Cancer is the uncontrolled growth of cells on or within the body. They are either localized to one part of the body as a visible mass (tumor) or spread throughout (metastasis). Cancer is common in dogs and cats, and the rate increases substantially as pets age. Dogs develop cancer at roughly the same rate as humans, while cats get fewer cancers. In one survey, approximately 1 of every 250 dogs and cats developed cancer during their lifetime, and cancer accounted for almost half the deaths of those pets over 10 years of age. Since the cause of cancer is not known, prevention is difficult.