Radiation therapy
What is radiation therapy?
Radiation therapy is a type of treatment that uses penetrating beams of high energy waves (gamma rays) or streams of
particles (generated by linear accelerators) that kill cancer cells by damaging their DNA. Intact DNA is necessary in
order for the cancer cell to continue dividing, therefore, radiation-induced damage will either eventually kill the cancer
cell or stop it from further multiplication.The treatment is not used to irradiate the whole body but rather focus it on the
area where the tumor is located.
Fast facts about radiation therapy
How does radiation therapy work?
Radiation in high doses will kill cells or prevent them from further multiplications. Cancer cells have been shown to
divide more rapidly in comparison to normal cells, therefore, radiation therapy will preferentially target these cancer
cells. Unfortunately, radiation will also damage some normal cells, but normal cells are usually capable of repairing
themselves and recovering from the effects of radiation. With the advancements of new imaging techniques such as
computed tomography (CT), magnetic resonance imaging (MRI), and nuclear scanning, oncologists are usually able to
shield as much normal tissue as possible while focusing the radiation beam to the target area where the tumor is
located.
Factors to consider before radiation
When is radiation therapy used?
Radiation therapy can be used on a variety of different cancers and is often used in combination with surgery and/or
chemotherapy. Radiation can be used prior to surgery to shrink the tumor to a smaller size or it can also be used after
surgery to stop the growth of any cancer cells that may have been left behind. Sometimes, radiation therapy is
administered during the surgery, a process called intraoperative radiation.
Radiation can be also used in combination with chemotherapy without surgery. It can be administered before, during or
after chemotherapy treatment, depending on the type of cancer, its location and size. When irradiating tumor before
chemotherapy, it shrinks its size and thus enables chemotherapy to become more effective on the remaining cells.
Sometimes the opposite is done - the chemotherapy treatment will be used to shrink the tumor, followed by radiation to
kill any remaining cells.
When cure is not an attainable goal, radiation can be used to shrink the tumor size to alleviate the pressure of the
tumor on neighboring tissues, pain or other symptoms. When any treatment is used to alleviate rather than cure
cancer, the process is referred to as palliative care (or palliation). While radiation won't be able to rid the body of
cancer, it can substantially enhance the pet's quality of life.
How often is radiation therapy administered?
When radiation therapy is used to try to cure the pet of cancer, it is given in small fractions over 2-5 weeks, depending
on the type, size and location of the cancer and pet's health. When radiation therapy is used for palliative reasons (to
alleviate symptoms without cure), it is given in large fractions, usually once for three weeks.
What are the risks of radiation therapy?
The high doses used to irradiate tumor can also damage or kill normal cells, leading to side effects. Fortunately, many
adverse reactions associated with irradiation are well known and can be treated.
Acute side effects usually occur during or shortly after radiation therapy, and involve tissues with rapidly dividing cells
(eg in head and neck region, intestines, eyes, and skin). Some pets may experience redness and irritation at the area
of irradiation and the injuries will resemble that of a sunburn. The area may be very itchy for the pet in which case anti-
itching medication is prescribed to prevent the pet from further aggravating the skin damage. The pet may experience
hair loss at the area of radiation, but the hair will grow back, although the color may change (see below figure). If
radiation is administered to the head and neck region, the pet may become reluctant to eat or drink in which case either
hand feeding of special diet or placement of a feeding tube along with subcutaneous administration of fluids may be
necessary to prevent dehydration and malnourishment. The pets may experience neurological signs if brain was
irradiated. When the radiation is targeting an area close to the eye, the pet may experience changes in vision and/or
loss of tears. There is always some risk associated with general anesthesia during the irradiation but the pets will be
monitored throughout the procedure and the duration is usually short. While the acute side effects may be unpleasant,
the pets usually recover with proper treatment within a few weeks.
Late effects of radiation involve tissues with slowly dividing cells (eg bone, lung, heart, kidneys, and spinal cord).When
they do occur, they may be more serious and can result in severe fibrosis, necrosis, loss of function, and sometimes
even death. The symptoms of late reactions should be treated by a doctor experienced in radiation injury.
Types of radiation therapy
External-beam radiation
The radiation used in cancer treatment is either in the form of gamma rays (emitted from a material called cobalt-60) or
in the form of charged particles such as electrons (generated by linear accelerators). The pet will be placed on a table
under the source of radiation and as soon as the treatment is over, the pet won't be radioactive. The radiation therapy
is typically given in fractions rather than all at once, requiring either daily or several times a week visits to the hospital.
