The diagnosis of
cancer is stressful for pet owners, and the prospect of
chemotherapy treatments can be equally difficult.
However, the fear that animals will spend most of their
last days sick from chemotherapy treatments is
unwarranted. Knowing how anti-cancer chemotherapy drugs
work and what to expect from the treatments can help pet
owners decide on whether such therapy is appropriate for
their pets.
When do we use
chemotherapy to treat animals with cancer?
Chemotherapy may be used as the sole treatment for
certain cancers or may be used in combination with other
treatment modalities, such as surgery and radiation
therapy. Chemotherapy is likely to be recommended for
cancer that has already spread to other areas of the
body (metastatic disease), for tumors that occur at more
than one site (multicentric disease), or for tumors that
cannot be removed surgically (nonresectable disease). In
some cases, chemotherapy can be used to try to shrink
large tumors prior to surgery or to help eradicate
certain types of microscopic cancer cells that cannot or
have not been completely removed surgically. For cancers
that are at high-risk for metastasis early in the course
of disease, chemotherapy can be used after surgery or
radiation therapy to help slow down the growth of cancer
cells in other parts of the body.
How does chemotherapy work?
Chemotherapy drugs
attack cells in the process of growth and division.
Individual drugs may work through many different
mechanisms, such as damaging a cell's genetic material
(DNA) or preventing the cell from dividing. However,
chemotherapeutic drugs cannot distinguish between
malignant cancer cells and normal cells. All rapidly
dividing cells are potentially sensitive to
chemotherapy. Toxicity to normal, rapidly growing or
self-renewing tissues in the body is the reason for most
of the side effects seen with chemotherapy. Fortunately,
these normal tissues continue to grow and repair
themselves, so the injury caused by chemotherapy is
rarely permanent.
What are the
typical side effects of chemotherapy?
Compared to
people who receive chemotherapy, pet animals experience
fewer and less severe side effects because we use lower
doses of drugs and do not combine as many drugs as in
human medicine. The normal tissues that typically are
most sensitive to chemotherapy are the intestinal
lining, the bone marrow (which makes red and white blood
cells), and hair follicles.
Toxic effects to the gastrointestinal
tract are responsible for decreased appetite, vomiting,
and diarrhea. These effects may be mild, moderate, or
severe. In most cases, these signs are mild and usually
resolve on their own or with oral medication given at
home. Although infrequent, some dogs (and cats) may
develop severe diarrhea requiring hospitalization and
fluid therapy. In many cases, the gastrointestinal side
effects from chemotherapy are not seen on the day of
treatment. They often occur 3 to 5 days later.
Suppression of the bone marrow by
chemotherapeutic drugs may cause a drop in the white
blood cell count, leading to increased susceptibility to
infection. The infection usually comes from the animal's
own body (such as bacteria normally found in the
intestines, mouth, skin, etc.). Severe infections may
require hospitalization for intensive supportive care,
including intravenous fluid and antibiotics. When a
chemotherapeutic drug is used that is known to have a
high potential for bone marrow suppression, a complete
blood count (CBC) is checked several days after the
treatment. If the white blood cell count is low but your
pet is feeling well, antibiotics are prescribed as a
preventative measure. Subsequent doses of chemotherapy
are adjusted based on the results of the CBC. Anemia
(low red blood cell count) is often a complication of
cancer but is rarely caused by the chemotherapy drugs
used in veterinary medicine.
Hair
follicle cell in dogs (and cats) that are wire-haired or
non-shedding may be particularly susceptible to
chemotherapy. Certain breeds of dogs, such as terriers
and poodles, will experience variable amounts of hair
loss. Hair loss often is most evident on the face and
tail. Whiskers and the long hairs over the eyes often
fall out in cats. The hair will regrow once chemotherapy
is stopped, but may initially have a modest change in
color or texture.
There are many
different types of chemotherapy agents and each has a
different likelihood of causing side effects. If your
pet is treated with drugs known to cause certain side
effects, we will prescribe medications to help prevent
these complications, such as antiemetics (anti-nausea
and vomiting medication). In addition, we will give you
instructions on what to do if and when a problem arises.
We seldom see severe side effects as described above; it
is estimated to be less than 5% of all pets receiving
chemotherapy. With proper management, most animals
recover uneventfully within a few days.
Please keep in mind that any animal can have an
unexpected reaction to any medication.
How is chemotherapy given?
How a chemotherapeutic
drug is administered, how often it is given and how many
treatments are given varies from case to case. The type
of cancer, the extent of disease, and general health of
the animal help the oncologists to formulate a treatment
protocol (type of drugs, dose, and schedule used)
appropriate for your pet.
Some
drugs are oral medications (pills) that you give at
home. Others are brief injections that require an
outpatient appointment. In some instances, slow
infusions or repeated treatments throughout the day may
require an animal to spend the day in the hospital. The
treatments are typically repeated from weekly to every
third week. Blood tests may be needed to monitor the
effects of chemotherapy during the weeks between drug
treatments.
The duration of
chemotherapy depends on the type of cancer and the
extent of disease. Some animals need to receive
chemotherapy for the rest of their lives. In others,
treatments may be spread out or discontinued after a
period of weeks to months provided that the cancer is in
remission, i.e., there is no detectable evidence of
cancer in the body. Chemotherapy can be resumed when the
cancer relapses.
We usually
recommend that every patient receive at least 2 cycles
of chemotherapy and then be evaluated for response
before we decide to continue the treatment, change drugs
or discontinue chemotherapy.
It is imperative that you, as a pet
owner, are committed to treatment and that you bring
your pet to the veterinary hospital when scheduled for
therapy.
What can be expected from chemotherapy?
In
many cases, we are unable to cure our veterinary cancer
patients. Our goal is therefore to improve a pet's
quality of life. To this end, chemotherapy can be used
to minimize the discomfort caused by a tumor or to slow
down the progression of the disease. For most (but not
all) types of tumors, the oncologist will provide
information on average life expectancy with and without
treatments.
The decision of
whether to pursue chemotherapy treatments can be
complex. Medical information, practical concerns (such
as need for repeated visits, your pet's temperament,
etc.), and financial responsibility all play a part in
this decision. We encourage you to discuss your concerns
with the oncologist and/or our social worker when making
this decision.