When horses develop lumps or tumours, the most common ways to treat them include laser removal, surgical removal or the application of chemotherapy creams. However, if the lump happens to be near an eye it makes treatment very difficult as we want to protect the eye from damage. That’s where radiation therapy can be helpful.

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How does radiation treat cancer?

Radiation damages the DNA within cells. As it prevents the cells from dividing and growing, it particularly targets cells that have a high activity. Cancer cells grow and divide rapidly so that the mass can grow quickly, causing damage to the surrounding tissues. Therefore, cancer cells are more likely to be targeted by radiation treatment. Although normal healthy cells are also affected by the radiation, the normal cells are able to recover from the damage more easily than tumour cells.

What are the side effects?

As radiation causes damage to the cells, the most common signs include inflammation and peeling of the skin which can occur in the days following treatment. However, in some cases, long-term damage to bone and skin can occur. This results in hairlessness (alopecia) in the treatment area or, more rarely, weakness of the bones.

How can radiation be used to treat horses?

There are two different types of radiation therapy.

Teletherapy is the term used to describe the use of radiation from a long distance

For example, in the treatment of a large, deep tumour. A machine is used to direct the radiation towards the tumour; but structures in the path of the radiation beam will also be affected. Careful planning of the procedure is required with thorough diagnostic imaging. General anaesthetic is also required throughout the procedure.

This is only rarely used in horses.

Brachytherapy is the opposite of teletherapy and involves the placement of the source of radiotherapy directly onto the tumour

A probe made from the radioactive isotope Strontium-90 can be placed onto the surface of the tumour, causing damage to the cells. However, the radiation cannot penetrate very deep into the tissue. And therefore can only be used for small superficial masses, usually less than 3mm in depth. Suitable tumours include those affecting the structures around the eyes. For larger tumours, Iridium-192 can be used in the form of wire implants which are left in the tumour for a period of time and then once the correct dose of treatment has been achieved, they are removed. The length of time that the implants stay in place will depend on the size and type of tumour, as well as the number and the placement of the implants. While the horse has these implants in place, they must be isolated to prevent radiation exposure to people and other horses. 

Another form of brachytherapy, called high dose rate (HDR) brachytherapy involves the placement of a number of catheters into the tumour. Iridium-192 is then inserted through the catheters into the tumour. The process targets a specific area in the tumour and can be performed remotely, reducing the risk to the handlers.

How successful is treatment?

Radiotherapy has an excellent long-term success rate. It should be the first choice for the treatment of tumours around the horse’s eye. But it can also be used to treat tumours on other areas of the horse if required.

What are the downsides of the treatment?

Unfortunately, the costs associated with radioactive treatment can be prohibitive. Not only is the treatment expensive but handling it comes with health risks and therefore extra protective measures need to be put in place. Some horses will need to be under general anaesthetic for the treatment to be performed, or hospitalised for periods of time during or following treatment, which also results in extra costs. Due to the special equipment and facilities required, there are not many veterinary hospitals that offer the treatment, so there can also be a long wait time before treatment can be performed.

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