Effective, but rarely used:
How does radiation therapy work in animals?
von Dr. Jan Kuntz & Team // 05. February 2025
Radiation therapy is still a comparatively rarely used therapy in our pets and domestic animals. The rays are not visible and the basic principle of this therapy is accordingly difficult to imagine. We clarify here the most important questions about the effect, technique and procedure. However, this information cannot replace a personal conversation in a specific case. If you have any questions, please do not hesitate to contact us.
What are the indications for radiotherapy?
Radiation therapy is used, among other things, for cancer and is one of the three classic pillars of oncology. It is therefore used in our animals, just as in humans, to combat benign and malignant tumors. Radiation therapy, like surgery, is a local form of treatment. This means that tumors and metastases are targeted for radiation, which is why the side effects of radiation therapy are also local.
Not all tumors and tissues are equally sensitive to radiation, which means they react differently to radiation therapy.
Individual treatment is crucial for every patient
The ideal therapy for a particular tumor can consist of several components. For example, some tumors are first operated on and then irradiated; this is known as adjuvant radiotherapy. The reverse is also possible: first radiotherapy, then surgery. This is called neoadjuvant. Sometimes chemotherapy following radiotherapy makes sense. There is even a direct combination, radiochemotherapy. In this case, the enhancing effect of chemotherapy during radiotherapy is utilized; the drugs used are called radiosensitizers.
Can radiation therapy be used for other diseases?
In addition to the treatment of cancer, radiation therapy is also successfully used to relieve pain. This is also referred to as radiation therapy for benign diseases. These include, for example, degenerative joint diseases, chronic inflammatory processes in the joints or osteoarthritis. Such pain arises, for example, in dogs in the long term due to elbow joint dysplasia, hip joint dysplasia, due to overweight, but also as a normal sign of wear and tear in older animals.
How does radiation therapy work?
Physically, radiation therapy is based on the application of ionizing radiation. This means that the radiation must have enough energy to knock individual electrons out of an atom or molecule. In our case, it is either high-energy electrons or ultra-hard X-rays.
The ionizations in the cell cause damage to the DNA, the blueprint of the cell. If the damage is so extensive that it cannot be repaired, the cell perishes and the goal of radiation therapy is achieved. Cells and tissues do not burn during radiation, they do not even heat up. The total energy is so low that heating is not measurable. Therefore, radiotherapy itself is not painful, there is no sensory perception of ionizing radiation. However, the side effects may cause temporary pain, similar to sunburn. Radiation therapy should not be confused with the use of lasers or visible light. Photodynamic therapy (PDT) is also based on a completely different principle. The patient does not become radioactive himself during radiation, so there is no additional danger to him after treatment.
Important to know
After radiotherapy, the patient is not radioactive. Therefore, radiotherapy is not a source of danger for the animal owner when handling the animal. However, especially when handling children, the animals should be observed until the anesthesia has completely worn off..
Radiation therapy: High-tech for maximum precision!
The ionizing radiation is generated at Equinox Healthcare using a linear accelerator. This irradiation device from human medicine accelerates electrons so strongly that they reach almost the speed of light. These high-energy electrons can now be used directly for radiation therapy, such as for many equine skin tumors.
When the electrons are decelerated again inside the accelerator in a metal plate, the target, bremsstrahlung, or ultra-hard X-ray radiation, is produced. This is physically comparable to radiation from a diagnostic X-ray tube, except that the energy is about a hundred times higher. The X-ray tube would therefore have to be operated at six million volts. These X-ray photons penetrate deep into the tissue and are therefore used to irradiate internal tumors, for example in the nose, brain or chest.
The millimeter-precise irradiation with a huge irradiation device requires a lot of planning and modern technology. When irradiating with electrons, individual apertures are often cast from a heavy metal to narrow down the irradiation field.
In the case of internal tumors, the radiation plans are drawn up on powerful computer systems. This ensures that the tumor receives the prescribed dose while the surrounding tissue is spared as much as possible. An irradiation plan contains instructions for the irradiation device, from which directions and with which settings to irradiate. An important component here is the multi-leaf collimator, or MLC for short. The individual leafs can be moved very precisely, quickly and computer-controlled, leaving a gap in the center for the beam to pass through. This means that exactly the desired area can always be irradiated, while organs at risk are spared as much as possible.
Computed tomography and patient positioning
The performance of a modern radiation therapy unit can only be fully utilized if the patient is positioned accurately and reproducibly. For this purpose, individual positioning aids are made for each patient. At Equinox Healthcare, we use vacuum mattresses, i.e. dimensionally stable pillows, and denture casts for positioning, among other things. Many aids are not commercially available. We either have them custom-fit or build them ourselves in our workshop.
We make the positioning aids for each patient at the very beginning, even before the computed tomography is performed for radiation therapy planning. To ensure that everything fits together in the end, our computed tomography unit has the same patient couch as the radiation unit. This means that the positioning aids can be easily and precisely attached to both systems.
Patient-specific positioning materials
Vacuum mattresses, bite impressions, screens and other additional constructions are used for precise and accurate positioning of patients. They remain unchanged for as long as the patient is undergoing radiotherapy.
Treatment planning
In planning CT, not only the tumors themselves but also many organs are marked. In radiotherapy, this is called contouring. Based on the contoured structures, the computer can calculate an optimal radiation plan in which the tumor receives the highest radiation dose and the organs at risk receive the lowest possible radiation dose.
Biological radiation effects
Ionizing radiation causes damage in the irradiated cells. Although ionization processes take place everywhere in the cell, they are biologically relevant only in the area of DNA, which is important for cell division. The structure of the DNA can be disturbed, strands can break or the DNA attaches to other structures. In many cases, this damage can be repaired; only a few actually lead to cell death. It is precisely this property that we exploit in radiation therapy. This is because the ability to repair is much more pronounced in healthy cells than in tumor cells. For this reason, we divide the total dose into small portions, known as fractions, which we irradiate at intervals of one to seven days. In the time between fractions, the healthy cells can repair themselves, while the tumor cells accumulate damage. For the damaged tumor cells, the defects in the DNA become a problem during one of the next cell divisions and they go into structured cell death. The cells therefore do not simply burst, but are recycled by the body's own functions.
How quickly will I see the results of the radiotherapy?
How quick a tumor responds to radiation therapy depends on the original cells from which it originated. Some tumors already shrink significantly under radiation therapy, while others take several months to respond. Palliative radiation is aimed in part at stopping or slowing further growth of the tumor and relieving pain. The appropriate radiation protocol depends largely on the indication, i.e., the tumor itself. A basic distinction is made between definitive and palliative protocols.
Definitive protocols aim to achieve the best possible tumor control over the longest possible period. They have relatively many fractions with moderate single doses.
Palliative protocols, on the other hand, are designed to improve the patient's quality of life without placing a prolonged burden on the patient. Here, we usually use few fractions with a high single dose.
Wherever possible, we base the selection of protocols on scientific studies. However, in veterinary medicine, there is not an adequate publication for every tumor in every species. In particular, in the horse, large studies are still lacking for most tumors. In these cases, we rely on findings from other animal species, from human medicine and on our empirical values. The therapeutic options and the protocol to be applied must be discussed individually in these cases. The question of whether to choose a palliative or definitive protocol is also an individual decision. However, it is not possible to adapt the therapy to the individual response of the tumor. Small tumors do not necessarily require fewer fractions than large tumors. Rather, adjuvant radiotherapy after macroscopically complete tumor removal is often performed in a very large number of fractions to achieve the best possible outcome.
