Mammary cancer occurs much less frequently in cats than dogs, but when it does occur, it is often malignant and difficult to treat. Mammary cancer is likely to strike 1 in 4,000 cats. While this is about half the rate as in dogs, when cats develop mammary cancer it is often fatal. There is a strong correlation between early spaying and a reduced incidence of the disease. If mammary cancer is caught early, the treatment is more often successful. This article will help familiarize the cat owner with appearance and treatment of this deadly cancer.Which cats are at risk for developing mammary cancer?
Any adult female cat can develop mammary cancer, but the average age is usually 10-14 years of age. Siamese cats appear to have a genetic predisposition for developing mammary cancer and are twice as likely to develop it as other breeds. Unspayed females are at a much greater risk of developing mammary cancer. Female cats that were spayed after having one to several heat cycles, with or without having kittens, are also at a greater risk than a cat that was spayed before her first heat cycle.
More here, none of it very positive.
I think I'd better not do any more searching.