How does radiation therapy work in animals?

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?

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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.
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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. PlusPlus
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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.
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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.
The patient-specific support materials, be they vacuum mattresses, dental impressions, shielding or additional constructions, remain unchanged as long as the patient is receiving radiation therapy. PlusPlus
Beißkeil und Vakuummatratze zur Lagerung eines Hundes

Treatment planning in Cats and Dogs

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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.
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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.

Equinox Healthcare GmbH
Strahlentherapiezentrum für Pferde und Kleintiere
An der Wann 8-10
63589 Linsengericht

T. +49(0) 6051 49098 – 10
F. +49(0) 6051 49098 – 11