As is the case with cancer in humans, some types of cancer are more common in pets than others. Breast (mammary) cancer, skin cancer, bone cancer, mouth (oral) cancer, connective tissue cancers (sarcomas), and lymphatic tissue cancers (lymphomas) are most frequently found in pets.
Cancers of the blood are known as leukemias. Cancers involving the blood-forming tissues are called lymphomas. Unlike cancers that form tumors, leukemias and lymphomas do not form a solid mass; they remain as separate cancerous cells. In the bone marrow and bloodstream, leukemia and lymphoma cells often crowd out and replace normal cells. Enlargement of multiple lymph nodes is often the first sign of lymphoma.
The cells that cover the surfaces of the body, produce hormones, and make up glands are known as epithelial cells. When epithelial cells mutate into cancer cells, the tumor is called a carcinoma. Cancers of the skin, lung, colon, stomach, breast, prostate, and thyroid gland all fall under the general category of carcinoma. Typically, younger animals develop carcinomas less frequently than older animals, although carcinomas can occur in animals of any age.
The cells that form muscles, connective tissues, and bones are collectively known as mesenchymal cells. When these cells become cancerous, the tumor is called a sarcoma. Bone cancer, also called osteosarcoma, is one well-known type of sarcoma.