By Dr. Alison Book, DACVIM Oncology

When your pet is diagnosed with cancer, hearing the word ‘chemotherapy’ as a treatment option can be scary. The truth is that chemotherapy can often improve or maintain quality of life in dogs and cats, but it’s not for every pet or for every owner. Here’s what I tell my clients when they are deciding if chemotherapy is right for their dog or cat.

What chemotherapy is…

The term chemotherapy refers to a group of anticancer drugs that target and kill cells that are actively dividing, or growing. This group of dividing cells is often called a tumor. Cells in the tumor that are in the process of dividing are more vulnerable to the drugs. All chemotherapy drugs are not the same. There are many different types, each of which have different applications, side effects, and ways in which they kill cancer cells.

What chemotherapy is NOT…

Chemotherapy is not limited to killing only cancer cells. It can also affect normal cells that are dividing rapidly. These include cells that line the intestinal tract, blood cells produced in the bone marrow, and in certain cases, hair cells.

It’s important to know that there are other anticancer drugs such as steroids, non-steroidal anti-inflammatory drugs (NSAIDS), and treatments that are NOT considered chemotherapy. These drugs may not control cancer as long as chemotherapy drugs can, but they may slow cancer growth and improve your pet’s quality of life.

When would chemotherapy be recommended for my dog or cat?

There are several different times when we might recommend chemotherapy for your pet’s cancer. These include:

  • Types of cancer that affect the whole body (examples: nervous system cancers or blood cell cancers)
  • If the type of cancer is highly sensitive to chemotherapy
  • As an additional treatment after surgery when the particular type of cancer has an increased risk of spreading, or metastasizing, to other parts of the body.
  • As an additional treatment after surgery to prevent the cancer from coming back when it was not possible to remove the entire tumor
  • To decrease the size of a tumor before surgery so that it’s easier to remove
  • As a palliative treatment, meaning that the treatment will not cure the cancer, but may help to slow its progression and relieve the pet’s of some symptoms.
  • To make radiation therapy more effective

How is chemotherapy given in pets?

Chemotherapy is most commonly administered into a vein, by mouth, or as an injection under the skin. The type of drug determines how the drug will be given and how often. Most pets do not need to be sedated for treatment. We try to make it a positive experience for your pet!

What are side effects of chemotherapy in cats and dogs?

Surprisingly, side effects from chemotherapy in pets are often much milder than they are in humans. There are probably several reasons for this, including differences in how chemotherapy is given and the way dogs’ and cats’ bodies handle drugs. Not all pets will experience side effects, and it is our goal as veterinary oncologists to ensure your pet’s good quality of life during treatment. Many owners will tell us that they can’t even tell their pet is receiving chemotherapy!

Possible side effects of chemotherapy:

  • Gastrointestinal upset: Side effects such as nausea, decreased appetite, vomiting, and diarrhea are most likely to occur 3-5 days after treatment. Most often, these are mild and self-limiting or preventable by administering supportive medications.
  • Bone marrow suppression: Neutrophils, a type of white blood cell, may become low, typically 7-10 days after treatment. Your veterinary oncologist will probably recommend checking your pet’s blood counts during this time. If these cells are low, most pets won’t feel any different but may need to take antibiotics. In rare cases (<5%), pets can develop a serious infection requiring hospitalization. Platelets, the cells that make blood clots, may also be susceptible to the effects of chemotherapy and are monitored closely by your veterinary oncologist.
  • Hair loss: Most pets will not lose their fur during chemotherapy. The exception to this is dogs with hair instead of fur (Poodle, Bichon Frise, etc). Also, fur that was shaved for a diagnostic or surgical procedure may be slow to regrow, and cats and dogs can temporarily lose their whiskers.
  • Extravasation: This means that a drug that was meant to be delivered into a vein leaked into the surrounding tissue. For some drugs, this can lead to serious tissue damage and is considered a medical emergency. For these drugs, perfect catheter or needle placement is required to prevent this. A veterinary oncologist has technicians that are highly trained in administering chemotherapy drugs in a safe fashion.

Risks unique to particular chemotherapy agents: Some drugs have unique side effects in dogs and cats. Your veterinary oncologist may want to check your pet’s liver, kidney, or heart health before administering particular chemotherapy drugs. Also, your doctor may use certain medications before or after treatment to reduce the risk of these side effects.

ow to decide whether your pet needs chemotherapy: We recommend chemotherapy based on a thorough evaluation of your pet’s medical history and the type of cancer that they have. We try to balance the risk for side effects with the benefits of therapy. Generally, if the benefits outweigh the risk, then we would recommend chemotherapy. Many times, this is the case and we can really help! It is best to have a conversation with a veterinary oncologist to make sure that you are making the best decision for your family.