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How To Better Understand Chemotherapy in Pets

Chemotherapeutic (anti-cancer) drugs are used in the treatment of several types of cancer in pets.



The type of cancer and extent of disease help us decide what protocol(type of drugs, dose, and schedule) to use for treatment.


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When do we use chemotherapy to treat animals with cancer?

We use it in the event of:

  • multicentric disease (tumors occurring are more than one site).
  • metastatic disease (cancer that has already spread).
  • nonresectable disease (tumors that cannot be removed surgically).
  • follow-up therapy after surgery when the tumor has not/cannot be completely removed.
  • follow-up therapy after surgery when we are treating tumors that usually metastasize early.

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Will there be side effects from the drugs?

Compared to people treated with chemotherapy, we see fewer side effects in pet animals receiving these drugs. In animals, we use lower doses, and do not combine drugs as often.

Most of the chemotherapeutic drugs are not specifically toxic to cancer cells, but to all cells that are dividing rapidly. This is why we see toxic effects in normally rapidly growing cells of the body, and many of the side effects are due to this.

The cells in the bone marrow, the intestinal lining, and hair follicles in some breeds of dogs (e.g. poodles, terriers) are rapidly dividing cells, and consequently more sensitive to chemotherapy.

The most common side effects are bone marrow suppression and vomiting/diarrhea. Whiskers of cats usually fall out, but regrow when chemotherapy is stopped.

Bone marrow suppression may cause a drop in the white blood cell count and increased susceptibility to infections. Severe infections may require intensive supportive care, including intravenous fluids and antibiotics.

The gastrointestinal signs may be mild, moderate, or severe. Although infrequent, some dogs may develop severe diarrhea requiring fluid therapy in the hospital.

We see side effects as described above very seldom (i.e., less than 5% of all pets receiving chemotherapy). With proper therapy, most animals recover uneventfully within several days.

Most of our patients experience only mild side effects, such as transient nausea, lethargy, reduced appetite, and mild diarrhea for a few days after treatment.

If your pet is treated with drugs known to cause side effects, we will give you instructions on what to do if there is a problem.

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How are the drugs given? How often and how long does treatment last?

This varies, depending upon what type of cancer we are treating and which drugs we are using. Some of the drugs are oral medication (pills) that you give at home, while others are injections or slow intravenous infusions that may require 1-2 days in the hospital. The treatments are usually repeated weekly, every other week or every third week.

It is most important that you, as an owner, are committed to treatment and bring your pet in when scheduled for therapy.

The duration of the chemotherapy depends on the type of cancer and stage. Some animals need to receive chemotherapy for the rest of their lives, but in others, treatment may be discontinued for a period (weeks to months) if the tumor is in remission (i.e., not evidence of disease or NED). Chemotherapy is resumed when there is a tumor relapse.

We usually recommend that every patient receive at least two cycles of chemotherapy and then be evaluated for response before we decide to continue the treatment, change the drugs, or discontinue chemotherapy.

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What can you expect from chemotherapy?

In many cases, we are not able to cure our patients with cancer. We are often talking about palliation, i.e., prolonging your pet’s life and slowing down the progression of the disease. From what we know about the type of cancer your pet has, we may be able to give you a prognosis about life expectancy with chemotherapy. We want to give your pet a long life while striving for a good quality of life.



Veterinary Hospital of the University of Pennsylvania. © Copyright, 1994 OncoLink


NB-In my experience, homeopathic palliation often provides a much better quality of life.–Dr. Jeff