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          Canine Radiation Therapy          

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Radiation Therapy
Radiation Therapy
Radiation Therapy

Radiation Therapy

What is it?
Radiation therapy (megavoltage) is supplied by a Linear Accelerator. This device generates high energy photons or electrons used to treat many kinds of cancer. Radiation therapy is typically applied to one site in an attempt to provide local therapy (local control or cure) of a cancer that only affects one part of the body.  It is designed to minimize damage to normal cells and inhibit the cancer cells from multiplying. 

What is the goal? When choosing radiation therapy for any patient, we have to consider what the goals of therapy are – are we trying to “cure” or eradicate all local tumor cells, or are we trying to shrink a cancer to make it operable or more amenable to chemotherapy, or are we simply trying to make pain control easier?

Does my pet have to be anesthetized for radiation therapy?
Every pet, no matter how obedient he or she is, must be sedated for radiation treatment. Anesthesia is necessary because pets are not able to stay perfectly still during treatment, and targeting a very precise area is critical to your pet’s radiation therapy. The entire treatment process is done very quickly so that your pet is not asleep longer than necessary.

Dogs are sedated using a fast acting anesthesia via injection. This is followed by the insertion of a trachea tube, which delivers Isoflourane, a gas that keeps the dog sedated for the entire procedure. Cats are not given the injection, but anesthetized with Isoflourane only. A small mask is placed over the mouth and nose to deliver the Isoflourane throughout the procedure. Depending on the area of treatment, it may also be necessary to use a trachea tube on a cat.

Because the eyes do not blink during sedation, a lubricant is applied to each eye to prevent them from becoming dry. This is the reason your pet may seem “teary-eyed” when you see him/her after the treatment.

During the radiation treatment, pets are in the therapy room by themselves for a brief amount of time, continuously monitored by closed circuit TV. Once the radiation treatment is complete, your pet will be woken up from anesthesia. Approximately 15 minutes after your pet is fully awake, he/she can go home.

Please be aware that the effects of the anesthesia may linger after the pet has finished its treatment and your pet may be more sleepy than usual. This is temporary and completely normal.

How does radiation therapy work?
Radiation, at levels thousands of times the amount used to produce a chest x-ray, destroys the ability of cells to divide and grow. Both normal and cancer cells are affected, but the radiation treatment is designed to maximize tumor effect and minimize normal tissue effect. Maximizing tumor effect is one reason radiation therapy is given in a series of treatments rather than one treatment.

What are the benefits of radiation therapy?
Radiation therapy can offer, in some instances, permanent control of tumors that cannot be controlled by surgery or chemotherapy,. In other cases, even when cure is not possible, radiation therapy can still bring a measure of relief. Shrinking a large tumor with radiation therapy may improve the quality of life by reducing pressure, bleeding or pain.

Are there risks involved?
There are some risks involved with any type of treatment for cancer. In addition to cancer cells, some normal cells will also be killed by the radiation. Some side effects may be apparent because of these normal cells being killed. Usually these side effects are temporary and outweighed by the benefits of killing cancer cells.

In addition, radiation therapy requires that the animal be perfectly still during treatment. Thus, general anesthesia is necessary for each treatment. There is always a slight risk associated with general anesthesia, but nearly all cancer patients accommodate to this aspect of the treatment very well.

How is the radiotherapy given?
All radiation therapy requires perfect positioning of patients to avoid side effects to portions of the body that should not be in the treatment field!  That means that each patient will have to be anesthetized for each therapy. Radiation therapy anesthesia protocols are designed for safety in repetitive use – often in an elderly patient population. Fentanyl, propofol and/or gas anesthesia are used to try to have patients back on their feet as quickly as possible. The treatment field may be shaved and marks placed on the skin during the first imaging session to be certain that the treatment field stays the same with each treatment.

High energy radiation is directed at the cancer and some normal tissue around it. At the UW-Madison's Veterinary Medical Teaching Hospital, a cobalt machine is used. When the cobalt machine is turned on, a source of radioactive cobalt moves over an opening in the storage head and the high energy radiation is directed onto the patient. Each treatment takes 5-10 minutes. When the daily treatment is completed, no radioactivity remains in the body.

