A fatty tumor, or a lipoma, says Dr. Lachowicz, is a benign proliferation of fat cells. Skin cancer, whether it’s a mast cell tumor or a type of soft tissue sarcoma, is a malignant, rapidly growing tumor that invades surrounding tissues and spreads to other parts of the body.“Most dogs will develop multiple lipomas, and they may get larger through time,” he says. “But they can be usually monitored. You really don’t have to do anything about those.”