Chemotherapy
Introduction
The goal of chemotherapy is to control the cancer and to prolong survival while maintaining a good quality of life for the
pet. In some cases, chemotherapy may result in a cure, but in most cases, it 'buys' time. It is critical that pet owners
understand the difference between remission (resolution) and cure. For example, dogs diagnosed with lymphoma
experience a rapid remission after initiating chemotherapy. Many pet owners might consider this initial response as cure,
however, the effect is not long term and many dogs will inevitably experience recurring cancer.
Chemotherapy uses
Chemotherapy is typically used for the following circumstances:
1. For pets with a tumor known to positively respond to chemotherapy
2. As complementary therapy to eradicate micrometastasis
3. As complementary therapy to prevent the recurrence of local tumor after incomplete surgical removal.
4. To ease the clinical signs associated with inoperative or metastaic cancer
5. To sensitize tissues to radiation therapy
6. If the localized tumor has a high likelihood of metastasizing
How does chemotherapy work?
Chemotherapeutic drugs kill cancer cells by several different mechanisms, depending on the class of drugs.
Unfortunately, the drugs also target normal cells which leads to significant side effects in the treated patient. The goal of
oncologists is to use as much drug as possible to increase the likelihood of killing cancer cells but without causing
substantial harm to the pet along the process. The pet will usually experience nausea, especially at the beginning of the
treatment.
The doses of chemotherapy at the beginning of treatment are usually high in order to induce remission (tumor
shrinkage) in the pet until no more visible tumor masses remain. After this period, the remaining cancers can be
removed surgically or the pet will have to continue to take chemotherapy, although less frequently and at lower dose, to
maintain the remission state.
Chemotherapy efficacy and resistance
There are many factors that will affect how effectively chemotherapy will work for your pet and resistance remains as one
of the biggest challenges in controlling cancer. Some tumors will contain cells that will be resistant to the treatment from
the beginning or will become resistant after being exposed to the treatment. Some tumor cells will begin to produce
molecules that will inactivate the chemotherapeutic drugs, or prevent the drugs from gaining access to the cells, or
prevent the cells from dying, all of which will render the therapy ineffective.
Chemotherapy vocabulary
Pet owners should familiarize themselves with the commonly used terms relating to chemotherapy treatment in order to
gain better understanding of the procedures performed on their pets.
Maintenance therapy - Continuation of a less intensive chemotherapy treatment for the purpose to maintain a remission
(resolution) and to prevent relapse (recurrence).
Complete remission (CR) - Complete resolution of measurable tumor after chemotherapy based on physical examination,
monitoring or imaging.
Partial remission (PR) - Reduction in the measurable tumor after chemotherapy without development of new tumors.
Stable disease (SD) - Stable disease shows neither worsening nor improvement.
Progressive disease (PR) - Progressive disease shows signs of tumor growth and/or development of new tumors.
Median duration of response/median survival - The point at which half the pets have relapsed or died and half are still in
remission and alive.
Adjuvant chemotherapy - Administration of chemotherapy after surgery or radiation therapy in cases with high risk of
recurrence of metastases.
Primary (neoadjuvant) chemotherapy - The initial treatment (typically for localized cancer) by chemotherapy, followed by
surgery or irradiation.
Maximal tolerated dose (MTD) - The maximum recommended dose of a chemotherapeutic agent based on toxicity.
Combination chemotherapy
It is generally believed that single drug treatment is not curative and the combination of several drugs has a better
chance of improving survival. By the time the tumor is detected, it will contain many different types of cancer cells, each
with a differential response to a particular drug. By combining several drugs, we are increasing the chances of killing all
the different types of cells present in the tumor. Combination therapies are designed so that maximum efficacy can be
achieved with acceptable toxic side effects.
Toxicity of chemotherapeutic drugs
Most chemotherapeutic drugs are well tolerated by most companion animals. Less than 5% of pets will have to be
hospitalized and less than 1% of pets will die as a direct result of drug-associated toxicity. There are, however commonly
experienced side effects within one week of beginning therapy such as nausea, vomiting, diarrhea and/or lack of
appetite. The use of anti-diarrhea or antiemetics usually offset some of these side effects.
Another commonly experienced side effect is myelosuppression, a term referring to the killing of blood stem cells in the
bone marrow. When the blood cell count becomes very low, chemotherapy is usually delayed until the count is restored
to acceptable levels. When the pet experiences severe side effects, the dose of chemotherapy is typically lowered but at
the same time, lowering of the dose also lowers the efficacy of the drugs in killing tumor cells.
Some dogs with rapidly growing hair coats (eg poodles, terriers, Old English sheepdogs) lose hair during chemotherapy
and cats may lose their guard hairs and whiskers while undergoing chronic chemotherapy. The hair will regrow after
discontinuation of chemotherapy, although it may differ in color and texture.
Chemotherapeutic drugs
The following table summarizes commonly used classes of chemotherapeutic drugs and their associated toxicities.
