Even after careful clinical and mammographic evaluation, cancer is found in the contralateral breast in up to 10% of women who have received treatment for unilateral breast cancer. We conducted a study to determine whether magnetic resonance imaging (MRI) could improve on clinical breast examination and mammography in detecting contralateral breast cancer soon after the initial diagnosis of unilateral breast cancer.

The importance of clinical breast examination and mammography in the diagnostic workup of a woman with recently diagnosed breast cancer is not disputed. However, mammography and clinical breast examination have limitations — both methods yield false negative results. A recent large study showed that screening magnetic resonance imaging (MRI) can improve on mammography by detecting otherwise occult cancers in 1.2% of women at high risk. However, this study did not include women with a current diagnosis of breast cancer.

Preliminary studies have suggested that MRI can detect otherwise occult contralateral breast cancers in an average of 5% of women with a recent diagnosis of breast cancer.The rate of detection for tumors not identified by other means ranged from 3 to 24%, the specificity of MRI was variable, and the studies lacked follow-up data to confirm the negative predictive values of MRI in such women. We conducted a study to determine the number of clinically and mammographically occult cancers in the contralateral breast that could be detected by MRI in women with recently diagnosed unilateral breast cancer.

A total of 969 women with a recent diagnosis of unilateral breast cancer and no abnormalities on mammographic and clinical examination of the contralateral breast underwent breast MRI. The diagnosis of MRI-detected cancer was confirmed by means of biopsy within 12 months after study entry. The absence of breast cancer was determined by means of biopsy, the absence of positive findings on repeat imaging and clinical examination, or both at 1 year of follow-up.

Results:  MRI detected clinically and mammographically occult breast cancer in the contralateral breast in 30 of 969 women who were enrolled in the study (3.1%). The sensitivity of MRI in the contralateral breast was 91%, and the specificity was 88%. The negative predictive value of MRI was 99%. A biopsy was performed on the basis of a positive MRI finding in 121 of the 969 women (12.5%), 30 of whom had specimens that were positive for cancer (24.8%); 18 of the 30 specimens were positive for invasive cancer. The mean diameter of the invasive tumors detected was 10.9 mm. The additional number of cancers detected was not influenced by breast density, menopausal status, or the histologic features of the primary tumor.

Conclusions MRI can detect cancer in the contralateral breast that is missed by mammography and clinical examination at the time of the initial breast-cancer diagnosis

The current standard practice for evaluating the contralateral breast in women with a recent diagnosis of breast cancer is to perform a clinical breast examination and mammography. In this prospective study, we estimated the additional diagonostic yield of MRI in such women. Among 969 women with a recent diagnosis of breast cancer and normal results of clinical breast examination and mammographic studies, 30 contralateral cancers were detected on MRI (18 invasive cancers and 12 ductal carcinomas in situ), for a diagnostic yield of 3.1%, with a sensitivity of 91%. In comparison, a recent assessment of mammographic and MRI screening in 1909 high-risk women documented 22 cancers that were detected only by means of MRI; the additional diagnostic yield of MRI over mammography was 1%, with a sensitivity of 80%.

We did not find that breast MRI had a low specificity, as previously reported. The specificity of MRI in our study was 88%; a biopsy was recommended on the basis of a positive MRI in 13.9% of the women, and 24.8% of the biopsies resulted in a diagnosis of breast cancer. The overall high accuracy of MRI (as measured by the estimated area under the ROC curve of 0.94) may reflect improved technology or improved interpretation of the results, especially in regard to how to distinguish benign from malignant patterns of enhancement on MRI scans. Our results should be widely applicable, since the participating sites represent a range of radiology practices, from academic centers to community practices, and a range of expertise in interpreting breast MRI studies, from extensive experience to moderate experience.

The negative predictive value of MRI in the population we studied was extremely high (99%). The risk of an occult cancer in the contralateral breast 1 year after a negative MRI was estimated at 0.3%, and all of the cancers that were detected at that time were ductal carcinoma in situ and were 4 mm or less in diameter. This information may be helpful to women and their physicians in discussing the relative value of bilateral mastectomy when only unilateral cancer is diagnosed after breast MRI. Some women with a diagnosis of unilateral breast cancer choose prophylactic mastectomy of the contralateral breast, but negative findings on preoperative MRI and mammographic studies might reduce the number of unnecessary mastectomies.

In our study, all of the cancers that were detected by means of MRI were node-negative, and 40% were ductal carcinomas in situ. The success of screening programs for breast cancer lies in their ability to detect early cancer, before it has spread to lymph nodes or metastasized to distant sites. Recent studies provide support for the benefit of detecting ductal carcinoma in situ, since this tumor is likely to progress to invasive disease if left untreated. In addition to early detection of in situ or node-negative invasive disease in the contralateral breast, MRI, if positive, can lead to simultaneous treatment of synchronous cancers rather than multiple treatments on separate occasions.

Our study shows that MRI can improve the detection of cancer in the contralateral breast when added to a thorough clinical breast examination and mammographic evaluation at the time of the initial diagnosis of breast cancer. The increased rate of detection of cancer comes with a false positive rate of 10.9% and a relatively low risk of detecting benign disease on biopsy (9.4%). The current cost of MRI precludes its widespread use in general populations, but this imaging tool appears to improve the detection of cancer in women at increased risk, such as women with a recent diagnosis of breast cancer.