A 48-year-old woman with a history of poorly differentiated adenocarcinoma involving her left breast presented with a mass in her right breast. A lumpectomy was performed, as well as an axillary lymph node biopsy. The lumpectomy contained an adenomyoepithelioma. The axillary lymph node specimen is shown below.
What is your diagnosis?
The axillary lymph node contains several subcapsular, cystic epithelial proliferations. One of these contains an exhuberant papillary proliferation seen at higher power in the last of the images. There is little or no cytologic atypia. Mitotic figures are absent and there is no evidence of necrosis. A distinct two-cell layer is clearly seen with the outer cell layer having predominantly clear cytoplasm. A smooth muscle actin stain, shown below, highlights the myoepithelial nature of the outer cell layer and supports the benign nature of this lesion.
This is an example of a mammary-type papilloma arising in ectopic breast tissue in an axillary lymph node. This is an uncommon but well-recognized phenomenon described in several publications, including Rosen's text on breast pathology. In the current case, in addition to the two cystic foci shown, several small subcapsular mammary-type glandular inclusions were also present, further supporting that the papilloma arose from one of these inclusions. Non-proliferative glandular inclusions in axillary lymph nodes are far more common than proliferative papillomas.
Although the current example should be recognized as benign on H&E sections, one can imagine that tangential sectioning of one of the papillary lesions, coupled with a thick slide could create diagnostic problems at the time of frozen section, perhaps leading to an overdiagnosis of malignancy and stimulating an unnecessary lymph node dissection. The presence of any ectopic glandular tissue in axillary lymph nodes may also cause difficulty if molecular or immunohistochemical studies are undertaken without appropriate light microscopic correlation.
At the other end of the spectrum, carcinomas have been described as arising in ectopic breast tissue in axillary lymph nodes. This, however, is at least somewhat of a diagnosis of exclusion, even if the carcinoma is closely associated with benign inclusions. An occult mammary primary must be ruled out.