Chemotherapy Options for Pets with Cancer

By Joanne Lynn Intile, DVM, MS, DACVIM on Aug. 14, 2013

It’s been two weeks since Duffy’s amputation surgery, and he’s coming in to see me for a check-up, and a final discussion on what our plan for his future will be. I anxiously awaited his arrival to my examination area, recalling how when he went home two days after surgery he seemed slightly sluggish with some impressive swelling and redness over his incision site.

Duffy definitely showed some difficulty rising from recumbency and even slipped once on the tiled floor of the waiting room while being discharged from the hospital.

Before I actually saw Duffy, I heard him — or rather I heard the rapid footsteps of one of our oncology technicians pattering down the hallway, the distinct jingling of tags swinging against a collar, and the heavy panting (which I truthfully could not distinguish as being canine or human in origin). The door to my examination area opened and in bounded a massive storm of golden fur and wet slobbery kisses attached to a giant pink tongue.

“Duffy’s doing great at home,” the technician stated as she adjusted her now lopsided ponytail while simultaneously catching her breath. “I think they are leaning towards chemotherapy!”

The fact that Duffy was bouncing around the room on three legs without hesitation did not surprise me at all. Most dogs recover from amputation surgery within a short period of time. Dogs who are overweight or have significant orthopedic disease may not be quite as agile as Duffy at this time point, and can benefit from post-operative physical therapy to strengthen other muscles and joints.

As an alternative to amputation for dogs with tumors located in the lowest portions of the radius or ulna (bones of the forelimb), metacarpal or metatarsal bones (longer bones of the paws), or digits (toes), owners also have the option of a “limb-sparing” surgery. In this surgery, the affected portion of the bone is removed, leaving the limb in place.

These can be technically challenging surgeries and complications can arise, including post-operative infections and regrowth if any portion of the tumor is left behind. Owners should really only consider this form of surgery if they are willing to commit to treating their pet with chemotherapy afterwards.

I typically discuss two main chemotherapy options for dogs with osteosarcoma: pursuing injectable chemotherapy versus pursuing treatment with a newer form of treatment called metronomic chemotherapy.

Injectable chemotherapy is the most well studied form of treatment for dogs with osteosarcoma. There are three drugs that are effective for this disease: doxorubicin, cisplatin, and carboplatin. Numerically, the outcomes are similar for each drug, though it is important to point out that no one has adequately performed the perfect study comparing the efficacy of each drug in a “head to head” fashion.

We generally say the prognosis for dogs treated with amputation alone is about 4-5 months. With additional chemotherapy with doxorubicin, cisplatin, or carboplatin, survival is extended to about 12 months, with approximately 10-15 percent of dogs surviving two years.

Doxorubicin is an intravenous drug given once every three weeks for a total of five treatments. This drug is usually well tolerated but has a moderate chance of causing upset stomach signs. There is a risk for toxicity to the heart; a problem seen when dogs receive more than six lifetime dosages, which is one of the main reasons we stop at five treatments.

Cisplatin is an intravenous form of chemotherapy administered once every three weeks for a total of four treatments. Of the three drugs, it is the one most likely to cause side effects in dogs. It is an example of a chemotherapy drug that can immediately cause nausea and vomiting, so it must be administered with anti-nausea medications. This drug can also be directly toxic to the kidneys, so it must be administered with an all-day intravenous fluid diuresis.

Carboplatin is also an intravenous drug given once every 3-4 weeks for a total of five treatments. Side effects are uncommon, but it can definitely cause lowered white blood cell counts.

Metronomic chemotherapy is also known as anti-angiogensis therapy. The idea behind this form of treatment is in order for tumor cells to grow, multiply, and spread, they need to grow their own blood supply. If this process can be inhibited, then it may be possible for dogs to live with tumor cells in their body, but the cells will not grow, or may grow at a slower rate.

This form of treatment is becoming a popular treatment options for pets with cancer, mainly because these are oral formulations of drugs owners can administer at home. There is limited research in veterinary medicine supporting the efficacy of this method of treatment, however a few small studies showed potential benefit for dogs with cancers other than osteosarcoma.

I discuss metronomic chemotherapy as a “stand-alone” option, but recommend the injectable protocols initially, and then consider stating a dog on metronomic treatment once the finish their protocol. More on this form of therapy will follow in a future article.

I completed my exam of Duffy and headed in to talk with his owners. We went over the pros and cons of treatment with chemotherapy versus starting Duffy on a close monitoring program. Ultimately, they elected to start treatment with carboplatin, knowing we were ideally committing to completing all five treatments, but knowing we were going to take things day by day.

Their fears were the same as any owner starting treatment with chemotherapy, but they knew they wanted to give Duffy every available chance for long-term survival.

Duffy received his first chemotherapy as planned, and is currently feeling great at home. He’s chasing squirrels, stealing cookies from the table, and acting generally “normal” at home.

From my perspective, Duffy is a true success story. He may not be breaking any records, but I feel confident he will continue to do well and share his happiness with his family for a long time.