Chemotherapy

Remission vs. cure

"Remission" is not a cure. It is a time interval during which there are no outward signs that the patient has cancer. Considering the relative life span of animals, increasing the period in which they can enjoy a good quality of life – for months or even years – is significant.

For example, in the case of canine lymphoma: 80% or more of dogs are expected to go into full remission, and the average overall survival time is approximately 12 months. Unfortunately, it is impossible to predict which dogs will achieve a full remission or for how long. We do know, however, that if no therapy is used, most dogs will die from their disease in a few weeks.

All of the drugs currently given to animals are human anti-cancer drugs.

They work by targeting and damaging or killing rapidly growing cells. For the patient, this means cells lining the stomach and intestine (high turnover rate), cells of the bone marrow that make up the immune system (white blood cells, in particular), and cancer cells. These medications are extremely potent, and must be carefully chosen on a case by case basis. Considerable expertise is needed to choose the most appropriate drugs, dosages and treatment protocol for each individual patient. Veterinarians that are board certified in medical oncology have specialized knowledge that is vital in making these critical decisions.

Most patients complete their course of cancer therapy without major complications.

Side effects tend to decrease in severity over time, as the unique drug sensitivities of each animal become better defined, as treatments are given less frequently, and once the cancer is in remission. Owners can be reassured that the most common side effects should be mild to moderate and may include decreased appetite, nausea, vomiting, diarrhea, and increased risk of infection caused by bone marrow suppression. Hair loss is uncommon in cats and most dogs on chemotherapy, although certain breeds such as terriers and poodles may experience more significant hair loss.

Normal cell populations, such as those in the intestine or bone marrow, can almost always regenerate themselves. This means that most chemotherapy-related complications that occur in these tissues resolve relatively quickly. At the same time, the less well organized malignant cells will suffer greater damage and have a decreased ability to repair themselves. However, even after treatment using higher doses of chemotherapy, microscopic clusters of malignant cells remain dormant but alive in the body, and will eventually give rise to drug-resistant cell lines. This is how malignant cancers recur and metastasize, and this is why many cancers that are very treatable with chemotherapy cannot necessarily be cured.

Two of the cancers most commonly treated with chemotherapy at the Western Veterinary Cancer Centre, and with the most success, are lymphoma and canine osteosarcoma.

Lymphoma

Lymphoma is one of the most common malignancies diagnosed in dogs and cats. This tumour arises from lymphocytes, and may occur in virtually any organ or tissue. However, dogs with lymphoma usually have dramatic lymph node enlargement, while lymphoma most often occurs in the gastrointestinal tract in the cat. Lymphoma is almost always a systemic cancer, with each patient having multiple sites of involvement throughout the body. Besides lymph nodes and the gastrointestinal tract, other organs that are commonly involved include the liver, the spleen, and the bone marrow.

Although complete cures are rarely achieved, treatment of lymphoma with chemotherapy is frequently very successful. There are many "protocols" or treatment schedules available for canine and feline lymphoma, although no single protocol has as yet been proven superior to all others. Most published protocols contain the same or similar class chemotherapy agents, with minor variations on the combination of agents and timing of treatments. Most veterinary oncologists choose treatment protocols based on the strength of the published evidence of their effectiveness, how likely they are to preserve the well-being of the patient, and what fits best with the lifestyle and expectations of the client.

A brief synopsis of the most common treatment protocols for canine lymphoma:

Canine Osteosarcoma:

Osteosarcoma is the most common primary bone tumour of dogs. Large and giant dog breeds are at highest risk to develop this malignancy. This tumour is locally destructive to surrounding normal tissues, and has a high metastatic rate (tendency to travel to distant sites in the body). It is typically quite painful when it is actively growing within a bone. Bones of the limbs that are infiltrated by osteosarcoma can be weakened to the point of a fracture (called a pathological fracture).

Fortunately, most dogs that undergo amputation for osteosarcoma improve dramatically because a source of constant, severe pain has been removed. However, surgical resection alone does not prolong the dog's survival time for more than 3 to 4 months, since osteosarcoma has almost always spread microscopically in the dog's body (usually to the lungs) by the time the diagnosis is made. We strongly recommend post-surgical chemotherapy to treat these metastases, and ensure as long a remission time as possible. Four to six doses of chemotherapy are given at 3 week intervals. Although they are not cured of their cancer, dogs receiving postoperative chemotherapy for osteosarcoma have an average survival time of 12 months, and 25% of dogs live 2 years or longer. Hospitalization for treatment-related toxicity is rare, and most dogs are lucky enough to have an excellent, pain-free quality of life at home with their family doing virtually all of the things they enjoy. Without treatment, dogs with osteosarcoma usually succumb to their disease within 1-2 months from the time of diagnosis.

The most common chemotherapy agents used to treat osteosarcoma are Doxorubicin or Adriamycin,¨ and platinum compounds such as Carboplatin or Cisplatin. Because Adriamycin¨ and both of the platinum drugs are synergistic with respect to their anti-cancer activity, they are often alternated in treatment protocols. Treatments are performed every 3-4 weeks, for a total of 4-6 treatments. Dogs receiving Adriamycin or Carboplatin are admitted as day-patients. Dogs receiving Cisplatin must stay in the hospital for one night to get intravenous fluids prior to drug administration. This is necessary to prevent the kidney damage that may otherwise be caused by this drug.