Brachytherapy
Another method of administrating radiation therapy is to deliver radiation directly to the tumor. This method is also
known as implant therapy or internal radiotherapy. In this method, the veterinarian will place radioactive implants
directly into a tumor, organ or site of tumor removal so that the radiation dose is concentrated in the small target area.
Depending on the tumor, it can be done either with a large needle or through minor surgery. The pet will remain
radioactive until the implants are removed and the pet will be required to stay in the hospital for about a week to
prevent radiation exposure to others. It has been used for dogs with nasal tumors and fibrosarcomas in cats.
Intraoperative radiation
This type of radiation therapy refers to a method which delivers large dose of radiation to a tumor and its surrounding
tissues during a surgical procedure.
Systemic radiotherapy
In this method, a radioactive isotope is given orally or by injection to travel through the body to find a target tissue to
irradiate. It has worked well with older cats for treatment of hyperthyroidism. Typically, the cat will be kept in isolation
until most of the radioactivity has been eliminated from its body to minimize others' risk of exposure.
Cases when radiation therapy won't be effective
Pet owners should keep in mind that radiation therapy may not be the appropriate treatment for some cases. Some
tumors are known to be resistant to radiation and some tumors are located in an area in which the normal cells present
also happen to be very sensitive to radiation such as cells in the gastrointestinal tract. In this case, the radiation would
kill not only the tumor cells but also the normal cells, leading to devastating consequences for the pet.
Which tumors are commonly treated with radiation?
Sources:
Radiation therapy is a type of treatment that uses penetrating beams of high energy waves (gamma rays) or streams of
particles (generated by linear accelerators) that kill cancer cells by damaging their DNA. Intact DNA is necessary in
order for the cancer cell to continue dividing, therefore, radiation-induced damage will either eventually kill the cancer
cell or stop it from further multiplication.The treatment is not used to irradiate the whole body but rather focus it on the
area where the tumor is located.
Fast facts about radiation therapy
- Radiation therapy is painless
- External radiation won't make the pet radioactive
- Skin in the treated area may become sensitive and easily irritated
- Side effects of radiation therapy are usually temporary and will vary depending on the area of the body being
treated - the pet will have to undergo general anesthesia
- list of radiation facilities is provided at www.vetcancersociety.org/pdf/radiation.pdf
How does radiation therapy work?
Radiation in high doses will kill cells or prevent them from further multiplications. Cancer cells have been shown to
divide more rapidly in comparison to normal cells, therefore, radiation therapy will preferentially target these cancer
cells. Unfortunately, radiation will also damage some normal cells, but normal cells are usually capable of repairing
themselves and recovering from the effects of radiation. With the advancements of new imaging techniques such as
computed tomography (CT), magnetic resonance imaging (MRI), and nuclear scanning, oncologists are usually able to
shield as much normal tissue as possible while focusing the radiation beam to the target area where the tumor is
located.
Factors to consider before radiation
- What is the likelihood of controlling the tumor with radiation therapy?
- What are the alternative treatments and how do they compare with radiation therapy in terms of improving your
pet's condition? - Do the benefits of radiation outweigh the risks?
- Is your pet's overall health in a reasonable shape to undergo this type of treatment for several weeks?
- Has the particular tumor type been successfully treated in other animals?
- Will your pet experience dramatic changes in appearance, function and/or quality of life?
- Should radiation be combined with other treatments such as chemotherapy?
- Does your pet have any other existing health problems that might affect the outcome?
- What is your level of time, effort and care commitment to complete the treatment?
- Do you have the financial resources or can you make arrangements to pay for the treatment?
When is radiation therapy used?
Radiation therapy can be used on a variety of different cancers and is often used in combination with surgery and/or
chemotherapy. Radiation can be used prior to surgery to shrink the tumor to a smaller size or it can also be used after
surgery to stop the growth of any cancer cells that may have been left behind. Sometimes, radiation therapy is
administered during the surgery, a process called intraoperative radiation.
Radiation can be also used in combination with chemotherapy without surgery. It can be administered before, during or
after chemotherapy treatment, depending on the type of cancer, its location and size. When irradiating tumor before
chemotherapy, it shrinks its size and thus enables chemotherapy to become more effective on the remaining cells.
Sometimes the opposite is done - the chemotherapy treatment will be used to shrink the tumor, followed by radiation to
kill any remaining cells.
When cure is not an attainable goal, radiation can be used to shrink the tumor size to alleviate the pressure of the
tumor on neighboring tissues, pain or other symptoms. When any treatment is used to alleviate rather than cure
cancer, the process is referred to as palliative care (or palliation). While radiation won't be able to rid the body of
cancer, it can substantially enhance the pet's quality of life.