Is radiation therapy ever used in combination with chemotherapy?
Yes, in situations where it is unlikely that any one method of cancer treatment will be effective (such as bulky or aggressive tumors), radiation therapy can be combined with chemotherapy or surgery. In some situations, combining all three types of treatment may be recommended. Details concerning these combinations can be provided by the cancer specialist.

Who gives radiation therapy treatment?
A cancer specialist will determine if radiation therapy will be useful for treatment of your pet's cancer. If so, a veterinarian specially trained in radiation therapy will carefully plan the treatment. Treatment for each patient is different. The radiation therapist (also called a radiation oncologist) will supervise the first radiation therapy treatment so that it is clear to everyone involved exactly how the treatment is to be given. After the first radiation therapy treatment, additional treatments will be given by a radiation therapy technologist. These individuals have had special training in the administration of radiation for cancer treatment.

Is radiation therapy expensive?
Radiation therapy can be costly  because it involves complex equipment plus the expertise of many health care professionals and it depends on the type and number of treatments required.

How long does the treatment take?
Radiation therapy is given in a series of treatments that encompass two to five weeks. This schedule helps protect normal healthy tissue by spreading out the total dose of radiation. The total dose used and the number of treatments, called fractions, in which the total dose is given depend on many factors. These factors include size and location of the cancer, the general health of your pet, and the type of cancer present.

Full course treatments.
Radiation therapy is delivered to a total dose of between 45-54 Gray for most types of cancer. This dose is more than 500 times what a diagnostic xray (radiograph) would be so it cannot be tolerated by any normal tissue as a single dose. We divide up that dose into many “fractions” which are then delivered on a Monday-Friday basis until the total dose needed for that cancer’s control has been given.  Radiation therapy is a key feature in the treatment of patients with the following cancers:
            Nasal carcinomas
            Nasal sarcomas
            Nasal lymphoma
            Laryngeal lymphoma
            Brain tumors
            Pituitary tumors
            Oral squamous cell carcinomas in dogs
            Skull tumors
            Incompletely excised (but not yet recurrent) mast cell tumors
            Incompletely excised (but not yet recurrent) soft tissue sarcomas (oral and other locations)
            Non-resectable plasma cell tumors

Treatment of these patients typically involves a CT-based 3D radiation treatment plan, and 15 – 18 treatments. Lymphoma/plasmacytoma therapy takes many fewer treatments as these tumor types are very sensitive to radiation therapy and we do not need to treat to as high a total dose.

Palliative: Radiation therapy can also be used in palliative fashion to try to slow an aggressive/non-resectable tumor’s growth, to shrink an oral tumor, decrease the vascularity of a cancer, or to kill pain associated with a cancer such as the following:

            Oral melanoma
            Bulky soft tissue sarcoma
            Bulky carcinomas
            Oral squamous cell carcinoma in cats
            Osteosarcoma

Typically, palliative radiation therapy is given on a 0-7-21 day basis (three treatments) or once weekly for four treatments. A two dimensional radiation treatment plan is usually used for this type of patient (may be radiographic based, or CT based).

What are the effects of treatment?
During treatment, the cancer specialist will monitor the effect of the radiation on the cancer as well as on normal tissue. It may be necessary to alter the plan because of changes in the tumor or normal tissue, but this is usually not necessary. Most side effects that occur during radiation therapy, although unpleasant, are usually not serious.

