Immunohistochemical staining characteristics of low-grade adenosquamous carcinoma of the breast.

BACKGROUND: Low-grade adenosquamous carcinoma (LGASC) is an uncommon variant of metaplastic carcinomas of the breast. The immunohistochemical profile of this entity has not been well characterized and is likely because of its seemingly inconsistent staining patterns when commonly used immunohistochemical stains are employed. We set out to further elucidate the immunohistochemical profile of this uncommon entity in a sizable cohort of patients.

MATERIALS AND METHODS: Thirty cases of LGASC were identified in our files. Commonly used immunohistochemical stains such as myoepithelial and cytokeratin markers used to evaluate a small glandular proliferation in the breast (the differential diagnosis of which includes LGASC) were utilized. The pattern and location of immunoreactivity were recorded in each case. Results were compared for staining trends.

RESULTS: All cases of LGASC demonstrated variable staining in both lesional glands and stromal cells for myoepithelial (p63, smooth muscle myosin, smooth muscle actin, CD10, calponin) and cytokeratin (CK AE1/3, CK5/6, CK7, CK 34βE12, Cam 5.2) markers. Within a single case, circumferential staining using myoepithelial markers was complete, discontinuous, and absent in over a third of the studied cases. A minority of cases showed either complete circumferential staining (or complete absence) by any single immunohistochemical stain. Lamellar staining of stromal cells surrounding glands was best highlighted using smooth muscle myosin heavy chain or calponin. Using cytokeratin stains, core staining (luminal glandular cells demonstrating distinctly stronger staining intensity than the basally located cells in the same gland) was observed in approximately half of the studied cases. These lesional stromal cells were negative for all cytokeratins, with the exception of 1, which was focally positive for 1 cytokeratin immunostain (CK7) while being negative for 3 others.

CONCLUSIONS: LGASC consistently stains in an inconsistent manner using commonly used immunohistochemical stains. In addition, we found lamellar staining and core staining using myoepithelial and cytokeratin stains, respectively, to be distinctive and therefore diagnostically valuable.