Source: Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
Sources:
The goal of chemotherapy is to control the cancer and to prolong survival while maintaining a good quality of life for the
pet. In some cases, chemotherapy may result in a cure, but in most cases, it 'buys' time. It is critical that pet owners
understand the difference between remission (resolution) and cure. For example, dogs diagnosed with lymphoma
experience a rapid remission after initiating chemotherapy. Many pet owners might consider this initial response as cure,
however, the effect is not long term and many dogs will inevitably experience recurring cancer.
Chemotherapy uses
Chemotherapy is typically used for the following circumstances:
1. For pets with a tumor known to positively respond to chemotherapy
2. As complementary therapy to eradicate micrometastasis
3. As complementary therapy to prevent the recurrence of local tumor after incomplete surgical removal.
4. To ease the clinical signs associated with inoperative or metastaic cancer
5. To sensitize tissues to radiation therapy
6. If the localized tumor has a high likelihood of metastasizing
How does chemotherapy work?
Chemotherapeutic drugs kill cancer cells by several different mechanisms, depending on the class of drugs.
Unfortunately, the drugs also target normal cells which leads to significant side effects in the treated patient. The goal of
oncologists is to use as much drug as possible to increase the likelihood of killing cancer cells but without causing
substantial harm to the pet along the process. The pet will usually experience nausea, especially at the beginning of the
treatment.
The doses of chemotherapy at the beginning of treatment are usually high in order to induce remission (tumor
shrinkage) in the pet until no more visible tumor masses remain. After this period, the remaining cancers can be
removed surgically or the pet will have to continue to take chemotherapy, although less frequently and at lower dose, to
maintain the remission state.
Chemotherapy efficacy and resistance
There are many factors that will affect how effectively chemotherapy will work for your pet and resistance remains as one
of the biggest challenges in controlling cancer. Some tumors will contain cells that will be resistant to the treatment from
the beginning or will become resistant after being exposed to the treatment. Some tumor cells will begin to produce
molecules that will inactivate the chemotherapeutic drugs, or prevent the drugs from gaining access to the cells, or
prevent the cells from dying, all of which will render the therapy ineffective.
Chemotherapy vocabulary
Pet owners should familiarize themselves with the commonly used terms relating to chemotherapy treatment in order to
gain better understanding of the procedures performed on their pets.
Maintenance therapy - Continuation of a less intensive chemotherapy treatment for the purpose to maintain a remission
(resolution) and to prevent relapse (recurrence).
Complete remission (CR) - Complete resolution of measurable tumor after chemotherapy based on physical examination,
monitoring or imaging.
Partial remission (PR) - Reduction in the measurable tumor after chemotherapy without development of new tumors.
Stable disease (SD) - Stable disease shows neither worsening nor improvement.
Progressive disease (PR) - Progressive disease shows signs of tumor growth and/or development of new tumors.
Median duration of response/median survival - The point at which half the pets have relapsed or died and half are still in
remission and alive.
Adjuvant chemotherapy - Administration of chemotherapy after surgery or radiation therapy in cases with high risk of
recurrence of metastases.
Primary (neoadjuvant) chemotherapy - The initial treatment (typically for localized cancer) by chemotherapy, followed by
surgery or irradiation.
Maximal tolerated dose (MTD) - The maximum recommended dose of a chemotherapeutic agent based on toxicity.
Combination chemotherapy
It is generally believed that single drug treatment is not curative and the combination of several drugs has a better
chance of improving survival. By the time the tumor is detected, it will contain many different types of cancer cells, each
with a differential response to a particular drug. By combining several drugs, we are increasing the chances of killing all
the different types of cells present in the tumor. Combination therapies are designed so that maximum efficacy can be
achieved with acceptable toxic side effects.
Toxicity of chemotherapeutic drugs
Most chemotherapeutic drugs are well tolerated by most companion animals. Less than 5% of pets will have to be
hospitalized and less than 1% of pets will die as a direct result of drug-associated toxicity. There are, however commonly
experienced side effects within one week of beginning therapy such as nausea, vomiting, diarrhea and/or lack of
appetite. The use of anti-diarrhea or antiemetics usually offset some of these side effects.
Another commonly experienced side effect is myelosuppression, a term referring to the killing of blood stem cells in the
bone marrow. When the blood cell count becomes very low, chemotherapy is usually delayed until the count is restored
to acceptable levels. When the pet experiences severe side effects, the dose of chemotherapy is typically lowered but at
the same time, lowering of the dose also lowers the efficacy of the drugs in killing tumor cells.
Some dogs with rapidly growing hair coats (eg poodles, terriers, Old English sheepdogs) lose hair during chemotherapy
and cats may lose their guard hairs and whiskers while undergoing chronic chemotherapy. The hair will regrow after
discontinuation of chemotherapy, although it may differ in color and texture.
Chemotherapeutic drugs
The following table summarizes commonly used classes of chemotherapeutic drugs and their associated toxicities.
Source: Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
Sources:
- Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
- Morrison Wallace B. Cancer in Dogs and Cats: Medical and Surgical Management. Baltimore: Williams&Wilkins,
1998.
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Chemotherapy
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Last updated 1/8/08
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