Side Effects:
 Radiation therapy cannot tell the difference between normal and cancer tissues – it simply kills cells as they divide. Since most cancers divide much faster than normal tissues, we get a selective effect on the cancer cells. We expect, however, to see side effects of radiation therapy. Acute (or short term) effects include loss of hair, moist desquamation of skin/mucus membranes (radiation burn like a blistering sunburn), and corneal irritation – if these tissues are in the treatment field. Common late side effects of radiation therapy include permanent hair loss, permanent hair color change, cataracts, retinal degeneration, keratoconjunctivitis sicca – again only if these tissues are in the treatment field.  Some organs have more difficulties with radiation than others – eyes are clearly sensitive, but tubular organs (esophagus, colon, rectum, urethra) can also be – they could heal from acute radiation effects by scarring (stricture). The heart and lungs can only tolerate a limited amount of radiation therapy – so we try to avoid these structures. Bone marrow is very sensitive to radiation if used in an “whole or half body” therapy.  Bone density within a treatment field decreases. One in 10,000 patients could develop a bone tumor in a treated bone 5-10 years following therapy.  The spinal cord and brain’s vasculature can also be sensitive to some types of radiation therapy (late effects). Because we want to minimize side effects, imaging and treatment planning are essential for most patients.
Area Treated
 Possible Side Effects
 Treatment
Abdomen, Colon,
Rectal Area
 Diarrhea
 Medication, Diet
Eye
 Dry eye, cessation of tears,
cataracts, blindness
 Topical Eye
Medications
Nasal Cavity
 Nasal discharge, possibly
mixed with blood, may
become dry and itchy
 Aquaphor, Aloe
Vera 100% Free
Throat

 Coughing or occasional
gagging

 Will disappear in
approx. two weeks
following the final
Radiation Treatment
Mucosal Tissue
(mouth, rectum)
 Redness, irritation,
itchy, bleeding
 Medication

source:http://www.rbvh.net/services/oncology/radiation_therapy_brochure.pdf

Symptom care for side effects:
Dogs and cats getting full course radiation therapy WILL develop acute side effects. Palliative or short course radiation therapy rarely causes acute side effects other than the hair loss needed in order to shave/mark the treatment field.

Dogs with moist desquamation (cats more commonly get dry desquamation with dry flaky skin and itchiness) need to have their treatment sites kept clean and dry (with water – not other products). We try NOT to bandage as we don’t want dirt and moisture to build up – that means that E-collars are often necessary as a dog’s licking will only make the side effects worse. If a pet is scratching, sometimes T-shirts, socks or hobbles must be used to prevent self-trauma. Anti-inflammatories and rarely narcotics can be used to help through this time period. Typically, these radiation burns form in the second to third week of full course therapy, and then form crusts, and then the skin heals under the crusts. The whole process takes about 2-3 weeks.

Mucositis is seen when the gums, tongue, cheeks, throat or other mucus membrane-lined tissue is in the treatment field (often with nasal or oral tumors). The mucosa will get very red, and may ulcerate or blister during the second week of full course  radiation therapy. Bad smell to the breath (halitosis), drooling, and difficulty eating can occur. Oral rinses are often helpful. Some cats and small dogs could require a temporary feeding tube if a large portion of their mouths are in the treatment field. Mucosa heals quickly following the conclusion of radiation therapy.

Ocular side effects are of concern if the eyes are in the treatment field. Acute side effects include dry eye and corneal irritation – so artificial tears and tacrolimus ointments are often needed. Eyes are checked for corneal ulcer formation at least weekly if they are in the treatment field – but let your oncology nurse know right away if your pet is squinting, or if the eyes look abnormal to you at home. Late side effects of radiation (permanent keratoconjunctivitis sicca or dry eye, cataracts, and retinal degeneration can be irreversible side effects of radiation therapy if the eye gets full dose or even scatter radiation.  It takes between 6-12 months for cataracts to form following radiation therapy – cataract surgery could be considered if the tumor is under control.

Certain types of delayed side effects are very dangerous or deadly (spinal cord malacia, kidney fibrosis or scarring, lung fibrosis, death of bone) so every effort to avoid late side effects is made in full course radiation therapy – the risks of developing these problems is small (5-10%). The multiple fractions of radiation therapy as well as careful treatment planning are the ways that we attempt to avoid late side effects.

How will my pet's skin be affected during radiation?
Occasionally a pet may experience some skin reaction during or after receiving radiation therapy. While most of these problems can be remedied with specific over the counter ointments, it is important to check with your veterinarian before applying anything to your pet's skin. Many topical oitments may actually cause a more severe reaction when used in conjunction with radiation, making your pet more uncomfortable.

