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Radiation Therapy

Radiation Therapy is becoming a prominent and demanded modality as a tool for treating canine and feline patients with cancer. It plays an essential role in both primary treatment and as an adjuvant to surgery for local control of tumors. Many, but not all tumors can be controlled with radiation therapy, either alone, or in combination with surgery and chemotherapy. Our ability to treat cancer patients is optimized with the availability of modern equipment and trained therapists and doctors. We are also continuing to learn how to better plan radiation dosages and schedules in order to optimize tumor control while limiting side effects. Side effects do occur because radiation treatment is not 100% specific for cancer cells, but these side effects can be managed and will heal without long-term consequences. Radiation patients are very specially cared for; we understand that they have a long road in battling cancer, and we are with them every step of the way.

What follows is a practical guide to radiation therapy: its indications, the nitty-gritty on how it’s delivered, and expected side effects.

Indications

Best for local control when surgery is not an option or surgery has failed (i.e. incomplete surgical margins)

As primary therapy:

  • Nasal tumors
  • Oral melanoma
  • Single site lymphoma in cats

As adjuvant therapy:

  • Soft tissue sarcomas
  • Mast cell tumors
  • Feline vaccine associated tumors
  • Some oral tumors
  • Anal sac (apochrine) adenocarcinoma

As palliation:

  • Osteosarcoma
  • Non-resectable mast cell disease
  • Cutaneous hemangiosarcoma

Logisitics

A gray is a unit of radiation dose. Most curative intent radiation doses to treat cancer are between 45 and 57 gray. (500 times what a diagnostic x-ray machine puts out). This amount of radiation cannot be tolerated at one time; therefore it is broken into many small doses, or fractionated. The dose of a fraction is usually 2.5 - 3.2 gray. Here at GSVS, we treat 5 days a week Monday through Friday. Therefore, a course of treatment usually will take 3-4 weeks. The first day is a simulation and planning day, where often the dog’s tumor will be imaged by CT scan. The field, dose and schedule will then be planned to optimize tumor coverage, while limiting normal tissue exposure. This is done via a state of the art computerized imaging and planning system and a trained radiation therapist. On the planning day, the field of treatment will be shaved and ink marks will be placed as reference for consistent beam alignment from day to day (treatment to treatment).

Patients can be treated as an outpatient if the owner’s wish, or they can remain in hospital during the week for convenience, and go home for the weekend. The cost of treatment is usually between $3500 - $4500 (all inclusive), depending on the animal’s condition, what monitoring needs to be done, expected side effects and site. (A nasal case will be more than a mast cell tumor on a limb).

Side Effects

Radiation side effects are divided into acute side effects and delayed side effects.

Acute side effects happen during, or immediately after therapy. These side effects are predictable, and happen to normal tissues that are in the field of treatment. Acute side effects will heal over time, but nursing care and pain management is required during the initial phases. Side effects vary with the site treated, as well as the total dose; some tumors are treated more intensely than others, which will alter the severity and duration of side effects.

Moist Desquamation describes the side effect seen to the skin. I liken it to a severe, blistering sunburn. The skin will get red, moist, itchy and painful. The most important steps to managing this side effect are:

  1. Hygiene - keeping the site clean and dry with warm water misting or rinsing. No cleansing products or moisturizing products should be applied. We also don’t like to wrap the site, as this may retain dirt and moisture.
  2. No licking! - E-collars are absolute necessities during this time. Constant moisture will slow healing. Also, that skin is very fragile and if it is torn or ulcerated, infection may get in and further delay healing. Sometimes, dogs may need to be sedated (benadryl often works) for a couple of days at the peak of itchiness.
  3. Pain management. I always start with anti-inflammatories, but add codeine or fentanyl patches (narcotic pain management) if needed.

As the healing process moves forward, skin will slough off, then the site will form crusts, which should be allowed to fall off on their own (or this can be encouraged with warm water rinsing which will loosen the crusts), and fresh new skin will be underneath. The whole start to finish process is about 2-3 weeks, with 3-7 days in the middle being the toughest. When the skin heals completely, the only noticeable persistent effect may be a change to the coat color in that area.

Note: Cats get dry desquamation rather than moist desquamation, which consists of dry, flaky skin and mild-moderate itchiness.

Mucositis describes the changes to the oral mucosa that may be in the field when treating oral or nasal tumors. The oral mucosa will get very red and painful, and ulcers may occur on the gums, lips or tongue. The signs that mucositis is beginning is halitosis (bad breath) and ropey saliva. Many dogs will continue to eat and drink through this side effect. Sometimes dogs will need subcutaneous fluids to ensure adequate hydration. Occasionally, a small dog or cat may need a feeding tube if a large portion of their mouth is in the field. Fortunately, the mouth heals very quickly, and once again, there are no long term effects once healing is complete.

KCS (keratoconjuctivitis sicca) or dry eye can happen when dogs undergo radiation therapy for a nasal tumor, since many nasal tumors extend back into the frontal sinus. Treatment with artificial tears and cyclosporine (to encourage new tear production) is often indicated and monitoring for and treating corneal ulcers is essential.

Delayed side effects happen many months to years after therapy. These effects are on slowly dividing tissues such as nerve, bone, and vessels. These tissues cannot easily repair, and may be life-limiting if they occur in a vital organ; therefore every effort is made to avoid vital structures such as the spinal cord, brain, heart, and kidneys. If these areas need to be in the field for tumor control, the dose given each day is decreased because it is that dose that determines late side effects. Where acute side effects happen every time, the occurrence of late, life threatening side effects is usually 5-10%.

Some more important delayed side effects that we are concerned with include:

  • Spinal Cord Malacia - the spinal cord is often included in the treatment field when a vertebral tumor, or cat vaccine associated sarcoma is treated.
  • Cataracts - Most commonly seen with nasal tumor radiation; formation takes 6-12 months. There is no contraindication to cataract surgery in long term survivors.

Radiation is an important tool in control of many local invasive tumors. Treatment requires a large time, emotional and financial commitment. As an owner, you need to have every tool necessary to make the best decision for you and your pet. It must be remembered that the rewards can be great, and the end often does justify the means.

During your visit to Garden State Veterinary Specialists, we will discuss your particular pet, and the special and individual needs and wishes of your family in battling this devastating disease.

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