Of the tumors affecting the eye in horses there is little doubt that squamous cell carcinoma (SCC) is the most important and one of the most frequently encountered in practice.
The disease (like most tumors) occurs most commonly in older horses (over 10 years) and affects Appaloosas and draft breeds with the highest frequency. Horses with light skin and hair coloring have a higher incidence of disease and although most cases affect one eye there have been instances of bilateral disease occurring.
The most frequently affected parts of the eye are the third eyelid and the medial canthus (area where the upper and lower eyelids come together on the nasal side of the eye), and the edge of the cornea (limbus). Over time if untreated these tumors can slowly invade the orbit or spread over the cornea.
Any chronic irritation may predispose to development of SCC - there is a very good correlation between longterm exposure to sunlight (ultraviolet radiation) and the development of the precursor lesions of SCC.
Lesions often start as an area of redness on the eyelids and then progress to ulceration with development of friable pappillomatous masses. These then may invade the tissues around the eye and extend into the orbit. On histopathologicl examination fours stages are identified - the earliest corresponding to the initial redness is the plaque (also called carcinoma in situ) which is limited to proliferation within the superficial layer of skin or conjunctival tissue. This progresses to a papilloma when the deeper connective tissue grows outwards into the epithelial layer. Next the tumor starts to show signs of malignant change in the cells but does not penetrate the deepest epithelial layer (non-invasive carcinoma) and finally once the tumor cells invade past the basal epithelial layer the tumor is recognized as invasive SCC.
SCC is a slowly growing tumor in almost all cases and its progression can often be slowed or stopped by aggressive therapy early in the course of the disease. Spread (metastasis) of the tumor
is very slow to occur - usually the tumor is more of a problem as it invades the local tissues. When it spreads it most often does so via the regional lymph nodes of the head, neck and eventually thorax (chest).
The main problem with SCC is the risk of local recurrence after treatment. It is very important to treat the disease aggressively from the outset and have regular folowup to ensure that recurrence is not present. In cases where owners are vigilant and prepared to aggressively treat the disease over many years the prognosis can be quite good. Tumors affecting the eyelids or which is already involving the orbit are the most difficult to treat without risk of recurrence. Tumors on the third eyelid or the cornea carry a better prognosis if treated early in the disease
Diagnosis of SCC is based on appearance, location and biopsy of the lesion(s). This is usually easily accomplished with sedation and local infiltration of anesthetic around the lesion. Horses are affected with several other inflammatory and neoplastic (cancerous) lesions of the eye and eyelids and these can easily be confused with SCC (see other tumors
). Incorrect diagnosis based on the clinical appearance without histological confirmation can have severe consequences if inappropriate treatment is used.
Various treatments are available for SCC - the main issue with most is that early lesions may extend beyond the locations where the SCC is most readily seen - thus treatment of one lesion may miss other early/developing lesions. Inadequately aggressive treatment of any SCC may result in recurrence. For this reason it cannot be stressed enough that aggressive early treatment of the lesions is important for a successful outcome. Treatment options include:
- Surgical excision of visible lesions with wide margins into apparently normal tissue with histopathological examination of the margins of the excised tissue.
- Medical therapy using chemotherapeutic agents (cisplatin injected into the lesion) or immunotherapy (BCG injected into the lesion) plus or minus oral piroxicam treatment.
- Cryotherapy - lesions are frozen with a double freeze-thaw cycle. SCC is fairly responsive to cryotherapy. This is often done after removing (debulking) large areas of the tumor.
- Radiation - Both beta- and gamma- radiation is occasionally used to treat SCC. The main problem with this treatment is the limited availability of the radiation sources and the risks to personnel and the likelihood of developing other ocular disease (corneal ulcers, cataract) form the radiation.
- Hyperthermia - Radiofrequency hyperthermia has occasionally been used in conjunction with surgical debulking to treat SCC.