A normal side affect of radiation therapy is sensitive skin. Normal reactions such as redness, peeling and oozing may worsen if subjected to products containing talc, alcohol, perfumes or unknown ingredients.

One of the best and most common applications to use on skin subjected to radiation therapy is an over the counter product called Aquaphor. This is a salve that may be applied as needed to the affected area. Another product is Aloe Vera 100% Free, which contains no additives. Though they are safe for your pet's skin, they should NOT be applied immediately before a radiation therapy treatment.

During the summer months, it is important to keep your pet's radiated skin out the sun. Exposure to the sun will enhance the normal radiation reactions, which may lead to discomfort to your pet.
What if patient also needs other types of treatment?
If a patient has a type of cancer that would benefit from multimodality therapy (surgery, chemotherapy, etc), then there are additional timing concerns – healing from surgery may be altered based upon when radiation therapy is started. Some chemo drugs make anti-cancer activity of radiation therapy stronger – while some cause worsened radiation burns. 

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 skin moistness, similar to eczema. This moistness may cause the animal to scratch, but it is important not to allow the pet to do so. Medication and/or physical means to prevent scratching may be prescribed by the veterinarian. It is not advisable for you to purchase over-the-counter drugs and treat these conditions yourself unless instructed to do so by your veterinarian. This skin moistness and itching will usually subside in 7-10 days.

 "Burning" is the major concern of owners. Although there may be some skin reaction, this either will not occur, or be very minor in degree. The remedy for this will be the application of Aquaphor or Aloe Vera 100% Free.

Will my pet lose its hair like humans do?
Fortunately, animals experience fewer side effects from radiation therapy than humans do. Fur loss in the treatment area may well happen, but it will grow back in approximately three to five weeks post therapy. There may be some change in the texture and color of the fur but this should not be a concern.

Hair loss in the treated area is common. This will occur after the radiation therapy is over. Hair loss in the treated area may persist for some time, but hair regrowth occurs in most patients. The color of the regrowing hair and skin in the treated area are likely to change. Dark hair and skin usually become lighter in color while light skin and hair may become darker.

It is unusual for animals to become nauseated and have vomiting or diarrhea as a result of radiation therapy. This will usually only occur if portions of the abdomen are irradiated.

When cancer of the oral or nasal cavity is treated with radiation therapy, a foul odor may develop as the tumor is destroyed by the radiation. This odor is usually temporary and decreases as the tumor dies.

If it is necessary to include an eye in the radiation field, a lack of tears may develop in that eye (dry eye) requiring artificial tear supplementation. Gradual loss of sight may occur in that eye.

Will my pet's diet/appetite change during radiation therapy?
 While your pet is undergoing radiation, his/her appetite and digestion may change. Although there may be several reasons for this, it is usually due to stress. Remember, your pet is being subjected to strange smells, sounds, animals, and people. These symptoms should disappear once the treatment series is complete.

If your pet experiences diarrhea during treatment and it worsens or becomes severe, your pet's diet may be changed or medication may be added to help make your pet more comfortable.

It is very important that you do NOT give food or water to your pet for at least an hour following radiation treatment. Due to the sedation that your pet receives, his/her swallowing reflex is temporarily impaired and needs time to return to normal. Otherwise, your pet may choke.

It is also critical that your pet NOT eat any food or water after twelve midnight the night before a radiation treatment, unless other arrangements have been made with the doctor. Feeding your pet beforehand may result in your pet aspirating (choking) on undigested food or water while under anesthesia. This can result in pneumonia, which is a life-threatening problem.

What happens after the treatment is over?
It is important for your veterinarian to examine your pet periodically after radiation therapy is over. This will allow normal tissue side effects to be detected before they become advanced, and the effect of the radiation on the tumor to be evaluated. It is the goal of radiation therapy (with the exception of palliative treatment) to completely eradicate the cancer. In some pets this happens and no evidence of the tumor persists. In other pets the cancer or lump may never completely disappear, but growth is arrested and the tumor is essentially controlled. The specific results to be expected depend on many factors. Specific details on likelihood of success for a specific tumor will be provided to you by the cancer specialist.

