Lobular carcinoma in ectopic axillary breast tissue.

Accessory mammary tissue is reported to occur in 2 per cent to 6 per cent of women and results from failure of regression of a portion of the galactic band, which runs from the axilla to the groin during embryogenesis. The differential diagnosis of a mass located along the course of the primitive milk line must include accessory breast tissue. This tissue is subject to the same physiologic and pathologic changes as in the breast proper. There are numerous case reports of masses arising in accessory breast tissue ranging from fibroadenomas and phyllodes tumors to invasive mucinous adenocarcinoma. The predominant pathology, as with the pectoral breast, is that of invasive ductal carcinoma. We present the case of a lobular carcinoma arising in axillary ectopic breast tissue. Although this has previously been reported, it is a rare entity.