Pet Health Library

Background

Radiation therapy is one type of treatment for cancer. It is often best used for localized tumors, or cancer that is only in one spot in the body. At times, it is very useful after surgery if there are cancer cells that surgery could not remove or if a mass is too big or in too risky of an area for surgery to be performed.

Radiation therapy uses high-energy beams (ionizing radiation) aimed at specific points on the body in an attempt to kill tumor cells. As it is important that these beams target the cancer cells and damage as few healthy cells as possible, pets need to be anesthetized and precisely positioned to deliver radiation treatment most effectively.

Conditions Treated by Radiation Therapy

Below is a list of common cancers treated with radiation therapy:

• Anal gland tumors
• Oral tumors (melanoma, squamous cell carcinoma, ameloblastoma, fibrosarcoma, carcinoma, lymphoma, plasmacytoma, osteosarcoma)
• Nasal tumors (adenocarcinoma, sarcoma, lymphoma, squamous cell carcinoma, lymphoma, hemangiosarcoma)
• Mast cell tumors
• Bone tumors (osteosarcoma, chondrosarcoma, MLO/multiobulated tumor of bone)
• Soft tissue sarcomas (fibrosarcoma, nerve sheath tumor, hemangiopericytoma, schwannoma, myxosarcoma, leiomyosarcoma, hemangiosarcoma, lymphangiosarcoma)
• Basal cell tumors
• Injection-site sarcomas
• Toe (digit) tumors (melanoma, mast cell tumor, sarcoma, plasmacytoma, carcinoma)
• Heart-based tumors (chemodectoma, hemangiosarcoma)
• Lymphoma

Below is a list of noncancerous conditions treated with radiation therapy:

• Stomatitis
• GME
• Arthritis
• Cystitis
• Epulus

Types of Radiation

There are a variety of methods to administer radiation treatments. The type used will depend on which cancer your pet has, the location of the cancer, as well as what options work best for you and your family. Your oncology specialist may also recommend chemotherapy, surgery, and/or immunotherapy.

• Stereotactic Radiation – uses pinpoint accuracy to avoid administering radiation to the normal area surrounding the tumor while giving a high dose of radiation to the tumor. Treatments are given on consecutive days for 1-3 treatments total.
• Conventional Radiation – uses small doses of radiation often given 3-5 times per week for 16-21 treatments.
• IMRT Radiation (intensity modulated radiation therapy) – uses pinpoint accuracy to treat resistant cancers using small doses of radiation administered 3-5 times per week for 16-21 treatments.
• Palliative Radiation – given to help improve quality of life by decreasing pain and inflammation. Treatment is often once a week for 3-6 weeks.
• Plesiotherapy – a wand-like device containing radioactive strontium-90 is used to apply a small amount of radiation directly onto the tumor.

What to Expect

After consultation with a radiation oncologist and after pre-radiation tests are completed, your pet will be scheduled for an RT set-up appointment where measurements will be taken, the treatment area mapped, and molds to hold your pet in an exact position will be made. General anesthesia is necessary for this stage of planning as accuracy is paramount. A CT scan may also be indicated. This data will then be processed, and your pet’s radiation plan will be created. This is a labor-intensive process which can take 1-7 days to complete. Once complete, your pet will be scheduled to begin RT. Based on the tumor type and goal of therapy, your pet may have 1, 3, 6, 10, or 18 treatments. With each RT treatment, your pet will need to be anesthetized to ensure accurate and reproducible positioning each and every time.

Side Effects

It is the goal of the radiation therapy team to minimize the side effects as much as possible. Unfortunately, there is always a risk of side effects with any medical treatment. Side effects occur only in the area that is receiving radiation.

• Acute Effects – these typically occur within the first few weeks of treatment up to a few weeks after radiation has been completed. Signs can include pink/red or moist skin, fur loss, inflammation/redness of the eyes, mouth, ears, bladder, and diarrhea (if these areas received radiation).
• Late Effects – these can occur months to years after radiation and can include no fur in the RT area, white fur regrowth, skin discoloration, dry/thin skin, strictures, scar tissue, lymphedema, and lysis of bone or fractures. While many patients will experience changes to their fur, they rarely will experience other late effects.

Treatment often includes anti-inflammatory medications, antibiotics, and/or topical creams. These will be discussed with you and prescribed if needed.

Prognosis

Your radiation oncologist will discuss treatment outcome expectations with you. The results of a thorough diagnostic evaluation, cancer type, chance of metastasis (spread of cancer), and the presence of other diseases will factor into the prognosis. As with any medical treatment, prognoses are merely guides as some patients do no do as well as predicted and some do much better than predicted.