Endosalpingiosis in axillary lymph nodes simulating metastatic breast carcinoma: a potential diagnostic pitfall.

Intraoperative assessment of sentinel lymph nodes at time of surgical excision of primary breast carcinoma is a crucial step in the determination of cancer extent and the need for further axillary dissection. Benign epithelial inclusions in axillary lymph nodes can mimic metastatic carcinoma and are a well-known pitfall during examination of these nodes in frozen or permanent sections. Most often, these inclusions consists of heterotopic mammary glands and are familiar to the practicing pathologist. Here, however, we present a rare case of endosalpingiosis in the axillary lymph nodes of a breast cancer patient and describe our experience and effort to characterize the lesion. Simulating a metastatic focus of invasive ductal carcinoma, the glandular inclusions lacked myoepithelial cells and failed to stain with myoepithelial markers. However, consistent with a Mullerian origin, the inclusions demonstrated strong staining with PAX-8 and WT-1. Although endosalpingiotic inclusions are not uncommonly encountered in subdiaphragmatic lymph nodes, they are an extremely rare finding above the diaphragm. Pathologists must be aware of these lesions and their ability to imitate metastatic gland-forming carcinoma during frozen section or permanent examination of axillary lymph nodes.