Feline mammary cancer is a serious disease. It may not initially look bad to you because it starts as a tiny pea-sized lump near a nipple, and you may feel like it can just be removed and be done. Not so fast. That little lump has a high probability of being malignant, and may have already spread to surrounding breast tissue or even the lungs by the time it is found. Treatment needs to be swift and aggressive to prolong life.