1 patient exhibited chromosome 17 polysomy on human epidermal growth factor receptor 2 (HER2) fluorescence in situ hybridization (FISH) analysis. Moreover, using immunohistochemistry and FISH analyses, two patients with concomitant thyroid lesions were found to demonstrate loss of phosphatase and tensin homolog (PTEN) expression, suggestive of possible Cowden syndrome. CONCLUSIONS: SPC of the breast is a rare tumor and generally carries a favorable prognosis. However, some cases may show unusual immunohistochemical expression patterns; in particular, the clinical significance of chromosome 17 polysomy detected using HER2 FISH remains unclear. In addition, occasional cases may be associated with Cowden syndrome, which warrants further investigation through genetic testing.