Just as in people, cancerous tumors and growths develop when something has happened to damage the genes in the body. This means that anything from heredity to genetic damage in areas
that control viral processes; some breeds actually have cancerproducing genes built into their anatomy.
Other contributors are age, sunlight, poor diet, lack of exercise, stress, and more. It’s difficult to pin down exactly what causes cancer in any given dog, but the true cause stems from the fact that cancer cells trick the immune system into believing that the cancer cells are its own normal cells. 
The immune system doesn’t destroy them as it does other abnormal cells, which means they’re free to multiply and spread across the body. Benign tumors, those that are not cancerous, grow very slowly, don’t spread, and only cause damage if they begin to block
necessary processes in the body. Malignant tumors, growths that are cancerous, can spread via blood or bodily fluid, grow very quickly, and often invade multiple areas of the body if not treated successfully.