Finally, it is important to realize that even though your pet may never be totally the same as before the cancer was diagnosed, it is possible in many pets to provide additional comfortable months or years of happy life through radiation therapy.
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Radiation therapy

PET CANCER CENTER
Comprehensive guide to cancer diagnosis and treatment in cats and dogs
 http://www.petcancercenter.org/index.html
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
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

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.
radiation1
radiation2
The cobalt-60 radiation unit.
Source: http://www.cvm.tamu.edu/oncology/faq/questions/radiat01.html

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.
radiation3
A change in hair color after radiation therapy.
Source: http://www.cvm.tamu.edu/oncology/faq/questions/side01.html

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.
© 2007 Pet Cancer Center. ALL RIGHTS RESERVED.
reprinted with kind permission from Tatiana Spikacova
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Radiation Therapy

Holly Nash, DVM, MS
Veterinary Services Department, Drs. Foster & Smith, Inc.
http://www.PetEducation.com
  
In veterinary medicine, radiation therapy was first attempted at the beginning of the twentieth century. During the last 50 years, large advances have been made. The use of histopathology, MRI, and CAT scans has resulted in accurate diagnosis of the type and location of tumors. New technology has increased the effectiveness and decreased the side effects and risks of radiation therapy.

Under what conditions is radiation therapy used?
Radiation therapy can be used in combination with surgery and/or chemotherapy to provide permanent control or death of a tumor. It is used for tumors that have not spread to other sites in the body and offers a potential cure for some localized tumors. In other cases, radiation therapy can be used for its palliative effect (relieving the signs of disease). Even if the tumor can not be destroyed, at least shrinking the tumor may improve the quality of life of the animal by reducing pressure, bleeding, or pain.

Which tumors are commonly treated with radiation therapy?
Oral tumors and tumors within the nasal cavity often respond well to radiation therapy. Brain tumors have been successfully treated, as have small skin tumors, including some mast cell tumors and squamous cell carcinomas. Lymphoma in both cats and dogs has responded to radiation therapy, and it is often used in conjunction with chemotherapy. The management of bone tumors, including osteosarcoma, has used radiation therapy as part of the regimen.
In considering good candidates for radiation therapy, issues that must be included in the decision-making process include:

The owner's wishes, commitment, time, and financial resources
The pet's overall health and presence of any other disease conditions
The probability of control of the tumor, which must take into account its size, type, location, and any metastasis
Availability of other treatments, e.g.; surgery, chemotherapy
Predicted outcome of radiation and other treatments in relation to cosmetic effects and function
How does radiation therapy work?

Using radiation therapy, beams of photons, electrons, or gamma rays are focused on the tumor. When the photon, electron, or wave hits the nucleus of a cell, it alters it, destroying the ability of the cell to divide and grow. The slower growth rate and ultimate death of the cancer cells causes the tumor to shrink over time. Radiation affects both normal and cancer cells, but the radiation treatment is designed to produce the maximum effect on the tumor and minimize the effect on normal tissue.

What are the types of radiation therapy?
There are many types of radiation therapy. Some radioactive particles can be injected into the body and localize in specific tissues. Radiation therapy using iodine, which collects in the thyroid gland, has been used to treat thyroid disease, such as hyperthyroidism in cats.

"Brachytherapy" is the term used to describe the radiation therapy that is administered through radioactive implants. Implants of iridium-192 are the most commonly used. Radioactive wire containing iridium-192 can be placed using a large needle, or through minor surgery. This type of therapy has been used to treat nasal tumors in dogs and fibrosarcomas in cats.

Beams of radiation are the type of therapy most people are familiar with. A linear accelerator can be programmed to produce varying levels of photon or electron beams that are focused on a certain area on or within the body. Large orthovoltage machines, which are very large x-ray machines, form beams of electrons. Cobalt-60 machines produce gamma electromagnetic waves. These beams, or waves, are focused on the tumor, and when the actual burst of radiation therapy is over, no radioactivity remains in the body.

