Immunostaining of Type IV Collagen and Smooth Muscle Actin as an Aid in the Diagnosis of Breast Lesions.

The immunohistochemical staining patterns of type IV collagen (TIVC) and smooth muscle actin (SMA) were studied in benign and malignant breast lesions in order to assess their usefulness in the differential diagnosis of difficult lesions. Eighty-six in situ breast carcinomas (8 with microinvasion; 1 diagnosed after immunostaining) and 58 invasive carcinomas, with associated benign lesions including 87 nonproliferative fibrocystic changes, 19 sclerosing adenosis, 17 atypical hyperplasias, 7 benign papillary proliferations, 6 radial scars, and 105 normal mammary tissue, were studied. TIVC and SMA were concomitantly positive throughout in 94% of benign tissues, in 16% of in situ carcinomas, but in none of the invasive carcinomas. Conversely, both markers were negative in 66% of invasive carcinomas, but in none of the benign breast tissue and none of the in situ carcinomas. In 6% of the benign tissues, in 21% of in situ carcinomas, and in 34% of invasive carcinomas only one of the markers was positive. In the remaining 63% of the in situ carcinomas there were discontinuities of the staining of both markers. It is therefore suggested that a diagnosis of invasive malignancy can be confirmed when both markers are negative, and ruled out when both markers are positive. This is particularly useful, in our experience, in the identification of small foci of invasion. The stains are useful when used in parallel, whereas they may be misleading when used singly. The staining pattern is not useful in the differential diagnosis of atypical hyperplasia and in situ carcinoma.