Seroma consisting of eosinophilic acellular fluid and outlined by a thick fibrous wall. Image

Scattered lymphoid cells are also present within the fibrous wall. There is no epithelial lining to this pseudocyst. Image

Seroma formation is the most common complication after breast cancer surgery, particularly if full axillary dissection is performed.

Most post-mastectomy seromas resolve within a few weeks. Some cases of prolonged seromatous effusion with thick walls require resection and closure of supplying regional lymph vessels.