Rarely, women present with adenocarcinoma meta stases in the axillary lymph nodes without an identified primary source of cancer.

 If the primary cannot be found through clinical breast examination and standard breast imaging, adenocarcinoma in the axillary lymph nodes is usually presumed to be caused by a primary cancer in the ipsilateral breast.

Historically, mastectomy was recommended in this situation.[33]
However, more recent studies have suggested that well-selected patients who undergo whole breast radiation without mastectomy, followed by continued mammographic screening of both breasts, can have equivalent outcome as measured by local recurrence in the breast.[34]

MRI helps resolve the clinical dilemma about mastectomy in these patients. MRI detects the primary cancer in up to 70% of these patients, changing the staging from T0 (unknown primary) to the defined T1 to T3.[35] Once the primary cancer is detected and histologically proven to be present in the ipsilateral breast, surgical planning can be made using standard criteria for patient management.

In several published series, MRI accurately identified the primary cancer in most patients presenting with axillary adenopathy, an unknown primary, and a negative clinical examination and mammogram.[35-41] Overall, MRI will identify the primary in approximately 59% of women with this diagnosis, allowing them to pursue more appropriate and focused therapy for the breast malignancy.

Therefore, for patients with biopsy-proven adenocarcinoma presenting in the axilla, a normal clinical breast examination, and a negative mammogram (TxN1-3), breast MRI is indicated to identify a primary cancer in the breast.