Deb;One of my grey mares had a pea sized lump on her front chest that grew into a ping-pong ball fairly rapidly, within a month or so. We first thought it was a kick injury from her foal and a bone chip or something. It progressed to where we couldn't get it removed on the property and she had to go to the hospital, for surgery under a general anesthesia. Pathology revealed that it was a mast cell sarcoma. While she was there, they found three melanomas on her tail and vulva that were removed.
I am also an oncology nurse. The rule of thumb is..wait and see, and you will watch it grow. It will not go away on it's own, and there is always a chance that the waiting will result in worsening disease. Biopsies are not done by squeezing it like a pimple..that is just a wild guess. If you suspect its cancer, not a bad idea to get rid of it. Beth