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Radiation Therapy

Radiation Therapy Options

WHAT IS RADIATION THERAPY?

Radiation therapy uses high-energy radiation from x-rays, gamma rays, electrons, neutrons, protons, or other sources to destroy cancer cells and shrink tumors. Radiation treatments vary and can be either non-invasive (treatments are administered from the outside of the body), or they can be systemic – radioactive materials are circulated throughout the body.

Most radiation treatments in veterinary medicine are conducted non-invasively, using a machine that directs a beam from outside the pet’s body (external-beam radiation therapy). Other forms of radiation therapy for specific tumor types are also available, including Strontium-90 radiotherapy for the treatment of superficial tumors, especially 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 the cancer cells.

Systemic radiation therapy uses a radioactive substance that circulates throughout the body. In veterinary medicine, the most commonly prescribed version of this therapy is radioactive Iodine-131 for the treatment of hyperthyroidism in cats.

HOW DOES RADIATION THERAPY WORK?

Radiation kills or prevents cells from growing and dividing by damaging their 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 different types 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 oncology. The most common use of radiation therapy is as an additional treatment for tumors that are incompletely resected or unresectable. These include sites were surgery isn’t always a good option, including: some brain tumors, nasal cavity, or mouth tumors. 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 associated pain.

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, in more serious cases it can be moist and/or 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 recover completely from this acute radiation skin reaction within a month of finishing their course of treatment. It can take many months for the hair to regrow at the treatment site, and it will often grow back thinner and white or grey in color. In some cases the hair will not grow back at all in the area that was treated.

Depending on the specific location of your pet’s treatment field, there may be other important acute side effects at sites (such as the eyes, mouth, or bone marrow). The veterinarian prescribing radiation therapy will discuss the side effects that may be particular to your pet when you come in for your appointment.

Stereotactic Radiation Therapy for Osteosarcoma

Nonsurgical methods for limb salvage such as 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 SRT Protocol Involves:

• A CT scan the week prior to generate the radiation treatment plan.
• After the radiation treatment plan has been  developed, 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 on 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.