Lymphosarcoma (lymphoma) is a malignant cancer originating in lymphocytes, which are found in lymph nodes and pretty much all the organs and tissues around the body. It is probably the most common type of cancer in cats and not uncommon in dogs. Like most cancers, middle-aged to older animals are most at risk. However, lymphoma is unique in that it can be associated with feline leukaemia virus (FeLV) infection in cats. Cats that are infected with FeLV can develop feline lymphoma as young as three years of age.