During radiotherapy radioactive or ionizing beams are aimed at the tumour, resulting in damage of quickly dividing cells (such as cancer cells) within the irradiated region. The objective of radiotherapy is to destroy tumour cells while further sparing healthy peripheral tissues. The sensitivity of cells to ionizing rays generally depends on three factors: (1) cellular oxygenation (the more perfused/rich in oxygen the tumour is, the better the radiation therapy works), (2) the phase of the cell cycle at the time of irradiation (cells are more sensitive to damage when dividing) and (3) the cell’s capacity to repair damage (cancer cells are less able to repair DNA lesions than healthy cells). That is why the cancerous tissues need to be irradiated multiple times.
The therapeutic principle of radiotherapy is to divide the total radiation amount in several doses (fractions). This splitting of the dose leads to a better tumour response, while preserving the functionality of the healthy tissues for repair.
Generally speaking, the greater the number of fractions, the higher the total dose administered can be, while reducing the risk of delayed side effects. This type of protocol is preferred for curative treatments (= treatment with the goal to cure the patient). Depending on the animal's situation, the radiotherapist can opt for conventional fractionation, hyperfractionation or hypofractionation:
Radiotherapy can be used as a sole treatment option or in combination with other treatment modalities.
Radiotherapy leads to shrinking or in the best case to complete destruction of the tumour. Some tumour types, such as lymphoma, are very sensitive to radiotherapy and can disappear in a few days, whereas most tumour types respond within 4 to 12 weeks to radiotherapy.
Radiotherapy with curative intent is typically administered over 3 to 4 weeks with 3 to 5 sessions per week (average of 12-20 treatment sessions). Palliative radiotherapy is mainly used to enhance quality of life, reduce pain and keep the hospitalization period as short as possible. E.g. for the treatment of tumours that cannot be surgically removed, when multiple anaesthesia sessions are impossible or in case of metastases. Because the patient needs to be perfectly still, the animal is sedated during the treatment session. The treatment session itself usually takes only a few minutes.
There are 3 main types of radiotherapy: Ortho/megavoltage radiotherapy, brachy- or Curietherapy, and metabolic radiotherapy
The beams penetrate the tissues more deeply than is the case with orthovoltage. With megavoltage, a lower percentage of the dose is delivered to the skin resulting in less possible skin damage. Megavoltage is suited for both superficial and deeper lesions. Of note, the units emitting megavoltage beams are mounted on an arm that can rotate 360° around the animal, which allows multiple angles via which the tumour can be irradiated. Megavoltage allows a dose distribution adapted to the tumour’s shape, whereas orthovoltage allows less flexibility for the dose distribution.
For tumours located deeply or in an anatomically complex location, a scan needs to be made that charts the tumour’s precise location and size. These scanner images are then used to create a radiation administration plan. Linear accelerators can come with various planning techniques (3D CRT, IMRT/VMAT, SRT) that allow a precise modelling of the tumour (not all linear accelerators have these technique capabilities). This means that based on the obtained model of the tumour, an even more precise irradiation of the tumour is possible and it allows a better sparing of surrounding healthy tissue. All planning techniques require a precise positioning of the animal: the animal must be positioned for the imaging in a way that can be closely replicated on a day-to-day basis for treatment.
The veterinary radiotherapy specialist will decide which treatment is best for your dog, based on their tumour's size and extent, the type of tumour and location.
Tumour types generally sensitive to radiation include:
Radiation is also used as palliative therapy for osteosarcoma, as it has a very efficient painkilling effect.
Side effects of radiotherapy can appear fast (acute) or can be delayed. The risks and benefits for the individual patient should discussed with the radiooncologist before treatment.
Acute effects are mainly seen in quickly replicating tissue: bone marrow, epidermis, cells of the gastro-intestinal tract, mucous membranes (inflammation) and cancer cells. The total administered dose, the intensity of the dose, the amount and location of treated tissue are all factors that influence the side effects on healthy tissue. These acute effects will appear during radiotherapy with a curative intent, but they subside as time passes and appropriate care is given. Inflammation of the mucous membranes generally occurs within 7 to 10 days after the start of the treatment, inflammation of the skin within 12 to 14 days.
Late effects are mainly seen in tissues that replicate slowly (such as the brain, the spinal cord, the muscle, the bone, the kidney and the lung). Because the damage is often irreversible, this limits the total dose of radiation that can be administered. To calculate the most efficient dose, a CT-scan is almost always necessary before the start of the irradiation.
Radiation with megavoltage radiotherapy is offered at
Radiation with orthovoltage radiotherapy is possible at
Radiation with brachy- or curietherapy is possible at
Radiation with metabolic radiotherapy (I131) for thyroid tumours is possible at