What is mast cell tumors? They are one of the most common tumors in dogs, more likely affect the skin but can appear in any organ, There are different grades of mast cell tumors, some benign and some aggressive. They are always concerning. They can fluctuate in size, one day it can get bigger, then get smaller again. They can cause an allergic reaction if irritated. They need to be addressed, more than likely by surgical removal.