Very uncommonly,  dogs may develop a tumor where they were given a vaccine.  One theory is that when your dog gets a vaccine, his body’s immune system will rush to that spot to fight off the invasion, and that inflammation can cause cells to become cancerous and spread, forming a tumor. Another idea is that getting vaccines every year (i.e. rabies) in the same spot is causing a buildup of medicine from the vaccine and scar tissue from repeated injections. Yet another group of experts claim the cause is from the aluminum the drug companies started including in the vaccines after the United States Department of Agriculture (USDA) insisted that the vaccines not include live virus in its components. While quite uncommon, vaccine associated sarcoma is a serious and fast-moving invasive cancer that usually does not have a good outcome unless caught very early. 

A sarcoma in general is a cancerous lesion or tumor made up of connective tissue cells that develops fast and spreads rapidly. Unfortunately, this type of cancer is resistant to treatment, tends to return, and is fatal in most cases.

Many dogs with sarcomas may not have any other symptoms other than a lump at the site of the injection that quickly grows and turns into a large ulcerated sore. It is when the cancer starts affecting other parts of the body that you might see these other symptoms:

Partly because the problem is so uncommon, the cause of vaccine associated sarcoma is controversial, as some veterinarians do not believe the cancer to be caused by vaccines and others claim that they are related. Even experts that claim that these sarcomas are caused by vaccines discuss several different reasons behind these claims. These claims are:

Inform the veterinarian about the symptoms that brought you to visit and that you think it may be caused by an immunization. They will try to determine if the immunization was the cause or if another reason exists for the tumor. A complete and comprehensive full body examination will be done first so the veterinarian can look for signs of any underlying illness that could be causing the symptoms you describe. Your dog’s weight, height, body temperature, blood pressure, pulse and respiration rate will be documented and then the veterinarian will order some tests.

The diagnostic tests your veterinarian might perform are blood chemical panel, complete blood count (CBC), blood titers (antibody count), urinalysis, and blood coagulation (clotting) test. They may also do a Coomb’s test (direct antibody test) to determine which antibodies are attacking your dog’s red blood cells. However, the best test to diagnose vaccine associated sarcoma is a biopsy of the tumor. The veterinarian can do this by taking a fine needle biopsy, in which he inserts a thin needle into the tumor and removes cells to be examined under a microscope. Images may also be needed of the area where the tumor is present as well as a whole body MRI or CT scan to determine if the cancer has metastasized (spread). An ultrasound may also be done to find the edges of the tumor as another verification before treatment is done.

The most effective treatment of vaccine associated sarcoma is surgery to remove the tumor and adjacent tissues and lymph nodes if it has not spread to other vital organs. Your veterinarian may decide to do radiation treatment before surgery, after surgery, or both, depending on your dog’s health and age. Chemotherapy doesn't tend to be effective for this kind of cancer.

After surgery, the chances of your dog’s recovery is fair, depending on whether the veterinarian was able to remove all of the tumor and enough of the surrounding tissue. In many cases, the tumor will return, and the best choice at that point is usually just palliative treatment (pain medication and fluids) to make your dog more comfortable in the time he has left. It is best to speak with your regular veterinarian about how often to vaccinate your dog, based on their lifefstyle and risks.