High dose rate brachytherapy appears to be an appealing option for treating skin tumors in horses, according to a clinical commentary.
High dose rate brachytherapy is a form of radiation therapy that uses implanted radioactive seeds to deliver maximum radiation to a tumor for a short period. Typically, it involves local anesthesia, with catheters placed under the skin and into or near the cancer.
The seeds, about the size of a rice grain, are inserted into the catheter for several minutes, and then removed. Patients may receive several treatments and are not radioactive after seed removal.
The procedure is an established technique for treating skin tumours in human medicine, Dr Anna Hollis noted in a commentary in the journal Equine Veterinary Education. Hollis, with the Department of Veterinary Medicine at Cambridge Equine Hospital, part of the University of Cambridge, said squamous cell carcinomas and basal cell carcinomas respond especially well, with 96–98% cure rates and excellent cosmetic outcomes. Results, she said, compare extremely favourably to surgery.
“Indeed, in human non-malignant skin cancer, high dose rate brachytherapy is often the treatment of choice for lesions that cannot be surgically removed without serious defects, which require cosmetic and/or reconstructive procedures.”
Compared to external beam radiotherapy, brachytherapy has favourable dose distribution, with improved tumour radiation coverage and better protection of the surrounding tissues at risk of secondary radiation damage.
“The ability to treat large tumours with minimal detrimental effects to the surrounding normal tissues and a high probability of cure with high dose rate brachytherapy makes it an interesting option for equine skin tumours,” Hollis says.
Interstitial brachytherapy, in which the radiation source is placed directly in the tumor, is theoretically the ideal technique for radiation treatment of equine tumours, she says, because a high radiation dose can be delivered precisely and safely to the tumour in a relatively short time.
The technique, she adds, is relatively simple, and the radioactive material can be delivered to the tissue without the need for any operator exposure, rendering it much safer and more practical than traditional low dose rate brachytherapy techniques.
In humans, CT imaging is performed as the standard once the applicators are in position to allow for accurate treatment planning. However, in horses, this is unlikely to be possible, financially viable or practical in most cases. Hollis says there are other imaging options. While they will have reduced accuracy compared to computer-assisted methods, they may be more appropriate in locations where the underlying tissues are less radiosensitive.
Hollis notes that high dose rate brachytherapy was briefly described for the treatment of equine skin tumours in 1998, and two small case series were published in 2017 on its use in treating equine sarcoids.
Hollis said she had also used the technique in horses with good success rates in more than 140 sarcoids and squamous cell carcinomas (her data is unpublished).
She notes that a variety of other tumours in dogs have been treated with the technique, including carcinomas, various sarcomas, mast cell tumours, lymphoma and melanoma, citing a 2019 paper. However, to her knowledge, there is only one report of its use for treating dermal hamartomas (local malformations made up of an abnormal mixture of cells and tissue) in any veterinary species.
In humans, hamartomas in various locations have been successfully treated with different forms of radiotherapy, she notes, including high dose rate brachytherapy.
“In the majority of veterinary reports of hamartomas, including those in horses, surgery was performed as the treatment of choice.
“However, in the case of recurrent lesions or those in which surgical excision is not possible due to their location, high dose rate brachytherapy appears to be a safe and successful method of treatment, where this technique is available.”
Hollis, A.R. (2022), High dose rate brachytherapy for the treatment of skin tumours in humans and animals. Equine Vet Educ, 34: 402-403. https://doi.org/10.1111/eve.13596