Oncology / Skin Tumours

The Animal Health Trust has several unique and pioneering techniques to treat difficult and unusual tumours in the horse.

We are primarily focused on the diagnosis and treatment of difficult skin tumours, but also offer referral and advice for the diagnosis and treatment of other types of cancer in the horse.

A horse receiving HDR treatment at the AHT

High Dose Radiation (HDR) Brachytherapy

There are many treatments for skin tumours in horses, with varying success rates. The most difficult to treat are those around the eye, as options are limited by the location, and these are often aggressive and invasive lesions.

An exciting new form of radiotherapy for tumours such as sarcoids around the eye of the horse is now available. The Animal Health Trust’s innovative Cancer Centre is pioneering the technique which gives new hope for these difficult lesions. This uses a high dose radiation technique known as HDR brachytherapy.

HDR brachytherapy uses specialised technology to drive the radioactive source from a special shielded safe into the patient via catheters that are implanted directly into the tumour. Specialist imaging techniques are used to form a 3D reconstruction of the tumour to allow treatment planning. During planning, computer software is used to calculate the dose given to the tumour and the technique is so advanced, different areas of the tumour can be given different doses enabling a completely bespoke treatment for each horse.

This method drastically shortens treatment times and it is possible to deliver the full radiation dose in 5-10 minutes for each session. The typical treatment consists of two sessions delivered a week apart, which is given under standing sedation with the horse suitably restrained in stocks. During treatment there is no operator exposure risk, and the horse is completely radiation free as soon as the treatment is completed. The results to date are impressive, and it is expected that this method will become the standard approach to the treatment of all tumours around the eye of the horse.

The AHT is currently the only veterinary clinic in the UK that can offer HDR. 

Cases can usually be seen 2-3 weeks after initial referral contact, and typical costs are well within the average insurance claim limit.

Contact: Equine Centre, 01638 751908, equine.centre@aht.org.uk

A horse receiving strontium treatment at the AHT

Strontium Plesiotherapy

Traditionally, Squamous cell carcinomas (SCC) is removed by surgical procedures. This can be an effective treatment, but it can be difficult to get good surgical margins and therefore recurrence and spread to lymph nodes remains a risk. Other treatments are often given alongside surgical removal (or instead of surgery if this is not possible). These are usually topical chemotherapy agents that are applied onto or injected into the tumours, or radiotherapy. Radiotherapy has several advantages over topical treatments, and is now available at the Animal Health Trust.

The most effective form of radiotherapy for most superficial SCC is called strontium plesiotherapy. Strontium plesiotherapy involves the direct application of a radioactive source to the tumour surface. The major advantage is that the type of radiation emitted penetrates only a few millimeters, meaning that a large dose of radiation can be applied to the surface without adversely affecting deep tissues. This means that the tumour tissue is destroyed, but delicate surrounding structures (such as the eye) are not affected.

Strontium plesiotherapy has been successfully used in the treatment of tumours located in many difficult areas in dogs, cats, and horses and represents a real advance in the treatment of these lesions. Strontium may be performed under standing sedation in some locations, and under a short general anaesthetic for others. Depending on the size of the tumour, treatments are usually given in 3 sessions, spaced 48 hours apart. Each treatment takes around 5-10 minutes depending on the size of the area being treated, and between treatments the horse can be treated as normal. This treatment is suitable for both tumours that cannot be surgically removed and those that have been removed with small or ‘dirty’ margins, and can also be used to treat some other types of skin cancers in the horse.

The AHT is currently the only veterinary clinic in the UK that can provide strontium plesiotherapy treatment.

Contact: Equine Centre, 01638 751908, equine.centre@aht.org.uk

Some horses, usually greys, develop skin melanomas which can be invasive and aggressive in their later stages.

Melanoma Vaccine

What is melanoma?

In horses, melanoma is a type of skin tumour that is usually associated with coat colour, with up to 80% of grey horses developing some form of visible melanoma by 15 years of age. These tumours are characteristic in their appearance, generally presenting as rounded, raised, black nodules of varying sizes, commonly found under the tail, in the perineal and genital region, salivary glands and on the lips. Whilst usually considered to be benign (non-cancerous), some will be malignant and may even spread throughout the body. Many of the benign lesions become large and obstructive and may also ulcerate and fistulate; forming a hole that oozes a black discharge with secondary infections, pain and functional problems occurring. In non-grey horses, melanomas are almost always malignant.

Unfortunately there is currently nothing that can be done to stop horses from developing melanomas. Unlike in people, sun exposure is not thought to be linked to the development of these tumours. Horses with ulcerated lesions will require excellent fly control in the summer to reduce the risk of secondary infection and subsequent pain.

How can we treat equine melanoma?

Traditionally equine melanomas have often been ‘left well alone’ rather than actively treated, because they are usually slow growing and do not tend to cause pain or other problems in the early stages of the disease. However, the chance of eventual malignancy or clinical problems is high, and we know that early treatment is likely to be beneficial to slow down or even stop the progression of the disease.

Surgical removal is often the preferred approach for early disease, but can be difficult depending on the location and size of the tumours, so is not always a feasible option. Some melanomas can also be treated with locally injected chemotherapy agents, but this can be difficult and results are highly variable and confined to the individually treated lesion, as well as bringing up many health and safety concerns for the horse, the treating vet and the owner.

A promising new treatment for equine melanoma is a vaccine called Oncept®, licenced in the USA for treatment of melanoma in dogs, which is an option we provide to at risk horses at the AHT. This vaccine works by causing an immune response against the melanoma cells, thus causing the body to fight the tumours. This means that all melanomas will be affected, including those that are not suitable for other treatment approaches. A study has shown that horses vaccinated 4 times at fortnightly intervals develop an immune response against the melanoma. If the horse shows a positive response, the vaccine is then given at 6 monthly intervals thereafter.

There is currently a large clinical trial underway in North America to further investigate this treatment, but initial clinical impressions are good, with a number of horses showing a reduction in growth or even regression of the tumours. This means that specialist vets in the UK, including our AHT equine oncologists, can now give owners a new hope for this common and sometimes very debilitating disease.

Contact: Equine Centre, 01638 751908, equine.centre@aht.org.uk

Spearheading cancer treatment in horses

Dr. Anna Hollis

Dr. Anna Hollis

Director of Equine Referral Clinic

Anna is pioneering new ways to treat difficult cancers in horses and is undertaking research to better understand why equines develop sarcoids, to help us understand this cancer further.

Read Dr. Anna Hollis's bio