From The New England Journal:
A woman with unilateral breast cancer has an increased risk of having cancer in the contralateral breast.1-6 In the 1990s, the role of mammography in improving the detection of contralateral cancers at the time of the initial diagnosis of breast cancer was firmly established; as compared with clinical breast examination alone, mammography resulted in a 1 to 3% increase in the number of cancers detected.7-9 Despite normal findings on clinical and mammographic examination of the contralateral breast at the time of the initial breast-cancer diagnosis, however, contralateral cancer was subsequently detected in up to 10% of women.1,3,5,6 When contralateral cancer is diagnosed after the initial treatment, the woman must undergo a second round of cancer therapy rather than the single round that would have been administered if the contralateral cancer had been detected at the time of the initial diagnosis.
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.10,11 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.10 However, this study did not include women with a current diagnosis of breast cancer.