Radiotherapy in Veterinary Patients
Radiotherapy (RT) is the use of high-energy x-rays to kill cancer cells. These are usually produced by a large machine called a linear accelerator (see photograph) which produces a beam of x-rays which can be directed at a tumour.
RT is typically used:
- After surgery, when all the visible cancer has been removed but microscopic cancer has been left behind at the surgical site and thus there is a risk of tumour re-growth (examples include mast cell tumours and soft tissue sarcomas)
- For treatment of melanoma in the mouth of dogs, either in combination with surgery or as an alternative to surgery
- For palliation/slowing down the growth of tumours that are not surgically removable – or where surgery would carry a significant risk to the patient
- For pain relief in dogs with primary bone tumours (eg osteosarcoma) or tumours that have metastasised (spread to) areas of bone
How is the treatment given?
The required radiation dose is not given as one single treatment, as the normal tissues surrounding the tumour would not be able to tolerate this. Each treatment is given under a very short general anaesthetic. In radiotherapy each treatment is known as a ‘fraction’.
Definitive Treatment (also known as curative-intent treatment)
This is usually the protocol of choice for most cancers as it gives the best chance of long-term tumour control.
Treatment in the UK is usually given every Monday, Wednesday and Friday for 4 consecutive weeks – i.e. 12 treatments over 4 weeks. Patients may also have to attend the RT centre for an initial session prior to treatment commencing,for an initial assessment, especially needed for tumours of the head and neck such as nasal cavity tumours, brain tumours, some complex oral tumours or unusual thyroid tumours. In these cases a head/neck mould will be created at the same time to allow accurate patient placement, beam alignment and to immobilise the head during therapy (if required). RT will require careful planning from MRI/CT scans and utilises computerised planning to target the dose to the tumour whilst avoiding the surrounding normal brain and other important structures such as the eyes, ears and inside the mouth. However, it is inevitable that there will be some dose to these areas, as a margin has to be added to target microscopic tumour cells away from the main tumour and allow for subtle patient movement during treatment.
Sometimes further scans/special computerised planning is not required – particularly for the treatment of simple post-operative scars.
The main reasons 12 small fractions are given, (as opposed to fewer treatments, each at a higher dose) is to limit both acute side effects, (brain swelling in the case of brain tumours or nasal cancers), and also to minimise risks of late side effects such as long-term brain damage. This way a greater total dose can be administered, which improves the probability of controlling the tumour for longer. Slightly different protocols may be given by certain centres either as their main protocol or modified for an individual patient.
Palliative Treatment (also known as hypofractionated treatment)
Treatment in the UK is usually given once a week for 4 consecutive weeks – i.e. 4 treatments in total. In certain cases, such as oral malignant melanoma in dogs, this protocol is preferred. The advantages of hypofractionated treatment are that there are fewer treatments, it is cheaper and the dose is lower so there are fewer side effects (e.g. sore skin/gums; see below). However, because the dose is lower there is less chance of controlling the cancer and there is a higher risk of late side effects (see below).
What are the possible side effects?
The vast majority of dogs receiving this treatment enjoy an excellent quality of life and continue with their normal daily routine. Because radiotherapy is a localised treatment, (i.e. directed to one small area of the body as opposed to the whole body being exposed), the side effects are limited to the area being treated.
Acute side effects (side effects which happen while the patient is being treated) are more likely to happen in the 3rd and 4th week of therapy and can sometimes worsen for a week or two after treatment ceases. Side effects will depend on the area being treated but can include localised hair loss, sore skin, sore gums/lips, a sore eye(s) and some discomfort. Medications such as tablets or topical creams/eye drops are sometimes prescribed to manage these. They are temporary and disappear within a month of stopping treatment in most cases.
Serious late side effects are fortunately very rare and the risk is reduced by the small dose administered with each fraction in the case of definitive therapy. These can occur months or even years after radiation therapy and may include skin changes, joint changes, cataracts and retinal changes, bone damage and very rarely even 2nd unrelated cancers in the radiation treatment field. Fortunately the only common late side effects that we see are the hair growing back white in the treatment field and some thickening/changes to the skin.
The vast majority of dogs, however, tolerate the course of radiotherapy with few problems and owners are often very pleasantly surprised at how little the treatment has impacted on their quality of life/normal daily routine.
Because the treatment involves several anaesthetics, patients should be free of significant heart-, kidney- or liver disease and not have any concurrent problems that would be problematic.
Currently centres which offer radiation therapy:
- Southfields Veterinary Specialists, Essex
- University of Liverpool Veterinary School
- University of Cambridge Veterinary School
- University of Glasgow Veterinary School
- University of Edinburgh Veterinary School
- Animal Health Trust, Newmarket
Our Oncology Specialists work closely with the Specialists at the Radiation therapy Facilities to provide the best outcome for your pet.
If you have any queries or concerns, please do not hesitate to contact us.