Heterotopic breast tissue versus occult metastatic carcinoma in lymph node, a diagnostic dilemma

Breast cancer is the leading cause of cancer in women and the third leading cause of cancer mortality in the United States. We report a case of a patient who underwent bilateral simple mastectomies and right sentinel node biopsy for invasive lobular carcinoma in the right breast. An ipsilateral sentinel lymph node showed a microscopic focus of ductal elements. Although residual lobular carcinoma was identified in the right breast, no ductal carcinoma was identified in either breast. The ducts were discrete distributed in a 3-mm focus in the lymph node parenchyma as well as the subcapsular sinus. By immunohistochemistry, the ducts were positive for cytokeratin, estrogen receptor/progesterone receptor and did not show a myoepithelial layer by P63 or smooth-muscle myosin heavy-chain staining. The differential diagnosis includes heterotopic epithelial inclusions and benign mechanical transport. Mechanical transport of the breast tissue was ruled out because primary tumor type in the breast and the ductal structures in the lymph nodes were of different types. The diagnosis of occult metastatic tumor was based on the lack of the myoepithelial layers associated with the ductal structures. The diagnostic dilemma of the differential diagnoses is discussed, and pertinent literature is reviewed.