Canine Cancer: Radiation Therapy
Service, Veterinary Hospital of the University of PA
How does radiation therapy work?
Radiation, at levels thousands of times the amount used
to produce a chest X-ray, kills cells. Both normal and
cancer cells are affected, but radiation treatment is
designed to maximize tumor effect and minimize normal tissue
effect. Maximizing tumor effect is one reason that radiation
treatments are given as a series of small doses rather than
one large dose.
What are the benefits of radiation therapy and when is it
Radiation therapy is used to treat localized disease. It
can be used in the management of cancers which cannot be
treated successfully by surgery or chemotherapy alone.
Typically, it is employed following surgery when there are
tumor cells remaining after excision, either because of the
nature of tumor growth, or because a complete surgical
removal of would involve a very extensive procedure
involving vital structures. In some instances, radiation
therapy may be employed before surgery or chemotherapy in an
attempt to shrink down a tumor to a more manageable size.
Radiation therapy can offer, in some instances, permanent
control of a tumor.
Even when a cure is not possible, radiation therapy can
still bring some relief. Shrinking a large tumor with
radiation therapy may improve a pet's quality of life by
reducing pressure, bleeding, or pain. This is called
Are there risks involved?
There are some risks involved with any type of cancer
treatment. In addition to cancer cells, some normal cells
will also be killed by the radiation. Some side effects may
be apparent because of normal cells being killed (e.g.
"radiation dermatitis"). Usually these side effects are
outweighed by the benefits of killing cancer cells.
In addition, radiation therapy requires the animal to be
perfectly still during treatment. Thus, anesthesia is
necessary for each treatment. There is always a slight risk
associated with anesthesia.
How is the therapy given?
In veterinary radiation therapy, a machine directs a
radiation beam toward the cancer and some normal tissues
around it. At VHUP, orthovoltage (relatively low-energy)
radiation is used. The equipment is very similar to
equipment used for standard X-rays, except that exposure
times are many times longer. While orthovoltage radiation is
suitable for many veterinary application#, some tumors
(dependent on size, location, or tumor type) are better
treated with megavoltage (relatively high energy) radiation.
In such instances, you will be offered a referral to a
veterinary institution with appropriate facilities.
Is radiation therapy ever used in combination with
chemotherapy or surgery?
Yes. In situations where it is unlikely that any one method
of cancer treatment will be effective, such as large or
aggressive tumors, radiation therapy can be combined with
surgery or chemotherapy. In some situations a combination of
all three types of treatment may be recommended.
How long does the entire treatment last, and what is the
Radiation therapy is given in a series of treatments
which encompass several weeks. This schedule helps protect
normal tissue by spreading out the total dose of radiation.
The treatment area is designed to include all of the cancer
and as little normal tissue as possible. The total dose used
and the number of treatments depend on many factors. These
factors include the size and location of the cancer (i.e.
which normal tissues will be within the treatment area), the
general health of your pet, and the type of cancer present.
The most important consideration is the total amount of
radiation that can be administered to a patient without
compromising the ability of healthy tissue to heal. The
treatment area will be marked with an ink-like fluid to
enable the treatment to be directed to the same area every
time. Please maintain the marks as directed.
At VHUP, most patients receive treatments on a
Monday-Wednesday-Friday schedule for a total of 12
treatments in 4 weeks. Weekends are reserved as a rest
period during which normal tissues have an opportunity to
recover from the treatment. At times, more extensive rest
periods are required. Patients may stay in the hospital from
Monday through Friday, or brought in for each treatment.
Outpatient treatments take 1-2 hours. Because of the
anesthesia required, outpatients should not be feed after
8pm the night before treatments, but water may be provided
throughout the night. Pets may be sleepy for several hours
following each treatment.
What are the side-effects of treatment?
During treatment the oncologist will monitor the effect
of the radiation on the cancer as well as on normal tissue.
Most side effects that occur during radiation therapy,
although unpleasant, are usually not serious, and are almost
always limited to the area being treated. Many animals
develop skin changes in the area being treated. A redness of
the skin may develop near the end of, or after, radiation
therapy. This may progress to a dry or moist skin reaction,
which resembles a severe sunburn or blistering rash. This
"radiation dermatitis" may cause your pet to rub or scratch,
but it is important you try to keep your pet from doing
this. Medication and/or physical means to prevent rubbing
and scratching may be prescribed by your veterinarian. Hair
loss in the treated area is common and may persist for some
time, but regrowth occurs in most patients. The color of the
regrowing hair and skin in the treated area are likely to
It is unusual for animals to become nauseated and have
vomiting/diarrhea as a result of radiation therapy. This
will usually only occur if portions of the abdomen are
irradiated. Side effects involving other tissues that may be
within the radiation treatment area (such as the eye, mucous
membranes and salivary glands of the oral cavity, and bone)
will be discussed with you on an individual basis. The time
from first appearance of acute side effects (i.e. those that
happen in the immediate treatment period) until their
resolution is usually 3-4 weeks. Other side effects, if they
occur, develop gradually over months to years.
What happens after the treatment is over?
It is important for your veterinarian to examine your
pet periodically after radiation therapy. This will allow
normal tissue side effects to be monitored, and the effect
of the radiation on the tumor to be evaluated. It is the
goal of radiation therapy to completely eradicate the
cancer. In some pets this happens and no evidence of the
tumor persists. In other pets the tumor may never completely
disappear, but growth is stopped and the tumor is
essentially controlled. The specific results to be expected
depend on many factors. Details on the likelihood of success
will be provided to you on an individual basis by the
clinical oncology department.
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