Radiation Therapy Options
WHAT IS RADIATION THERAPY?
Radiation therapy is the treatment of disease using high-energy radiation from x-rays, gamma rays, electrons, neutrons, protons, or other sources to kill cancer cells and shrink tumors. Most radiation treatments in veterinary medicine are done using a machine that directs a beam from outside of the body (external-beam radiation therapy). There are other forms of radiation therapy used here as well for specific tumor types, including Strontium-90 radiotherapy for treatment of superficial tumors, used for feline squamous cell carcinomas of the nose and ear tips. In this instance, the radiation comes from a radioactive source of material placed next to cancer cells. Systemic radiation therapy uses a radioactive substance that circulates throughout the body. In veterinary medicine, the most frequent form of this is radioactive Iodine-131 for the treatment of hyperthyroidism in cats.
HOW DOES RADIATION THERAPY WORK?
Radiation kills cells or keeps them from growing and dividing by damaging DNA, the genetic material of cells. Because cancer cells grow and divide more rapidly than most of the normal cells around them, radiation therapy can successfully treat many kinds of cancer. Normal cells are also affected by radiation but, unlike cancer cells, most of them recover from the effects of radiation. To protect normal cells, doctors carefully limit the doses of radiation and spread the treatment out over time. They also shield as much normal tissue as possible while they aim the radiation at the site of the cancer.
WHEN IS RADIATION THERAPY RECOMMENDED?
There are many useful applications for radiation therapy in veterinary medicine. The most common use of radiation therapy is as an additional treatment for tumors that are incompletely resected or unresectable (including certain types of tumors in the brain, nasal cavity, or mouth that are also best treated with radiation therapy, because sometimes surgery at these sites is not a good option). There are many other cancers where radiation may be useful in controlling disease, your veterinarian will discuss these with you. When tumors are painful, radiation can also be a useful treatment to decrease the pain associated with those tumors.
WHAT ARE THE SIDE EFFECTS OF RADIATION THERAPY?
Effects from radiation therapy are broken down into acute (during or shortly after finishing treatment), and late effects (occurring many months or years after treatment). These effects are generally limited to the area treated and depend on the tissues that are in the radiation beam. Often, the skin will react to radiation, which can take about a month to heal. This can look very similar to severe sunburn and in more serious reactions, can be moist and develop crusting at the site. Preventing dogs and cats from licking or traumatizing the site is critical because self-trauma can dramatically worsen these reactions and can cause them to take much longer to heal. Most pets are completely recovered from this acute radiation skin reaction within a month of finishing treatment. It can take many months for the hair to stereotactic radiation therapy (SRT) can be considered in dogs who are not deemed suitable candidates for amputation. In the published literature about this technique in dogs, the reported outcome and survival was similar to dogs who underwent treatment with amputation followed by chemotherapy.
Typical protocol involves:
- A CT scan the week prior to generate the radiation treatment plan.
- After the radiation treatment plan has been generated, the use of a bisphosphonate 24-72 hours prior to the first dose of radiation therapy is recommended.
- The currect SRT protocol utilizes 3 doses of radiation om 3 consecutive days.
- Chemotherapy (usually carboplatin) is administered at the time of the first or second radiation therapy treatment and is continued every 21 days for a total of 4-6 doses of chemotherapy.
Pathologic fracture is the most common complication following this procedure. This is mostly due to the amount of preexisting tumor associated osteolysis and the amount of postirradiation bone necrosis. Typically smaller and more blastic lesions are better candidates for this procedure over large and more lytic lesions. Pathologic fracture rates can range from 30-45%. Other side effects include hair loss, pigmentation of the underlying skin, and dermatits (skin burn from radiation). Long term side effects from radiation include fibrosis and necrsis within the radiation site.
PALLIATIVE RADIATION FOR OSTEOSARCOMA
The goal of palliative radiation therapy is to address the pain and swelling associated with the primary tumor. We see relief in up to 70% of patients for some period of time. This therapy is given either as 2 consecutive treatments on back-to-back days, or once weekly for 4 treatments. Palliative radiation may have some anti-tumor benefit, however these effects are likely short-lived with minimal to no tumor control noted, and no benefit provided to lower the risk of metastasis. Side effects of this therapy are rare, however the treatments do involve low-level anesthetic events.
Without chemotherapy, survival times using palliative radiation therapy, with the addition of bisphosphonate (used to strengthen the bone) is generally around 3-4 months. The addition of chemotherapy may contribute some anti-tumor benefit, potentially lower the risk of or delay the onset of metastasis, and extend some patient survival times.