The type of treatment changes depending upon the stage of the tumor, whether it has intensely spread or not and the health condition of the patient. Three common and best treatment options are surgery (removal of infected tissue), chemotherapy (reduces the count of cancer cells) and radiation therapy (targets rapidly dividing cells and avoid spreading of cancer cells).
Some of the cancer treatment options for your cat may include surgery, medication and chemotherapy. You'll want to take your cat's age and general health into consideration, as these treatments can be very painful and are often quite expensive. Please consult with your vet before making any decisions.
Quality of life has to be considered in this situation and the best place to start is by having an in-depth serious talk with your vet. The side-effects of needed medical treatment as well as the issue of providing necessary nutrition for your cat are just two issues that should be discussed.
Your vet could operate to try to remove the tumor, but this is expensive, very painful for the cat, and not guaranteed to work. It's hard because your cat is a family member, but it might be time to let it go.