Chemotherapy is often used in the treatment of a variety of cancers

Side effects in companion animals are much less severe and frequent compared to what humans experience

How often the medicine is given, for how long and how it is administered is based on your pet’s type of cancer and individual circumstances

About Chemotherapy

• Chemotherapy is often used in the treatment of a variety of cancers in companion animals. The type of cancer and extent of disease helps your oncologist decide what treatment protocol (type of drug, dose and schedule) to use. Chemotherapy can be used as the primary treatment, along with surgery and radiation therapy, or if no other treatment option exists. The following is a list of situations where chemotherapy may be recommended for you pet.
  
  • For tumors that are sensitive to chemotherapy – lymphosarcoma
    
  • For tumors that occur at more than one site – multicentric disease
    
  • For tumors that have spread to distant locations – metastatic disease
    
  • For non-resectable tumors that cannot be removed surgically
    
  • As follow-up therapy for tumors that are incompletely removed at surgery
    
  • As follow-up therapy to surgery or radiation therapy for locally invasive tumors that may spread

Dose and schedule

• The dose and schedule of chemotherapy treatments in companion animals are much less aggressive than they are in people. Unfortunately, that is why the cure rate in veterinary medicine is lower, however, it is also why companion animals do not have nearly the side effects that people can have.
• The type of drug chosen, how it is administered, how often it is given and how long it takes to give are all dependent on the diagnosis, the protocol your oncologist chooses and what chemotherapeutics constitute that protocol.
• There are injections that go under the skin (subcutaneous), oral tablets, oral capsules, injections that go in the vein quickly (over a few seconds to 10 minutes) and injections that go in the vein over an extended period of time (over four to six hours).
• Treatments are typically given every week to every three weeks although this varies depending on the diagnosis and protocol. The duration of the protocol also depends on the type of cancer and the stage of disease. Some pets need to receive chemotherapy for the rest of their lives, while treatment may be discontinued in others for a period of weeks to months if the tumor is in remission (there is no detectable evidence of disease). Chemotherapy is resumed when there is tumor relapse. If we are treating a visible tumor that can be measured, we will typically begin with about two treatments or four to six weeks of therapy to determine if a particular drug is effective.

Side Effects

• Most chemotherapeutic drugs are not specifically toxic to cancer cells, but to all rapidly dividing cells. That is why we see toxicity in normal, rapidly dividing tissue such as the hair follicles, intestines and bone marrow.
• Companion animal’s fur grows differently than human hair. Their hair follicles are not rapidly dividing like a human’s are, so your pet is not going to go completely bald with chemotherapy treatment. Some wire-haired or non-shedding breeds such as terriers and poodles have fur that grows more like human hair, so they can develop bald patches. Whiskers may fall out in both dogs and cats. Fur and whisker growth will resume when chemotherapy is stopped.     
• The cells along the lining of the intestines are also rapidly dividing and can be affected by chemotherapy. Mild, transient and self-limiting decreased appetite, vomiting and diarrhea are possible about three to five days after therapy. These are typically mild and go away on their own, however, there are medications that we can put your pet on to make them feel better. Additionally, we may recommend withholding food and water for a short period of time to allow your pet’s stomach to rest, followed by feeding a bland diet. Your pet is NOT going to be sick, in the hospital, lethargic, not eating, vomiting or having diarrhea during their entire treatment protocol and it is extremely rare that these intestinal signs are severe enough to require hospitalization.
• In order to replenish expiring blood cells, bone marrow cells are also rapidly dividing and thus can be affected by chemotherapy. Mild, moderate or severe decreases in total white blood cell level, neutrophils (infection fighting white blood cells) and platelets (produce clots and prevent bleeding) may occur about 7-10 days after therapy. Most of the time these are mild decreases that do not cause any clinical signs and take a few days to resolve. However, severe decreases can pre-dispose animals to sepsis (severe, systemic bacterial infection) and we may recommend prophylactic antibiotic therapy to try to prevent a serious infection. Additionally, routine white blood cell level checks are performed with a complete blood count (CBC) to monitor the white blood cell levels. It is rare that a pet becomes septic or sick enough to have to be hospitalized secondary to a decrease in white blood cell level from chemotherapy, although septic animals do require aggressive supportive care with intravenous fluids and antibiotics in the hospital. Certain chemotherapeutics may have other potential side effects that are specific to that drug and your oncologist will discuss these with you at the appropriate times.

Prognosis

• From what we know about a variety of cancers in pets, we may be able to give you a prognosis about life expectancy with and without treatment. Some tumors (lymphosarcoma) are very responsive to chemotherapy and prognosis can be very good with chemotherapy alone, while others are very resistant to chemotherapy and if surgery is not an option, chemotherapy can be used in a palliative manner. Palliation means attempting to slow the progression of the disease and maintain a good quality of life.