“When we diagnose cancers,” says Tufts veterinarian Michele Keyerleber, DVM, DACVR (Radiation Oncology), “occasionally they’re already in a very advanced state. The tumor is too large or has spread to other sites and carries a very poor prognosis, so that surgery to remove the mass or chemotherapy to kill cancer cells in different tissues — we’re kind of beyond that. Even if we treated the cancer as aggressively as possible, we would not be able to cure it” and would therefore be putting the animal through the rigors of a difficult operation or a chemo regimen with little hope of survival.
Why do people wait so long to bring a cancer-ridden dog to the vet’s office? “Signs can be hidden for a while,” says Dr. Keyerleber. Some of the signs are non-specific and come on gradually so aren’t necessarily taken as life-or-death symptoms. These may include weight loss, loss of appetite, an offensive odor, or loss of stamina. Couple such signs with the fact that cancer is more common in older dogs, and it’s easy to understand that some people confuse plain old aging with warnings of a dire disease. Then, too, even signs that are not as subtle — abnormal swellings that persist or grow, bleeding or discharge from any of the body openings, sores that don’t heal — often go unheeded precisely because they’re unnerving. People are afraid to hear bad (and potentially expensive) news.
It’s understandable. Nonetheless, cancer is in many pets’ futures. “At least 50 percent of dogs over age 10 will experience cancer at some point,” notes Dr. Keyerleber.
Fortunately, for those dogs whose cancers are too far advanced or whose tumors for whatever reason will not be responsive to surgery or chemotherapy, there’s an option that can make the rest of their time with you as comfortable and asymptomatic as possible.
When palliative radiation therapy becomes an option
Some cancers can be cured. A diagnosis of a malignant tumor does not always signify that a dog’s life will be cut short. For instance, some dogs with mammary cancer experience complete recovery. It’s the same with low-grade mast cell cancer, as well as with soft tissue sarcomas — cancerous tumors arising from connective tissue in muscles, nerves, and the like.
In cases where the chance for cure is reasonably high, what’s known as definitive radiation may sometimes be employed to kill any potential remaining cancer cells at the site where the bulk of the tumor was removed via surgery. In those cases, where “cure” or long-term remission and a significantly greater survival time is the goal, the dog will have to come to the hospital for “daily radiation treatments, Monday through Friday, for about three or four weeks, sometimes longer,” says Dr. Keyerleber, herself a radiation oncologist who designs and administers radiation therapy for dogs with cancer. The treatment itself, or “beam-on” time, as Dr. Keyerleber calls it, is less than five minutes. “But,” she adds, “veterinary patients need to be put under general anesthesia. You can’t tell a dog to stay still on top of a table in a room with no one in it. So altogether the appointment takes about an hour, both to administer the anesthesia and to let it wear off once the radiation is complete.”
When the cancer can’t be cured — “if life expectancy is on the order of six months or less,” says Dr. Keyerleber — “we don’t want to put the dog, or the owner, through all that.” Nor is the veterinarian going to submit a dog whose illness is definitely terminal to an operation or a long course of definitive radiation therapy.
Enter palliative radiation therapy. “It’s a shorter course of radiation treatment at a lower dose,” Dr. Keyerleber says, “because you’re just treating clinical signs to make the dog more comfortable, to feel more like herself and without pain for as long as possible.” How short a course of radiation are we talking about?
“There are lots of protocols out there,” explains Dr. Keyerleber, “and not many studies to show there’s a big difference between them in terms of efficacy. So it often boils down to logistics — what works well for an owner without compromising the good that can be done for the dog. Maybe it’s daily treatment for five days or even one day instead of 20. Or once a week or twice a week for a series of four or five weeks. Or you can have four treatments over the course of two days, with six to eight hours between treatments. That can be very appealing for clients coming a very long distance and having to stay in a hotel. There are a lot of options in terms of helping. Sometimes even one treatment alone can provide pain relief.”
The nature of the relief
These short courses of radiation therapy are “really effective,” Dr. Keyerleber says. “The majority of dogs will experience significant relief.”
One of the cancers from which dogs find pain relief with palliative radiation therapy is osteosarcoma, or bone cancer. Bone cancer is notorious for causing excruciating pain, but radiation for that kind of tumor helps in two ways. In the short term, it leads to the release of molecules in the body called cytokines that can “modulate pain locally,” says Dr. Keyerleber. Between 75 and 95 percent of patients will experience pain relief, which comes as quickly as seven to 14 days after the first radiation treatment. But there’s also a longer-term response in that palliative radiation therapy can induce bone remodeling. “When a cancerous tumor eats bone, it becomes like a block of Swiss cheese,” Dr. Keyerleber comments. “But radiation can change the environment in such a way that it leads to recalcification,” which means filling in the holes — and ratchet down the pain that way.
Any tumors impinging on an outflow pathway from the body — such as an airway or tracheal cancer; a urinary tract tumor that interferes with the ability to urinate; or a GI tumor such as a rectal tumor that impedes the ability to defecate — also tend to respond well to palliative radiation therapy. “These types are probably the ones we treat with palliative radiation therapy most often,” Dr. Keyerleber says.
Into this group of “outflow track” tumors fall nasal tumors. “This type of tumor is quite a big thing with dogs,” says Dr. Keyerleber. “Long-nosed dogs in particular seem to get it disproportionately. Potentially it has to do with their having a lot more developed nasal passages. They’re scrolled up in there, and when a dog sniffs in environmental toxins, it’s theorized that they can settle down in the nose and lead to cancer.” (Short-nosed dogs, she notes, tend to get lung cancer — they don’t have the extensive nose filter, so the toxins are believed to go straight to their lungs.)
One dog Dr. Keyerleber treated with palliative radiation therapy for a malignant nasal tumor was an Australian cattle dog named Tazman. “She was almost nine years old when she was diagnosed,” says her owner, Romona Kirker-Head. “We saw that she was slowing down,” Ms. Kirker-Head relates, “but then she developed a backward sneeze. I thought she was trying to clear her throat. Then she started getting nosebleeds. That’s when we brought her in.
“The tumor was quite large. It was in her nasal cavity and eating through a part of her skull. From there we were referred to the oncology vets at Tufts. It was so stressful. The tumor was so far gone, but we wanted to buy her some time so we decided to have her treated with the palliative radiation therapy — a short blasting course.
“Very quickly the bleeding stopped — they had not promised me it would. We went on to have some very good time together. We had six good months that I wouldn’t have otherwise had. She started to perk up and feel better. The backwards sneezing stopped. She wasn’t a young dog to start with, so it wasn’t like she had a ton of energy, but I live on a farm and she was back out doing chores with me; she was more herself.
“She was my best friend in the world. It’s still pretty fresh for me,” says Ms. Kirker-Head, crying. “But I’m very glad I did it. We had really good time right up until her last month.”