histiocytoma

Hello, I have a 2 year old vizsla who developed a histiocytoma in March. It got to about the size of a dime. The vet did a fine needle aspiration and said there were no malignant cells and said it would go away on its own. Sure enough a few weeks later it started fading away and completely disappeared and all the hair grew back, No trace of the histiocytoma. Fast forward 2 months later and on the same site I noticed a new one coming in, this time much smaller. Went to a veterinary dermatologist who told me it could be a histiocytoma or mast cell but she was more inclined towards histiocytoma. She also did a fine needle aspiration and collected 5 round cells and also said she didnt see anything malignant and that raised her suspicion even more that it was a histiocytoma. So far its been almost 4 weeks since it appeared and it has stayed about the same size (5mm).  I have been extremely anxious about the possibility that it could be mast cell especially since the tumor appeared in the same spot where it had regressed earlier this year. I guess my question is how likely is it to be a histiocytoma especially knowing that both aspirations came back negative for anything malignant.

Hello!  I'm afraid that I don't know the numbers for that.  However, it is usually cited that histiocytomas can take 3 months to disappear, and as far as I know this is no quicker for smaller lumps.   As you know, cytology is only as good as the sample of cells from the lump is representative, but mast cells are easy to identify on a slide if some are sucked up.  I hope this helps you with your decision making as to what to do next, but it's always good to discuss lumps with the vet or, by proxy,  their pathologist, if you need for information.  Wishing you the best of luck and hoping that that worrying lump disappears soon for you!    

In your case, both fine needle aspirations have come back negative for malignancy, which is a good sign. While no test is perfect, a negative result does reduce the likelihood of a tumor being malignant. The fact that the growth has remained about the same size for four weeks is also somewhat reassuring, as malignant tumors often grow more rapidly.