Portland Veterinary Oncology Center

13655 SW Jenkins Road
Beaverton, OR 97005



What are the potential side effects associated with radiation therapy?

The possibility of adverse side effects is a major concern for many pet owners when seeking cancer therapy for their pet. Although there are some side effects associated with cancer treatment in pets, they are generally much less frequent and less severe than most people anticipate. Pets very rarely experience nausea, vomiting, diarrhea or inappetance. With radiation therapy, the side effects usually resolve quickly, are severe in few patients, and are limited only to the tissues in the irradiated treatment field (which includes the tumor and a margin of normal tissue around it). 

More Specifically:

– Potential side effects to the skin include localized hair loss, hypo-or hyper-pigmentation, and temporary moist or dry dermatitis. Most pets will experience some or all of these effects, however they will occur only in the irradiated field. The extent of these effects depends mainly on the amount of tissue treated, the radiation protocol used, individual patient sensitivity, and the ability to prevent the patient from traumatizing the area. Hair loss is usually temporary in cats, but it may be permanent in dogs (again, only in the treatment field). Hair will take months to re-grow and it may come back in a different color. Moist dermatitis (reddened, weepy skin) usually begins 3-4 weeks after the onset of radiation therapy. Because it is often accompanied by itchiness, protective methods including use of shorts, T-shirts, patted bandages on the paws, or and Elizabethan collar may be needed to prevent self-trauma. It is very important to prevent your pet from scratching, licking or chewing at the irradiated area as the skin in the field is easily injured. You may gently clean the skin with warm water and a wash cloth. In addition, topical or oral medications may be dispensed if necessary. The affected area usually heals 2-3 weeks after radiation ends. Bathing and swimming are not recommended until the skin is healed.

– You may notice increased salivation and some tenderness in the mouth if the tumor was located in the oral cavity, nose or throat. Therefore, some extra pampering and feeding soft, low salt food may be helpful. We also may dispense a medicated wash or recommend a dilute tea rinse. Inappetance occurs rarely, however it is more likely to affect cats and small dogs. If necessary, a feeding tube may be placed to ensure adequate nutritional intake. These effects should subside within 1-2 weeks following the end of radiation therapy.

– If the nose is in the treatment field, sneezing and nasal discharge are common. However, if your pet is experiencing bloody nasal discharge from a nasal tumor, the bleeding should actually subside within 2 weeks of starting radiation.

– The eyes are excluded from the treatment field if at all possible. However, sometimes the location of the tumor, such as those of the nasal cavity, requires partial or total inclusion of the eyes into the field. You may notice during and shortly after radiation therapy a reddening of the eyes, and discharge from the eyes. Eye drops or ointments may therefore be prescribed. It is imperative that your pet does not rub or scratch at their face or eyes as this may delay healing or injure the cornea. If your pet should start to squint, please bring it to my attention immediately as this may indicate the development of a corneal ulcer, which needs to be treated and monitored closely. Reduced tear production can occur and may be permanent requiring daily treatment with eye wetting agents or other medications. Other effects that may develop months to years after radiation include cataracts and changes in the retina. Visual impairment may result, but rarely leads to complete blindness.

Bones and joints
– The bones in the irradiated field can be weakened and also more susceptible to infection. This is usually not a concern except when the field includes bones of the mouth, or in bones already weakened by tumor (such as with osteosarcoma). It is important to prevent trauma and the possibility of secondary infections to these irradiated bones. For example, patients who have had bones of the mouth irradiated have special needs when it comes to dental procedures and playtime. Please contact Dr. Cyman prior to any scheduled dental work so she can inform your veterinarian of these special needs. In addition, these patients should not play tug of war, chew on hard objects or catch hard Frisbees. Arthritis or fibrosis may result if the radiation field is over a joint; however, this rarely becomes clinically apparent.

– The brain is rarely in the field except when treating brain, inner ear or nasal tumors. The most common side effects start 2 weeks to 3 months after treatment and usually last for 2-3 weeks. These effects include lethargy and sometimes with brain tumors, temporary recurrence of the original neurological abnormalities. Cortocosteroids (prednisone or dexamethasone) may be prescribed to alleviate symptoms.