Benign Tumors Stay in One Place
Benign tumors simply grow in one location. Though benign tumors can become malignant in rare cases, the biggest danger of a benign tumor is that it will grow enough to impede your pet's life. Rapidly growing benign tumors can be deadly because of proximity to organs.
Benign tumors are made of the same kinds of cells that make up the organ they grow from, unlike malignant tumors that are made of other types of cells. Benign tumors are usually easily removed as a consequence of the fibrous tissue that enclose the growth.
A lipoma is a benign tumor in the fatty deposits under the skin or in your dog's muscles. The tumors are painless, soft and spherical, and do not need to be removed unless your dog's movement is obstructed.
About half of mammary tumors are benign, so you shouldn't feel too alarmed if you come across one. Still, dogs that have not been spayed by the first heat cycle are seven times more likely to develop mammary tumors than those that have not, so keep your animal's cancer risk in mind. 25% of un-spayed females will develop mammary tumors.
Malignant Tumors Spread
Malignant tumors are deadlier than benign tumors due to their ability to metastasize. Tumors that metastasize spread to other parts of your dog's body.
If not treated early, using radiation, surgery or chemotherapy, secondary tumors can develop in other organs, and are difficult to treat. Your vet will most likely suggest euthanasia for a dog with later stages of malignant cancer.
In the later stages of cancer, malignant cancers will form a fibrous coating similar to the tissue that surrounds benign tumors. Although you can excise the tumors, at this point, secondary tumors will have formed in other organs that cannot be so easily dealt with.
Malignant cancers that develop in the skin, liver, pancreas, lungs, eyes, gastrointestinal and central nervous systems are known as carcinomas. Cancers that develop in the musculoskeletal system, blood vessels, connective tissues, and urinary tract are called sarcomas.
On a negative note, dogs that have been neutered quadruple the chance of developing prostate cancer. Dogs that have been fixed are also twice as likely to become obese, which increases the chance of urinary tract cancer, osteosarcoma and hemangiosarcoma.