Observations on the histopathologic diagnosis of microinvasive carcinoma of the breast.

Our histopathologic criteria for diagnosing microinvasive carcinoma of the breast may be enunciated as follows: (1) cytologically malignant cells in the stroma associated with in situ carcinoma, (2) absence of basement membrane and myoepithelial cells around the invasive cells, (3) frequent accompanying stromal alterations in the form of myxomatous change and loosening of connective tissue, and (4) the frequent presence of an inflammatory cell infiltrate composed of lymphocytes and plasma cells. Most or all of these four features are present in cases of ductal microinvasive carcinoma of the breast, but the lobular type is not likely to be accompanied by stromal changes or a lymphoplasmacytic cell infiltrate. The minimum information regarding microinvasive carcinoma of the breast that should be conveyed in the final pathology report includes size as measured by the ocular micrometer or a statement that microinvasion refers to a lesion smaller than 1 mm, the number of foci of invasion, and the spatial distribution of the invasive foci. The nuclear grade of the invasive cells and the size, type, and nuclear grade of the accompanying DCIS should be specified. The status of margins, presence of vascular channel involvement (a rarity in microinvasive carcinoma of the breast), and degree of proliferative changes in adjacent nonneoplastic breast tissue should be reported. Immunostains for basement membrane and myoepithelial cells may be helpful in the diagnosis of microinvasive carcinoma of the breast. Sclerosing lesions such as radial scar and sclerosing adenosis can simulate microinvasive carcinoma of the breast, especially when the latter is associated with in situ carcinoma. Caution should be exercised in cases wherein in situ malignant cells may be dislodged by needling procedures or during dissection of the excised specimen. Cautery-induced artifacts also hinder optimal histologic assessment. In some cases, it is virtually impossible to determine if true invasion is present, and the statement "microinvasive carcinoma of the breast cannot be entirely excluded" may be employed as a last resort. We consider the latter diagnosis to be the last refuge of the diligent pathologist and do not recommend it unless all diagnostic measures, including examination of deeper levels and supplemental stains, have been exhausted. It may be necessary to seek an expert opinion in "difficult" cases, particularly in the event that therapeutic decisions are to be based on the determination of invasion. From a clinical perspective, the management of microinvasive carcinoma of the breast ought to be dictated by the individual circumstances in each case. Based on currently available data, which admittedly suffer from lack of diagnostic uniformity, the vast majority of patients with microinvasive carcinoma of the breast will be node-negative and can look forward to an excellent prognosis. It is hoped that since the UICC has adopted a previously recommended definition of microinvasive carcinoma of the breast, prospective or retrospective studies with uniform diagnostic criteria will be conducted that will enable more definitive conclusions regarding the treatment and prognosis of microinvasive carcinoma of the breast.