There is a long list of man-made chemicals that are known to cause mutations in the DNA of cells and result in cancer. Viruses can cause a mutation in a gene. However, there are very few viruses that have been proven to cause cancer in dogs, and at this time it does not appear that viruses are a significant cause of cancer in dogs. Cancer caused by irradiation is uncommon in domestic animals except those individuals without pigmentation in their skin. Lifetime exposure to the sun can cause cancer on the white ears or white face of dogs.
Inherited cancer is somewhat different.
One copy of the gene comes from the dam and one copy of the gene comes from the sire. If the dog with only one normal copy of the gene suffers a single chemical insult, the affected cell will lose control of cell division and it will become a cancer cell. Generally, there are three methods by which cancer can be treated. Surgery is a good choice for treatment if the cancer is localized to a single area and the veterinarian has the opportunity to completely remove the cancer. Likewise, another cancer may have occurred originally in the skin, but has now spread to several different areas of the lungs.