In the latest revision of the TNM staging system, microinvasive carcinoma of the breast (MICB) has been defined as follows: “the extension of cancer cells beyond the basement membrane into the adjacent tissue with no focus more than 0.1 cm in greatest dimension.” 1 In this system, the presence of multiple foci of MICB is to be noted, but the addition of sizes of individual foci of microinvasion is not allowed.
Microinvasive carcinoma of the breast is among the most commonly misdiagnosed entities in breast pathology.2 False-positive diagnosis of MICB may be due to misinterpretation of lobular cancerization, radial sclerosing lesion, sclerosing adenosis, and fibrosing entrapment of epithelium; its false-negative diagnosis may be due either to masking of the invasive focus by inflammatory cells on lower magnification (Figure, A) or mistaking of tumor cells for histiocytes on higher magnification (Figure, B).3 The presence of a dense lymphocytic infiltrate is one of the histologic hallmarks of MICB. In equivocal cases, the invasive tumor cells are readily distinguished from inflammatory cells by cytokeratin AE1/3 immunostaining (Figure, C, detail in inset). The invasive nature of these tumor cells may be further corroborated by the lack of a smooth muscle actin–positive layer of myoepithelial cells, which typically border not only lobules and ducts, but also in situ carcinoma (Figure, D, detail in inset). Use of histochemical stains (eg, reticulin for basement membrane) and other immunohistochemical stains (eg, heavy-chain myosin for myoepithelial layer, collagen IV for basement membrane, or even double-immunolabeling for smooth muscle actin and cytokeratin) may provide further confirmation of microinvasive carcinoma.3
Microinvasive carcinoma of the breast is most commonly encountered in the sixth decade of life and presents as a palpable lesion in more than one half of the cases. The illustrated case is that of a 53-year-old woman with a breast mass, in whom the accompanying ductal carcinoma in situ was of the solid type (Figure, A), with intermediate nuclear grade (Figure, B). However, it is the comedo type of ductal carcinoma in situ that is most frequently associated with MICB.2,3
Cumulative prognostic data suggest that MICB behaves in a manner equivalent to, or only slightly worse than, high-risk ductal carcinoma in situ. There is a low, but not negligible, rate of local recurrence and axillary nodal metastases, the latter occurring in less than 10% of cases (sentinel lymphadenectomy was negative in the illustrated case). Patients with MICB may be expected to have a cure rate approaching 100% with local treatment alone.2,3