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 more perfused/rich in oxygen the tumours are, the better the radiation therapy works. Radiotherapy leads to shrinking or in the best case to complete destruction of the tumour.

Radiotherapy can be used as a sole treatment option or in combination with other treatment modalities.

  1. As a sole treatment: radiotherapy is mostly used for superficial tumours that cannot be removed completely via surgery or when surgery is not an option for other reasons.
  2. As a complementary treatment: radiotherapy can be used together with/instead of chemotherapy, or following surgery. If administered before surgery, the tumour will be more oxygenated and therefore more responsive to radiotherapy. Furthermore, when the tumour region has been irradiated before surgery, this reduces the risk of tumour spread as a potential consequence of surgical manipulations. However, it’s not always possible or useful to irradiate before surgery. Potential disadvantages include wound healing complications and delay of the surgical procedure.
Types of radiation therapy

Conventional radiotherapy is typically administered over 3 to 4 weeks with 3 to 5 sessions per week (average of 12-20 treatment sessions). Higher doses are used when the goal of the radiotherapy is to cure the patient or to prolong his survival (curative radiotherapy).

Palliative radiotherapy is mainly used to enhance quality of life, reduce pain and keep the hospitalization period as short as possible. Because the patient needs to be perfectly still, the dog 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 (external radiation):
These machines produce X-rays that penetrate the tissues only superficially (orthovoltage) (suitable for superficial tumours located in the skin or mucous membranes) and at megavoltage penetrate the tissues more deeply (suitable for tissues that are not located superficially). With megavoltage, the maximal deposit of energy is at a depth of 0.5 to 1 cm. Therefore, this treatment does not damage the skin.
Brachy- or curietherapy:
The source of radiation (radioactive needles or seeds) is placed inside or next to the tumour. This way, the radiation source will mainly irradiate the tumour tissue and affect the surrounding healthy tissue as little as possible. Because these needles or seeds irradiate over a very limited distance (the level of radiation is negligible at a distance of 5 mm from the needle), this type of radiotherapy is very well suited for locations close to important structures such as the eye, the lungs or the intestines. Because of the reduced radiation on healthy tissue, the side effects are reduced as well. This type of radiotherapy usually requires a hospitalization of a few days. The procedure depends on the tumour to be treated, the kind of implant and the dosage of radiation. The implants are placed under local anaesthesia or general sedation. This type of radiotherapy is used separately or complementary to surgery and/or external radiation.
Metabolic radiotherapy:
This therapy is mostly used for thyroid tumours, as most thyroid tumours capture iodine molecules. For radioactive iodine radiotherapy, we make use of this property by administering radioactively charged iodine molecules (I-131). The radioactive iodine accumulates in the thyroid (tumour). As the radioactive compound decays, radiation is released and local irradation of the thyroid cancer cells occurs. The treatment requires a hospitalization of 7-21 days (the duration of the hospitalization varies between countries).
Tumour types sensitive to radiotherapy

Tumour types generally sensitive to radiation include:

  • brain tumours (meningioma, schwannoma, choroid plexus tumour, astrocytoma, glioma, pituitary macroadenoma and adenocarcinoma)
  • nasal tumours (mainly lymphoma, sarcoma, carcinoma)
  • oral tumours (acanthomatous epulis, squamous cell carcinoma, fibrosarcoma, melanoma)
  • tumours on the limbs/body (small soft tissue sarcoma, lymphoma, mast cell tumour, ceruminous gland tumour, thyroid carcinoma, bladder tumour, prostate tumour, perianal adenoma, anal sac adenocarcinoma).

Radiation is also used as palliative therapy for osteosarcoma, as it has a very efficient painkilling effect.

Side effects radiation therapy

Side effects of radiotherapy can appear fast (acute) or can be delayed.

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.

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.

Where is veterinary radiotherapy available? (non-exhaustive list)

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 for thyroid tumors is possible at

  1. LaRue SM, Gordon IK. Radiation Therapy. Withrow and MacEwen's Small Animal Clinical Oncology, 5th edition, Chapter 12 (p 180-195).
  2. Dierenkliniek Randstad,