Oncology
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Staff
Miss Meredith A Smith MA VetMB CertES (Soft Tissue) DipECVS MRCVS
About Oncology
Horses do not suffer from neoplasia (cancer, tumours) as frequently as other companion animals. Neoplasia of the various internal organs does occur, and in these cases the tumour may cause a secondary presenting complaint such as colic, weight loss or lethargy. Tumours affecting the skin are more readily identified by owners, and these are summarised below. For more information on conditions of the eye please follow the ophthalmology links.
Sarcoids
Sarcoids are the most common skin tumour in horses. They are the most common tumour of horses, donkeys and mules world wide. These tumours are non metastatic, although they may be locally invasive, and occur in five recognized forms; nodular, fibroblastic, verrucose, occult, mixed and malevolent forms. Some horses are affected with just a single sarcoid, whilst other animals may be covered in multiple tumours, and significantly debilitated as a result.
Treatment must be planned and executed with caution, and there are many treatment modalities described, which is testament to the difficulties clinicians can be faced with in successfully treating these cases. Surgical excision often results in aggressive recurrence of the tumour, and so is rarely advocated as a sole treatment. Cryotherapy (topical freezing), Bacillus Calmette-Guerin (BCG) immunotherapy, and CO2 laser excision have all been used with varying success. Intralesional treatment with cisplatin or with a cytotoxic cream developed at the University of Liverpool may be options where the sarcoid is in a location suitable for topical treatment. Brachytherapy with Iridium-192 impregnated wires is a gold standard of treatment particularly suited to periocular tumours, and for tumours in other locations where the tissue affected must be preserved, such as over joints. Iridium-192 treatment is offered at the Animal Health Trust.
Squamous cell carcinoma
Squamous cell carcinoma occurs in the conjunctiva and third eye lid of the eye, and is the most common type of tumour of the eye and associated structures. Squamous cell carcinoma also occurs on the penis and vulva, and in the oral cavity. Squamous cell carcinoma is diagnosed by biopsy, by location, and by appearance which is often ulcerative and locally invasive. This tumour has the potential to metastasise (spread to other organs). Wherever possible, complete surgical excision is the treatment of choice. For structures in the eye this may not always be possible and so radiotherapy with strontium-90 may be performed in addition to surgery. Iridium-192 brachytherapy is also successful in treating these tumours. Squamous cell carcinoma of the reproductive organs may be treated successfully with intralesional and topical chemotherapy.
Melanomas
Melanomas appear to occur exclusively in horses with grey coats, and in horses that become dapple grey with age. These tumours are diagnosed on their typical appearance of firm, black nodules, and are mostly best left untreated unless they become ulcerated or start to interfere with function. Up to 80% of grey horses over 15 years of age are estimated to have melanomas in some form. Melanomas occur as benign nodules, often under the tail and around the anus, or this tumour may be malignant, and spread internally throughout the body systems with old age. Partial success in treating melanomas has been reported using cimetidine, and there is active research into the development and use of autologous vaccines for treatment of melanoma. Where these tumours are interfering with function, treatment may be attempted using iridium wire brachytherapy.
Mast cell tumours
Mast cell tumours are uncommon in horses. Arabs are reported to be the breed that is most affected. Typically this tumour presents as a single, hairless or ulcerated nodule, often located on the head. Mast cell tumours also occur within structures of the eye such as the conjunctiva. An alternate form of mast cell tumour is an area of firm swelling on the lower limb. These tumours do not metastasis and can be treated with surgical excision, sublesional steroid injection or cryotherapy.
Haemangiosarcoma/Haemangiomata
Haemangiosarcoma/Haemangiomata are benign tumours of the endothelial (lining) cells of blood vessels and occur in the limbs and also in the conjunctiva of the eye. They usually present as single, firm or fluctuant swellings in a lower limb. They are benign, but may ulcerate and bleed. Surgical excision is the treatment of choice, but is often difficult and recurrence is common.
Cutaneous lymphoma
Cutaneous lymphoma is rare in horses. Most affected horses have another underlying systemic illness with presenting symptoms such as weight loss, anorexia and anaemia. These tumours look like patches of raised tissue beneath the otherwise normal skin, and resemble urticaria (“allergy bumps”). Steroids may be beneficial although a thorough investigation should be carried out first, to rule out an underlying systemic disease. Systemic lymphosarcoma carries a poor prognosis.
