Aftercare and quality of life
Does radiotherapy in animals have risks and side effects?
von Dr. Jan Kuntz & Team // 24. January 2025
Radiotherapy is an effective method of treating many types of cancer in both animals and humans. In order to make the effect on the tumor as effective as possible, side effects are also accepted. With the right management, many side effects can be controlled very well and subside quickly. The benefits of radiotherapy then far outweigh the risks. In this article, we provide an overview of the most important side effects and address the question of why side effects are so important for radiotherapy protocols.
What are the side effects of radiotherapy for tumors?
In radiotherapy, we distinguish between early and late side effects, i.e. side effects that occur during or shortly after radiotherapy and those that sometimes only play a role years after radiotherapy.
Early side effects are mainly reactions to the skin and mucous membranes. They occur more frequently with definitive protocols with many sessions and a low single dose.
Late side effects can include tissue remodeling processes, weakening of connective and supporting tissues such as bone and the formation of scar tissue. These reactions play a role above all in palliative protocols with few high-dose fractions. As they only become relevant after months to years and this period usually exceeds the life expectancy of the animals, we accept this theoretical risk.
The most common side effect of radiotherapy is desquamation of the skin, which can be associated with redness and pain. This reaction is very similar to sunburn, both in terms of pain and healing.
What are the most common radiation therapy side effects?
When irradiating tumors, the priority is to irradiate the tumor tissue with a defined dose. In order to achieve this goal safely, the area to be irradiated is deliberately chosen to be larger than the clinically clearly visible tumor. This means that healthy tissue is always deliberately irradiated with the full dose. The normal tissue reacts to the radiation towards the end or after completion of the therapy with changes such as swelling, redness, hair loss and pain. These reactions are very similar to sunburn and are mainly seen on the skin and mucous membranes. The fur often falls out locally at the irradiated area. The strength of the radiation reactions depends on the radiotherapy protocol selected.
This sunburn-like change usually heals just as quickly as a sunburn. After three weeks, the areas are covered with intact skin in the vast majority of cases, which is often still thin and sensitive. It takes longer for the hair to grow back, the new coat is gray or white after radiotherapy and often grows back a little thinner.
To ensure that the painful areas heal without complications, it is particularly important that the animals do not scratch or lick. This causes additional injuries, irritation and bacterial colonization, which can also lead to inflammation of the skin, known as dermatitis. If dogs or cats scratch in the radiation field, they must always wear a collar!
If early side effects occur, consistent action is required:
The animal must not scratch or lick the irradiated area under any circumstances. If there is the slightest suspicion, the patient must consistently wear a neck collar until healing has taken place. This allows the reactions to heal and prevents secondary injuries or inflammation.
If the animals have a good prognosis and the life expectancy is several years, we work with long, so-called definitive protocols, which have a high total dose and many fractions. After this type of irradiation, we see weeping and dry desquamation of the skin and the formation of fibrin deposits on the mucous membrane. The main aim of palliative protocols is to improve the animal's quality of life and make its last months as comfortable as possible. These protocols do not cause too many short-term side effects. In the long term, long-term side effects may occur in some cases after several years, but in the vast majority of cases these are no longer experienced due to the life expectancy of the animal.
What other side effects are possible?
In principle, further and also serious side effects are possible. It is particularly critical if organs at risk are very close to the tumor. We try to spare them as much as possible, but in some cases they still have to receive the full dose in some parts. This is particularly obvious in the case of tumors in or even on the eye.
In the case of a tumor on the eye, in this case a squamous cell carcinoma, the eye is in acute danger from radiotherapy. We then speak of an organ at risk. However, this risk is rarely an argument against radiotherapy, as the alternative to this treatment is enucleation, i.e. removal of the eye, and often, as in this case, the organ at risk is spared completely;
Other side effects can also occur with internal tumors. If the intestines are also irradiated, for example, diarrhea and temporary digestive problems are possible. Sometimes food supplements can help during this time. We discuss these and other questions individually with the patient owners.
Even though we are pleased with the rapid response of the tumor in many cases, this can be a risk for some patients. If tumor tissue retracts too quickly and uncontrollably, the healthy tissue may not have time to close the resulting gaps. In these cases, fistulas can occur. These are also possible, for example, between the oral cavity and the nasal cavity, which can lead to problems and restrictions.
What si the role of side effects in the development of radiation protocols?
The aim of radiation is to damage the tumor tissue as efficiently as possible in order to improve the patient's prognosis as much as possible. The aim here is to irradiate with the highest possible doses. The side effects would then be correspondingly severe. For protocols that cause no side effects at all, the dose would have to be reduced so much that tumor control would also be much worse. When developing protocols, we always look for a compromise: the side effects should only be as severe as possible so that management is possible and the animals can tolerate the side effects well. At the same time, we want good tumor control. In general, it can be said that radiation reactions are actually a good sign that the most possible has been done in the unfavorable situation of the tumor diagnosis.
Do side effects also occur with pain radiotherapy?
Due to the low dose and the small number of fractions, the occurrence of side effects is extremely rare when irradiating osteoarthritis and chronic pain. Interestingly, however, we do observe hair loss in the irradiated area in some patients and some breeds, usually without severe reactions or changes to the skin. In order to record this accurately, it is important for us that you let us know about such side effects, even if they occur a long time after the radiation treatment.