How often is radiation therapy administered?
When radiation therapy is used to try to cure the pet of cancer, it is given in small fractions over 2-5 weeks, depending
on the type, size and location of the cancer and pet's health. When radiation therapy is used for palliative reasons (to
alleviate symptoms without cure), it is given in large fractions, usually once for three weeks.
What are the risks of radiation therapy?
The high doses used to irradiate tumor can also damage or kill normal cells, leading to side effects. Fortunately, many
adverse reactions associated with irradiation are well known and can be treated.
Acute side effects usually occur during or shortly after radiation therapy, and involve tissues with rapidly dividing cells
(eg in head and neck region, intestines, eyes, and skin). Some pets may experience redness and irritation at the area
of irradiation and the injuries will resemble that of a sunburn. The area may be very itchy for the pet in which case anti-
itching medication is prescribed to prevent the pet from further aggravating the skin damage. The pet may experience
hair loss at the area of radiation, but the hair will grow back, although the color may change (see below figure). If
radiation is administered to the head and neck region, the pet may become reluctant to eat or drink in which case either
hand feeding of special diet or placement of a feeding tube along with subcutaneous administration of fluids may be
necessary to prevent dehydration and malnourishment. The pets may experience neurological signs if brain was
irradiated. When the radiation is targeting an area close to the eye, the pet may experience changes in vision and/or
loss of tears. There is always some risk associated with general anesthesia during the irradiation but the pets will be
monitored throughout the procedure and the duration is usually short. While the acute side effects may be unpleasant,
the pets usually recover with proper treatment within a few weeks.
Late effects of radiation involve tissues with slowly dividing cells (eg bone, lung, heart, kidneys, and spinal cord).When
they do occur, they may be more serious and can result in severe fibrosis, necrosis, loss of function, and sometimes
even death. The symptoms of late reactions should be treated by a doctor experienced in radiation injury.
Types of radiation therapy
External-beam radiation
The radiation used in cancer treatment is either in the form of gamma rays (emitted from a material called cobalt-60) or
in the form of charged particles such as electrons (generated by linear accelerators). The pet will be placed on a table
under the source of radiation and as soon as the treatment is over, the pet won't be radioactive. The radiation therapy
is typically given in fractions rather than all at once, requiring either daily or several times a week visits to the hospital.
Brachytherapy
Another method of administrating radiation therapy is to deliver radiation directly to the tumor. This method is also
known as implant therapy or internal radiotherapy. In this method, the veterinarian will place radioactive implants
directly into a tumor, organ or site of tumor removal so that the radiation dose is concentrated in the small target area.
Depending on the tumor, it can be done either with a large needle or through minor surgery. The pet will remain
radioactive until the implants are removed and the pet will be required to stay in the hospital for about a week to
prevent radiation exposure to others. It has been used for dogs with nasal tumors and fibrosarcomas in cats.
Intraoperative radiation
This type of radiation therapy refers to a method which delivers large dose of radiation to a tumor and its surrounding
tissues during a surgical procedure.
Systemic radiotherapy
In this method, a radioactive isotope is given orally or by injection to travel through the body to find a target tissue to
irradiate. It has worked well with older cats for treatment of hyperthyroidism. Typically, the cat will be kept in isolation
until most of the radioactivity has been eliminated from its body to minimize others' risk of exposure.
Cases when radiation therapy won't be effective
Pet owners should keep in mind that radiation therapy may not be the appropriate treatment for some cases. Some
tumors are known to be resistant to radiation and some tumors are located in an area in which the normal cells present
also happen to be very sensitive to radiation such as cells in the gastrointestinal tract. In this case, the radiation would
kill not only the tumor cells but also the normal cells, leading to devastating consequences for the pet.
Which tumors are commonly treated with radiation?
- Brain tumors
- Soft tissue sarcomas
- Mast cell tumors
- Ceruminous gland tumors
- Localized lymphomas
- Nasal (nose) tumors
- Oral tumors
- Bone tumors
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.
The cobalt-60 radiation unit.
Source: http://www.cvm.tamu.edu/oncology/faq/questions/radiat01.html
Source: http://www.cvm.tamu.edu/oncology/faq/questions/radiat01.html
A change in hair color after radiation therapy.
Source:
http://www.cvm.tamu.edu/oncology/faq/questi
ons/side01.html
Source:
http://www.cvm.tamu.edu/oncology/faq/questi
ons/side01.html
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Radiation Therapy
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Last updated 1/8/08
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