The Dog Cancer Survival Guide (39 page)

Palliative radiation therapy uses far fewer sessions, usually one to five, which means there is less risk of an anesthetic accident. These sessions are also less complicated and less demanding on your time for after-session care. There is also, of course, less expense, because there are fewer sessions and a shorter stay in the vet hospital.

Because radiation equipment is so expensive to purchase and maintain, radiation oncologists usually practice in large urban centers or veterinary medical schools.

Generally, it is easier for normal tissues to repair from small doses of radiation than from larger doses. For this reason, radiation oncologists typically split up the total radiation dose into several small treatments. These small treatments are called fractions, and they ensure that cancer cells are killed and normal tissues have a chance to repair themselves. Most curative radiation protocols recommend fifteen to twenty daily treatments, over the course of three to four weeks, while most palliative protocols call for somewhere between one to five treatments (depending upon the tumor type and case), usually once a week.

Radiation therapy is usually given on a daily basis, Monday through Friday, requiring daily trips to the hospital. For convenience, some guardians choose to board their dogs in the hospital during treatment days and take them home for weekend breaks.

Dogs must be anesthetized every time they get a treatment. This is to make sure they are completely still and do not move a healthy body part into the damaging radiation beam. The repeated, daily use of anesthesia is a very important factor for guardians to consider.

(As we can sit still on our own, humans usually do not get anesthesia during radiation.)

The most common radiation machine in veterinary use is the linear accelerator, or LINAC. The linear accelerator is a large machine that produces short, rapid bursts of radiation. The dog is laid on a treatment table, which stays completely still, while the linear accelerator revolves around it, in a full 360° circle, to administer the radiation. This machine can treat from any angle around that axis, but not from any other angle, which means the radiation may fall on normal tissues. Typically, 25-50% of the total dose of radiation is delivered to normal tissues, which could cause life threatening side effects. The problem radiation oncologists have is this: how do we deliver the entire dose without causing life threatening side effects to the normal tissues in the treatment area? The solution is to break up that total dose into fifteen to twenty (or more) doses (called fractional treatments), given over the course of three to four weeks, all of which call for anesthesia.

Medical researchers have long been looking for a way to deliver radiation with more precision and less risk to the patient, and recent advances have made progress. For example, a technique called “radiosurgery” aims at getting the radiation energy directly to the tumor – even inoperable tumors – while avoiding healthy body parts that might be damaged by the beam. First, a CT (computed tomography) scan or MRI is run, to get a very detailed three-dimensional image of the tumor. Then, a computer program helps the oncologist and/or radiation oncologist to plan precise coordinates for the radiation beam. New, modified linear accelerators can generate very controlled, narrow beams that deliver radiation only to those exact coordinates. By the way, there is no actual cutting during radiosurgery – the name is meant to imply that radiation is accomplishing what surgery would do, if it could actually get to the tumor.

At the time of writing, this type of radio-surgery is only practiced at the University of Florida, Colorado State University, and the Animal Specialty Center in New York (Dr. Ettinger’s practice). Dr. Ettinger’s practice owns a particularly advanced radiosurgery machine called the CyberKnife. The machine has five “arms” and rotates around several axes. It can approach a tumor from almost any angle selected by the radiation oncologist and, while a traditional linear accelerator has two to four ports (holes where the radiation exits), the CyberKnife has hundreds. This allows for very narrow beams of energy that can be precisely directed to the tumor. Using this machine, normal tissue receives 1/100 or even just 1/200 of the total dose (instead of 25-50%). The entire dose can be given in one to three treatments (instead of fifteen to twenty) and, in Dr. Ettinger’s experience, acute, or immediate, side effects are greatly reduced. Dr. Ettinger’s radiation oncologist colleague uses this machine to treat otherwise inoperable tumors including brain tumors, nasal tumors, spinal tumors, osteosarcoma, prostate tumors, and some oral tumors. On the other hand, radiosurgery is not recommended for microscopic cancer cells, like those left after a surgery, mast cell tumors (even non-resectable mast cell tumors) or soft tissue sarcomas (except in a few non-resectable cases).

There is a relatively new radiation therapy called “brachytherapy” (which translates to “short therapy”).

Instead of using a machine to beam radiation into tumors, therapeutic radioactive materials are sealed inside small bundles, shaped like seeds, needles or tubes. These bundles are then surgically implanted in the tumor itself or right next to it. The surgical site is closed up and, after a period of time, a second surgery removes the bundle. This allows the radiation direct contact with the tumor, causing much less effect on normal tissues. If you’re interested in brachytherapy, ask your oncologist whether it could help your dog. To find a radiation facility near you that offers brachytherapy, check
www.VetCancerSociety.org.