How is beam radiation therapy administered?
During the treatment, the animal will need to be anesthetized because he cannot move during the procedure. During the first session, the animal is placed on the table. Using radiographs (x-rays), MRIs, and CAT scans, the exact location of the tumor is located. Using mathematical formulas and mapping techniques, the machine is programmed and positioned to focus the beam on the tumor. Various points on the animal's skin may be marked to provide "landmarks" for subsequent treatments. This machine setup usually requires 30-60 minutes. The actual treatment time during the following sessions is very short, and the animal is usually anesthetized for only 10-15 minutes.

How often is radiation therapy administered?
When administered with curative intent, radiation therapy is given in small fractions over 2-5 weeks, depending on the size and location of the cancer, the pet's general health, and the type of cancer they have. For palliative effects, radiation therapy is given in large fractions, usually once weekly for three weeks.

What are the risks and side effects of radiation therapy?
Severe toxic effects of radiation therapy in pet animals are rare, and occur in less than 5% of the animals treated. Because the radiation affects all cells, some normal cells with be killed. Side effects of radiation therapy occur more often with curative attempts, and can be categorized into acute and chronic problems. Acute injuries begin during or shortly after the completion of therapy. They arise in tissues within the radiation therapy field that are growing and dividing rapidly. The most common acute effects are skin problems that mimic a severe sunburn, which sometimes causes the animal to scratch. Since the scratching will further traumatize the skin, medications may be prescribed to reduce the itchiness. Hair loss often occurs at the area where the beam penetrates and leaves the body. The hair will grow back with time, but may be darker or lighter.

Oral and nasal tumors may develop a foul odor as they die.
Chronic side effects arise from damage to tissues that slowly replace old, dying, or damaged cells (i.e., bone, retina, brain). Because these cells reproduce slowly, it takes a longer period of time for them to be replaced. Clinical syndromes such as the formation of bony sequestra, retinal lesions, and neurologic signs may appear, depending upon the area of the body being treated. If the eye is near the radiation field, a loss of tears (keratoconjunctivitis sicca) or vision may occur. Chronic side effects are dose limiting, meaning the dose of radiation may need to be limited if chronic side effects are observed. Veterinary radiation therapy protocols are designed to minimize long-term problems.

There is always a slight risk associated with general anesthesia, but the pet is monitored carefully while it is anesthetized, and the duration of anesthesia is generally short. Prior to radiation therapy, each animal is assessed through laboratory tests and a physical examination.

Radiation therapy in pets usually does NOT cause systemic side effects (tiredness, loss of appetite, nausea).

How much does radiation therapy cost?
Depending on the type and number of treatments, radiation therapy may cost $900 - $3,500. The cost is a result of the expensive and complex equipment needed, anesthetic and laboratory costs, and the expertise of the many veterinary health care professionals involved.

References and Further Reading
Burk, RL; King, GK. Radiation Oncology. Veterinary Clinics of North America - Small Animal Practice. W.B. Saunders Co. Philadelphia, PA. July 1997.
LaRue, SM; Gillette, EL. Radiation therapy. In Withrow, SJ; Macewen, EG (eds.) Small Animal Clinical Oncology. W.B. Saunders Co. Philadelphia, PA. 2001
Kansas State University Press Release: Pet Health News. CT, MRI and radiation therapy. June 2002.
Stachewicz, S. Manager, Radiation Oncology, Marshfield Clinic, Lakeland Center, Minocqua, WI. Personal communication. July 2002.
University of Wisconsin School of Veterinary Medicine. Radiation therapy. http://vmthpub.vetmed.wisc.edu/hosp_services/rt/default.htm.
 
(c) 2008 Drs. Foster and Smith, Inc.
Reprinted as a courtesy and with permission from
PetEducation.com (http://www.PetEducation.com)   On-line store at http://www.DrsFosterSmith.com
Free pet supply catalog: 1-800-323-4208

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The above information is simply informational. It's intent is not to replace the advice of a veterinarian nor to assist you in making a diagnosis of your pet. Please consult with your own veterinarian for confirmation of any diagnosis. Your pets life may depend on it.