Utility of breast magnetic resonance imaging in patients with occult primary breast cancer

Background: Although carcinoma presenting as axillary metastases is assumed to be due to breast cancer, identification of the primary lesion may prove problematic. We investigated the ability of breast magnetic resonance imaging (MRI) to identify the primary tumor, thereby confirming the diagnosis and broadening treatment options. Methods: From 1995 to 2001, 69 patients at our institution presented with occult primary breast cancer. All patients had negative breast examinations and mammograms and underwent breast MRI. Results: Of 69 patients, 55 had axillary adenopathy without evidence of distant disease (stage II); 14 had stage IV disease. In patients with stage II disease, MRI revealed suspicious lesions in 76% (42 of 55). In 62% (26 of 42), the MRI finding proved to be the occult primary tumor. Of these, 58% (15 of 26) were candidates for breast conservation. MRI did not identify the primary tumor in 25 women; 12 underwent mastectomy. Cancer was found in 33% (4 of 12) of these. Thirteen patients were treated with primary breast irradiation; three were lost to follow-up, one developed distant disease, and nine were without evidence of disease with a median follow-up of 4.5 years. In women with stage IV disease, MRI identified the primary tumor in 5 of 9 patients with regional adenopathy and 2 of 5 patients with distant disease (overall 50%; 7 of 14). MRI identified the primary tumor in women with both mammographically dense (19 of 44; 43%) and less dense (10 of 20; 50%) breasts. Conclusions: Breast MRI detects mammographically occult cancer in half of women with axillary metastases, regardless of breast density. MRI is a powerful tool for stage II and stage IV patients with occult primary breast cancer.