DCIS-like invasive carcinoma of the breast with tumour-associated microvasculature mimicking myoepithelium: a diagnostic pitfall.

AIMS

Ductal carcinoma in-situ (DCIS)-like invasive carcinoma of the breast is an underdiagnosed pattern of invasive carcinoma that is challenging to separate from DCIS. We have observed some cases of DCIS-like invasive carcinoma in which smooth muscle myosin heavy chain (SMMHC) or calponin are strongly expressed in vessels tightly encircling the periphery of the tumour nodules, closely mimicking myoepithelium and potentially leading to misinterpretation as in-situ disease. We aimed to assemble and characterise a series of DCIS-like invasive carcinoma, with particular attention to presence and immunostaining of encircling vessels.

METHODS AND RESULTS

We collected 23 cases of DCIS-like invasive carcinoma and performed SMMHC, calponin, p63, CK5/6, SMA, CD31 and D2-40 immunohistochemistry. All the cases showed no true myoepithelial cell staining, supporting invasive disease. Eighteen of the cases displayed vessels tightly encircling at least 25% of the tumour, which we termed tumour-associated microvasculature (TAM). Of the TAM cases, 11 had at least some SMMHC vascular expression (termed tumour-associated microvasculature mimicking myoepithelium, TAMMM). The vascular SMMHC was strong, circumferential and closely mimicked myoepithelial staining in three of the TAMMM cases. Eleven cases had focal/patchy, often weaker, calponin staining in the vessels. CD31 was universally positive in TAMMM, supporting that the observed SMMHC or calponin expression encircling the tumour nodules was due to vascular staining. D2-40 was negative in all cases, confirming the lack of lymphatic endothelium.

CONCLUSIONS

Our findings illustrate TAMMM as a rare but important pitfall of myoepithelial cell immunohistochemistry in the distinction of DCIS-like invasive carcinoma from in-situ disease and support a panel approach to myoepithelial immunohistochemistry.