Accepted Answer

Hi tarabrawn,
Unfortunately, there is no way to know what a lump is definitively, unless it is removed surgically and sent in for biopsy. One way to try to find out is by doing a test called a fine needle aspirate. During this test, the vet places a needle inside the mass and pulls out some of the cells and other materials inside. This is then sprayed and smeared on a glass slide and is stained for viewing under a microscope. A pathologist can tell what a lump is 50% of the time with this test. The major benefit of this test is that it may give you an answer without having to put your dog under anesthesia. In your dog's case, it sounds like this test was in the 50% that come back inconclusive. I can tell you, however, that a fluid-filled cyst (that deflates when fluid is removed and then fills back up again later) is by definition, benign. The only way to get rid of the cyst, however, is to remove the entire thing surgically.
Most people opt to surgically remove the lumps, even when benign, because this will give a definitive diagnosis, and the lump will then be gone if it is malignant (cancerous). Furthermore, many benign lumps do continue to get bigger and this can be cumbersome, unsightly, and sometimes uncomfortable for the dog. Cysts, specifically, will often rupture, bleed and can become infected. So, in these cases, surgical removal is best.
I hope that this information is of help to you, and I wish you the best of luck with your dog. Please let me know if I can be of further assistance.

Dr. K

Expert:	Dr. K
Pos. Feedback:	99.6 %
Accepts:	2959
Answered:	4/20/2008

Veterinarian

9 years experience as Veterinarian