"Radiation Therapy and Pain Control"
GULF COAST VETERINARY ONCOLOGY'S TUMOR TIDBITS Volume 1, Number 11: October 1, 2000
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THIS MONTH'S FEATURE: RADIATION THERAPY AND PAIN CONTROL
DON'T FORGET TO VISIT OUR WEB SITE: CARING FOR PETS WITH CANCER, a
comprehensive web site for veterinarians and pet owners caring for pets with
cancer. Drug handouts, tumor handouts, case examples, pet loss information,
and much more at:
Editor: Kevin A. Hahn, DVM, PhD, Diplomate American College of Veterinary
Internal Medicine (Specialty of Oncology) & Overall Nice Guy; Gulf Coast
Veterinary Oncologists, Houston, TX 77027, drhahn@gulfcoastvetspec.com
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RADIATION THERAPY AND PAIN CONTROL
Introduction
Radiation therapy is an invasive nonpharmacological intervention used to
control pain in cancer patients. Although radiation therapy can cure primary
disease, over one-third of radiation therapy is considered palliative. The
purpose of palliative treatment is to relieve pain quickly and maintain
symptom control, allowing the patient quality of life. Radiation may enhance
the effectiveness of analgesic drug therapies by directly targeting the
cause of the pain. Noninvasive analgesic therapy should accompany or precede
invasive palliative treatment of metastatic pain.
What is Radiation therapy?
Radiation therapy involves the use of high-energy ionizing radiation to kill
cancer cells. Cancer cells are destroyed in the anatomic area being treated
so it is considered a local therapy. Radiation breaks down one or both
strands of the DNA molecule inside of cells, preventing cell growth and
division. Normal cells may be affected by radiation, but they are better
equipped for repairing damages to DNA. Radiation therapy is delivered with
machines administering high ionizing radiation. The machine used will depend
on the tumor itself. Examples of these machines include the Cobalt-60, the
first mega voltage machine and the linear accelerator, which uses
high-energy x-ray beams.
Dosage and Treatment Planning
The dosage of radiation used for palliative pain control is determined by
several factors; radiosensitivity of the tumor, normal tissue tolerance, and
the volume of tissue to be irradiated. Radiation therapy is planned
according to clinical status. The desired dosage of radiation should be
delivered in the least amount of fraction possible in order to promote
patient comfort. In palliative radiation, high doses with small daily
fractions are administered over one or two weeks. However, studies have
shown that low dose, short course radiation is just as effective as high
dose protocols.
The first phase of radiation is known as simulation. A simulator (X-ray
machine) is used to visualize and define the exact area to be treated.
Healthy tissue may be protected from radiation therapy by using customized
shields, known as blocks. Reference points on the skin are marked with
temporary dye or permanent tattoos, about the size of a small freckle. The
purpose of these markings is to ensure that the exact area will be treated.
The first phase may last longer than the following treatments.
Indications
Radiation therapy is used for relief of metastatic pain, especially
metastatic bone pain. Symptoms from local extension of primary disease may
also be controlled. Indications for radiation therapy in bone metastasis
include pain relief and prevention of pathological fractures. Spinal cord
compression from metastatic tumors requires immediate surgery, followed or
preceded by high dose steroids and radiation therapy. Radiation therapy is
also used to relieve headaches caused by brain tumors.
Types of radiation
Radiation therapy used for localized bone metastasis is known as external
beam radiation. This may be directed locally or if the pain is widespread it
may be treated with Hemibody irradiation (one-half of the body at a time). A
third way to deliver radiation is through the use of Radiopharmaceuticals.
Examples of these include Iodine- 131 used in treating multiple bone
metastasis from thyroid cancer, Phospphorous-32-orthophosphate used for pain
relief associated with bone metastasis from breast and prostate cancer, and
Strontium 89 is very effective for bone pain.
Side effects
Radiation therapy causes side effects due to the damage of normal tissues.
The main side effects include skin toxicity, fatigue and anorexia. These may
occur with treatment to any site, while other toxicities are more specific
to the site being treated. Volume of tissue being treated, total daily dose
(fractionation) of therapy, method of treatment and individual factors will
affect the severity of side effects. Acute reaction to radiation occurs
during treatment itself, while delayed side effects may occur months or
years after treatment and may lead to chronic problems. Radiation may also
affect the bone marrow, resulting in anemia thrombocytopenia, and
neutropenia. Routine blood work should be monitored during treatments.
Does It Work?
In one study of 24 dogs with a variety of advanced malignancies irradiated
using 800 cGy fractions given on days 0, 7, and 21, palliative response was
assessed using a quality of life instrument developed for veterinary use.
This pain score was based on owner response to questions regarding analgesic
requirement, activity level, appetite, and degree of lameness in the
affected dogs. Seventeen (74%) of the 23 dogs experienced complete pain
relief, and 3 (13%) obtained partial relief. Of the 17 dogs that achieved a
complete pain relief response, pain recurred in only 8 of these dogs at a
median time of 70 days. Six dogs were alive and free of pain up to 557 days
after irradiation. The 0-7-21 protocol was well tolerated; pain relief
occurred quickly, and acute radiation reactions were negligible.
In a larger study of 58 dogs with osteosarcoma, palliative radiotherapy
using 60Co photons evaluated. Each dog received 10 Gy on days 0, 7 and 21.
Seventy-four percent experienced pain relief following treatment. In dogs
experiencing pain relief the median duration of response was 273 days.
So What Do We Do At Gulf Coast?
Of course the best approach to managing the dog with osteosarcoma is to consider amputation (for pain relief) and adjuvant chemotherapy. However,
there are often times where amputation is not appropriate. Chemotherapy
should still be considered as the best measure for delaying the onset of
complications that may arise from metastatic disease. Palliative radiation
therapy should be considered in addition to radiation therapy to provide
pain relief and additional use of the affected leg. Given these results
clinical trial results, a 9-12 month prognosis will full use of the leg
could be considered.
For further information on radiation therapy in dogs and cats, please call
us to schedule an appointment (713-693-1166) or email Dr. Hahn at
drhahn@gulfcoastvetspec.com .
For more information on the web, use Medline at: