A seroma is a swelling caused by a build-up of clear fluid, serum, in the tissues.

Seromas are quite common after surgery for breast cancer, particularly when the lymph glands have been removed.

When a breast cancer is removed, whether by local surgery - like a lumpectomy, or by removal of the whole breast by a mastectomy, fluid collects under the skin at the site of the operation.  Sometimes the surgeon will put in a small tube at the time of the operation to drain any fluid that may collect.  These drains are removed a day or two later.  Usually any remaining fluid is rapidly absorbed by the surrounding tissues.

But sometimes the fluid continues to build up, and this causes a swelling, a seroma, at the site of the operation.  The fluid in the seroma can easily be drained away by inserting a fine needle through the skin and drawing the serum off with a syringe.  This may need to be done two or three times in the weeks after surgery before the fluid finally stops building up.

When a seroma does develop the fluid formation usually stops on its own within a few weeks of the operation.  But very occasionally it continues to develop and this may need another operation to put in a fine tube for a while, so that the fluid can continually drain away.

Seromas are not a sign of the cancer coming back, and are not dangerous in any way, and they do always settle down eventually.  But they can be a bit of a nuisance for a few weeks before they finally disappear.