Brachytherapy and Plesiotherapy in Horses

Brachytherapy and plesiotherapy are methods of delivering radiation therapy to treat cancerous tumors in horses. Radiation works by disrupting DNA in affected cells. In cancerous cells, disruption of the DNA prevents cancerous growths from multiplying and results in the cancer tissues being unable to spread. However, radiation also disrupts the DNA of the body's healthy cells, therefore resulting in damage to normal body tissue. Brachytherapy and plesiotherapy deliver radiation to cancer cells in a highly localized way, thereby, minimizing damage to your horse’s healthy tissues. Brachytherapy is the delivery of radiation immediately adjacent to or within the tumor itself, referred to as interstitial. This is performed by implanting radioactive “seeds” directly into the cancerous tissue. The dosage released is low, however because it is left in place, the cumulative radiation directed at the cancerous growth is high, while minimizing the exposure to tissues not immediately adjacent to the implanted radiation. 

Plesiotherapy is the application of beta radiation therapy on the surface of a tumor; sometimes it is called surface brachytherapy. It also accomplishes delivery of radiation specific to the cancerous cells while minimizing the effect on healthy tissues. Brachytherapy and plesiotherapy may be used alone or in conjunction with other cancer therapies or surgeries. These procedures require administration by a veterinarian under controlled circumstances as radiation must be carefully handled and contained so as not to cause unwanted contamination. Sedation or general anesthesia is often used to prevent movement of the horse during the therapy which would result in exposure to radiation by non-target tissues.

For brachytherapy, sedation and local anesthetic may be used, or general anesthesia if required to immobilize a non-cooperative horse, or if the horse is undergoing surgery to address cancerous tissue simultaneous to the implantation of a radiation “seed”. Precautions must be taken by humans involved in delivering interstitial radioactive seeds to ensure they are not contaminated with radioactive materials. Once your horse is sedated or anaesthetized, a seed with iridium 192 is implanted into the tumor or surrounding tissue using a nylon catheter to deliver the implant using long handled forceps. Seeds may be placed temporarily or permanently. Seeds deliver radiation for one to two weeks. Although the emitted dose is low, it is delivered directly over a long period resulting in a large localized dose. 

Permanent seeds are left in place and the radiation decays over time. Temporary implants are removed after the prescribed dosage has been released.

Plesiotherapy uses strontium 90, and is administered in an ophthalmic applicator or hand held beta emitting device. It can treat an area of 5 to 8mm and penetrates 3mm deep, making it appropriate for surface lesions. Treatment of each area takes 10 to 15 minutes. A larger area can be treated by multiple contiguous treatments to cover the required area. Because treatment requires that your horse does not move during administration, they may be given heavy sedation or general anesthetic in order to deliver this form of radiation therapy.

Brachytherapy and plesiotherapy have been used successfully to treat sarcoids and squamous cell carcinomas. Smaller tumors are more successfully treated and surgical removal of cancerous tissue may be performed in conjunction with radiation treatment. Due to the danger from exposure with interstitial implants, few veterinary hospitals still administer this form of treatment. 

Plesiotherapy is highly effective for treating small surface tumors and is also often used in conjunction with surgical tumor removal.

Dosage and location treated are critical to the effectiveness of the treatment.

Exposure to radioactive materials for handlers and personnel during the procedure is an issue and precautions to limit exposure must be taken. In addition, radioactive implants can become dislodged, causing contamination of the housing area and risk to the horse and others. Because of these risks, many veterinary universities and hospitals do not offer this treatment and it is not legal for use on equines in many states. 

If treatment is occurring around an eye it will require special precautions to shield the eye tissue during plesiotherapy.