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Multiple tumors in the liver are not always cancers. The livers of older dogs become nodular although this does not have any clinical effects. Multiple nodules (regenerative nodules) with an accompanying increase in fibrous tissue may be the result of previous liver damage including inflammation of the liver (hepatitis). When the fibrosis affects the whole liver, it is called cirrhosis. At this stage, it is progressive and not curable.

The liver has a massive blood supply so many cancer cells from elsewhere arrive within it and start to grow. In dogs metastatic tumors are three times as common as primary tumors and over 30% of malignant cancer is said to metastasize to the liver. Cancer of the lymph cells of the immune system (lymphosarcoma or lymphoma) is the most common type followed by tumors of the blood vessels (haemangiosarcoma) and then tumors of epithelial tissue, particularly from the pancreas. The liver has multiple functions and a variety of different cells within it so primary cancers may arise from the neuroendocrine cells (carcinoids) and tumors of histiocytic, fibrous and muscle tissue also occur. These are discussed in more detail elsewhere.

Lymph is a fluid which circulates in the body, transporting cells of the immune system (lymphocytes and macrophages) and draining areas where excess fluid or debris has accumulated, for example in inflammation. Macrophages (literally "big eaters") are cells that "eat" (phagocytose) and digest other cells, infections, and debris. They also collect and carry antigens to the lymphocytes. Macrophages are filtered out of the lymph in swellings called lymph nodes (glands). Within lymph glands there are concentrations of lymphocytes. The ’B’ type lymphocytes, through their derivative ’plasma cells’, make antibodies to the antigens brought by the macrophages (humoral immunity); and the ’T’ type lymphocytes prepare themselves to attack the foreign antigens (cellular immunity).