Chemotherapy in Small Animals
Chemotherapeutic drugs (anticancer
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.
I. When do we use chemotherapy
to treat animals with cancer?
- multicentric disease (tumors occuring are more than one site)
- metastatic disease (cancer that has already spread)
- nonresectable disease (tumors than cannot be removed surgically)
- as follow-up therapy
after surgery when the tumor has not/cannot be completely removed.
- as follow-up therapy
after surgery when we are treating tumors that usually metastasize early.
II. 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
- 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 patient
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.
III. 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 2 cycles of chemotherapy and then be evaluated
for response before we decide to continue the treatment, change the drugs,
or discontinue chemotherapy.
IV. What can you expect
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.