Cancer Considerations

Spayed  females had more than 5 times greater risk than intact bitches of developing
cardiac hemangiosarcoma and neutered  males had  1.6 times higher risk than intact males had of developing  cardiac  hemangiosarcoma.

Spayed females had 2.2 times increased risk for developing  splenic  hemangiosarcoma
In a second study, spayed/neutered dogs  had  a 2.2 times higher  risk of developing
bone cancer than intact dogs.

Neutered dogs had a 2.8 times greater risk for developing any prostate than intact dogs.

Neutered dogs had a 4.3 times higher risk of developing prostate carcinoma.

Neutered dogs had a 3.6 higher risk for developing transitional cell carcinoma of the bladder
than intact dogs and a 3 times greater risk of developing any bladder tumor.

Spayed/neutered dogs had more than 4 times greater risk for developing transitional cell carcinoma of the bladder than intact dogs.

One study indicated a slightly increased risk of mammary cancer in female dogs after one heat cycle (8% increase), greater risk with two heats (26% increase) and increased risk with each subsequent heat.

However, a recent  systematic review  of the publications that advocate neutering to reduce the risk of mammary  tumors in dogs indicated that 9 of 13 reports had a high risk of bias and the remaining 4 had a moderate risk of  bias. This study concluded that the evidence that neutering reduces the risk of mammary cancer is weak and do not constitute a sound basis for firm recommendations.

Additionally, at the time when several of these studies were conducted (late 1960s), it was found that incidence rates for all malignant neoplasms were 4/100,000 female dogs. Mammary tumors accounted for half of these tumors, 8/100,000. Thus, the actual overall risk  at that time of any bitch getting a mammary tumor was only 0.2%.

In any case, the figures for increased risk of mammary cancer must be compared with the 200 to 400% increased risk of other cancers in spayed females.

While about 30% of mammary cancers are malignant as in humans, when caught and surgically removed  early, the prognosis is very good)

This is in comparison to the other cancers listed, such as hemangiosarcoma
and bladder cancer, which are often fatal. 

Given the balance of cancer risks listed above, owners  should strongly consider leaving the ovaries intact for at least two heat cycles. In addition, the veterinary  field should be developing programs for regular examinations  including imaging to facilitate early diagnosis of mammary cancer in all intact female dogs, as has been performed in women for decades.

Behavior Considerations
Early age gonadectomy was associated with an increased incidence of noise phobias and undesirable sexual behaviors, such as mounting. Significantly  more behavioral problems in spayed and neutered bitches and dogs, with  fearful behavior being most common in spayed bitches and aggression in neutered dogs.

In 2011, researchers at the University of Georgia College of Veterinary Medicine found that cancer was the most common cause of death in older dogs. (It’s also the leading cause of death for 71 of the 82 breeds studied.)

what your vet says should take precedence over this - I will touch on some of the more common tumors/cancers as we've had several dogs over the past 4 decades with cancer and more with lipomas which while worrisome are not something to panic over -but should be monitored letting your vet know of rapid changes.

Not all tumors in dogs are cancerous, but all of them should be evaluated by your veterinarian.

Lipomas
Christine Swanson, DVM, a veterinary medical oncologist and assistant professor at the College of Veterinary Medicine at Michigan State University, calls this benign fatty tumor very common and notes that many different breeds are prone to developing them. Lipomas usually feel like a relatively soft mass that can be moved around underneath a dog’s skin.
They vary in size, and most of the time, they’re not a serious issue. A fine needle aspirate is done to confirm the benign nature of the tumor, and the tumor is usually only removed if it’s bothersome to the dog’s normal movement or activity

Mast Cell Tumors
mast cell tumors are one of the most common type of canine skin tumors. These grow quickly and are usually red and very itchy.

Melanoma
This is a form of cancer of the pigmented cells of the skin of dogs, and like melanoma in people, these tumors are typically black or dark brown. Many skin masses are benign, but those in the mouth and at the nail bed can be very aggressive, she adds. In the case of the latter, the toe is typically swollen and may be painful. Following an x-ray, it may be determined that the affected toe must be amputated in order to fully remove the cancerous mass.

The risks with this specific type of melanoma don’t end there. It may metastasize to places such as the lymph nodes in the area and the lungs, liver, or other internal organs. Once evidence of such metastasis has been identified, some combination of surgery, radiation therapy, and immunotherapy (a therapeutic vaccine for canine melanoma has been licensed by the USDA)

Lymphoma
Lethargy, decreased appetite, and coughing may accompany swollen lymph nodes in dogs of all breeds with this type of cancer, although some individuals initially show little in the way of symptoms other than lymph node swelling. this swelling is most noticeable under the jaw, in front of the shoulders, and behind the knees.
A fine needle aspirate and/or tissue biopsy is done to reach a diagnosis. Then, a veterinary oncologist will conduct something called a staging test to determine where else in the body these cells might be. The most common treatment is chemotherapy.

Papilloma
These benign tumors are warts in dogs, and Swanson says they can be uncomfortable and problematic. When this infection develops, multiple hard, pale, cauliflower-like warts are noted typically on the lips, inside the mouth, and around the eyes. The warts can be painful and severe infections can make chewing and swallowing difficult.

Papillomas will go away after a few weeks, sometimes months—though if they’re causing major problems for the dog in question, they can and should be removed by a veterinarian.
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These benign tumors are caused by a virus (called papillomavirus) that is transmitted by direct contact with an infected dog or contaminated objects like bedding or toys. While it’s best to keep affected dogs isolated from unaffected ones, the incubation period often lasts months, so by the time symptoms make themselves known, it might have already spread to other dogs in a household.