Most dogs can go home right after their last radiation treatment, whether it’s traditional radiation, palliative radiation, or radiosurgery. There is some recovery time involved, depending upon how intensive the therapy was and where the radiation was applied. Depending upon the case, some dogs might develop skin burns that need topical treatments (like aloe), or bandaging. If the mouth was irradiated, the tissue might be quite sore, in which case only soft food would be given. Some dogs might require antibiotics and/or anti-inflammatory medications. If the skin over a joint is burned, its movement might need to be limited and an Elizabethan collar might be necessary to keep the wound from being licked.

One of the main things to consider when evaluating radiation therapy is how often your dog will need to be treated. As you’ll remember from the sidebar on
page 118
, there are inherent risks every time your dog undergoes anesthesia, and those risks apply to every radiation treatment, including all twenty rounds experienced in many traditional radiation protocols.

There are also side effects from the radiation itself, which oncologists group into early (acute) reactions and late reactions.

Acute reactions usually happen during or soon after treatment, and are very common. Most of them have to do with what is called “die off,” or the death of rapidly dividing normal tissues within the path of the radiation beam. About 80% of dogs will experience some degree of the following
acute reactions
immediately after or within days of receiving traditional radiation therapy:

• Skin conditions:
  The skin is made up of several layers of rapidly dividing cells. As these cells die off or try to repair themselves, you may see redness, flaking, or blistering (like severe sunburn). Hair may fall out and, in extreme cases, the skin may even suffer necrosis (death and sloughing), leaving a hole (ulcer) that will need to be filled in by the body’s natural repair processes (if possible). Sometimes dogs lose their fur or hair, or it changes color.
  
• Mouth sores:
  When they fall in the path of the radiation, the mucous membranes in the mouth can swell or develop sores, making eating painful. If the sores get infected, antibiotics may be required.
  
• Dry eye, Corneal Ulcers:
  When radiation treatments occur near the eyes, the lachrymal gland or tear ducts can be damaged. If this happens, a condition called
  keratoconjunctivitis sicca
  (dry eye) may occur. In this condition, the eyeball does not receive enough natural tears or lubrication. This leads to extreme inflammation and a thick greenish discharge. As you can imagine, chronically dry and swollen eyes are very uncomfortable. To keep your dog’s eyes healthy, you will need to apply lubricating ointment twice a day for the rest of his life. If the eyeball itself is in the radiation field, corneal ulcers (holes or craters) are likely to develop, causing pain and squinting. Ulcers may require ointments or antibiotics until the ulcer heals.
  

About 5% of dogs experience
late reactions
, which show up one month, three months, six months, and even a year or two after radiation treatments. Which effects show up will depend upon what areas of the body were treated, usually involving cells that grow more slowly. If your dog is older, you may never see any of these effects – but if your dog is young, they may pop up later in her lifetime. Here are some late side effects:

• Bone and ligament damage,
  due to the necrosis, or death, of cells in these areas. This can result in limping or other painful movements.
  
• Nerve or spinal cord injury,
  which can cause instability or wobbliness while walking, difficulty getting up, weakness, or difficulty moving around.
  
• other cancers,
  which can arise from the damaged DNA in cells that manage to survive and replicate. For example, 3-5% of dogs whose bones are exposed to radiation develop osteosarcoma three to five years later.
  
• Brain infarction,
  which is a sudden loss of blood flow to the brain. Depending upon which area of the brain is affected, some signs of an infarct are: drifting to the side while walking, holding the head sideways, or having a facial droop on one side. These can be permanent changes, but sometimes dogs can make a recovery.
  
• Blindness:
  if the eyes were within the field of treatment, within a year or two, cataracts or retina problems inevitably show up, which could lead to permanent blindness.
  
• Kidney injury,
  which can cause weight loss, decreased appetite, vomiting, decreased energy, and increased thirst and urination (the kidneys are rarely included in the field of treatment for this reason).
  

Radiation therapy is incredibly powerful and increasingly more precise, and in the right case, its use can make sense. However, it is very expensive and carries high risks. I approach its use with caution, especially in younger dogs, and I am very concerned about minimizing risks and side effects. Make sure to weigh all these factors with your veterinarian and oncologist.

Historically, the word chemotherapy referred to the medical use of any drug. Today, we use it to mean the use of drugs in cancer treatment. Dr. Ettinger discusses the most common drugs in Part V, “ From the Oncologist.”

There are many chemotherapy drugs, which are usually combined with each other and administered in a series of treatments, called protocols. Like radiation therapy, chemotherapy targets rapidly dividing cells. All cancer cells divide rapidly, but so do some normal body cells (for example, those in the skin, lining the digestive tract and the bone marrow). For this reason, side effects are possible and even probable, in some cases.