Cancer is the uncontrolled growth of cells that develop into masses, commonly known as tumors. They may invade healthy tissue sometimes as a more diffuse blanket of cells throughout the tissue similar to roots of a tree spreading throughout the ground. The tumors themselves release chemicals into the body that cause several different side effects such as weight loss, depression, anorexia, fever, anemia just to name a few. Once a cell type or tissue becomes cancerous it can spread through the blood or lymph system to all regions of the body and become tumors anywhere.
List of Basic Cancer terms:
oma- Latin for "tumor or morbid growth"
Benign- not malignant, from Latin for "good or kind"
Malignant-showing invasiveness and metastasis, from the Latin root for "evil"
Carcinoma-abnormal tissue growth of epithelial tissue or skin
Lymphoma-abnormal tissue growth in lymph nodes
Melanoma-abnormal tissue growth in pigmented cells, such as skin cancer
Metastasis- when cells leave a cancerous mass and spread, forming new tumors
Neoplasia- abnormal growth of cells, either malignant or benign
Papilloma-small benign epithelial tumor, such as a wart
Sarcoma-abnormal tissue growth involving tissue, such as a muscle
List of Some Cancers Found in Horses
MELANOMA
This type of skin cancer can occur in any dark-skinned horse, but is most common in gray horses. Melanomas are tumors originating from the cells that produce skin pigment (melanin). A high percentage of gray horses over 15 years of age develop melanomas. They may appear any time after age 4 or 5, and growths that appear on a relatively young horse are more likely to become malignant more quickly than those that appear on older horses. Lumps may appear singly or in groups.
SQUAMOUS CELL CARCINOMA
This type of skin cancer is usually quite visible and readily detected. It often appears around the anus or genitalia. It also occurs around the eyes or eyelids of the horse.
It is most common in areas of unpigmented skin, especially those that have little hair covering—such as under the tail, around the mouth, or on the sheath. Cancer of the eyelid is fairly common in Appaloosas, Paints, Pintos or any horses with white faces, large white markings with pink skin around the eye or any horse with light skin. Tumors surrounding the eyeball are fairly common in sunny regions; intense sunlight irritates the eyes and encourages growth of this type of cancer. Unpigmented skin has little protection against the harmful effects of the ultraviolet rays.
Cancer of the third eyelid can spread to the surrounding tissues and may necessitate removal of the eye to save the horse. If caught early, the removal of the small tumor can prevent eye removal and long term resolution.
Horse owners should try to be aware of any changes around the eyes. The growths often occur on the third eyelid or on the lower eyelid’s inner surface. They may appear as a single raised bump or a raw surface that looks like a runny sore. A bump or red area is not always cancer, but if it becomes larger, redder or more irritated, the horse should have immediate veterinary attention.
This type of cancer will continue to grow and can spread to nearby tissues, eventually killing the horse unless removed.
SARCOID TUMORS
Cancer can be defined as an uncontrolled proliferation of cells, and this certainly describes a sarcoid. The good thing about sarcoids is that they don’t usually spread internally; they don’t travel through the bloodstream to other parts of the body. They stay in their local area, even if they get huge. If not removed early, however, the tumor may become so large that it disables the horse.
Sarcoids are one of the most common skin tumors of horses. They are believed to be caused by the bovine papilloma virus. They can be treated with chemotheropy, such as cisplatin, or removed surgically. Unfortunately, sarcoids are known to return if not all of the effected tissue is removed.
LYMPHOSARCOMA
This is the desease of the lymph tissue. The lymph tissue becomes cancerous, often starting with one lymph node and spreading to others. The lymphatic system is the body’s defense network and lymph nodes are small nodules throughout the body that filter lymph, destroy infections, and produce white blood cells. People often refer to these nodes as “glands.” Along with the lymph nodes, there is also lymph tissue that is diffusely spread throughout organs such as the intestines and the skin. Any one of these “glands” or lymph tissue can become cancers, hence the symptoms of the disease can vary greatly depending on the location of the cancerous tissue. There are four general types of lymphosarcomas which are categorized on location. The four main types are generalized (multi-centric), intestinal, mediastinal and cutaneous.
The generalized form is the most common and includes multiple peripheral and internal lymph nodes. Basically, it is tumors through the lymph nodes of the body. Common sites are the nodes (glands) around the throatlatch, in between the jaw, at the base of the neck, superficial inguinal, mesenteric, and the pectoral region. The most common clinical sign of this type of lymphosarcoma is large masses on the chest, at the base of the neck, under the jaw, and at the throatlatch. Ventral edema is very common as well as weight loss. Sometimes the diffuse lymph tissue within the skin can also be affected which manifests itself as a severe ulcerative dermatitis, where the skin literally erupts with crusting sores that don’t heal. This form also has the lowest survival rates, often only weeks to a few months.
Intestinal lymphoma involves the diffuse tissue within the intestinal wall. Involvement in this area causes malabsorption problems of the intestines. This leads to severe weight loss, diarrhea and sometimes colic. Mediastinal refers to the lymph nodes within the chest in between the lungs. Tumors in this region can cause coughing, increased heart rates, fluid on the chest and even fluid within the chest.
The best and least deadly form of this disease is the cutaneous form which, is lymph nodes that turn into tumors under the skin and don’t migrate throughout the body. Horses with these types of tumors typically do very well and live fairly long with minor clinical symptoms.
Unfortunately, if large tumors are not visible, diagnosis can be difficult. There is also overlap of the different types, which leads to further diagnostic challenges. A biopsy of the tumor can confirm the diagnosis, but can sometimes introduce tumor cells into the blood stream which could cause the cancer to spread further. An ultrasound of the tumor is typically very characteristic and diagnostic. Often a diagnosis is made from clinical signs such as weight loss, masses, depression, edema of belly and legs, fever, diarrhea, skin crusting and scaling, coughing, high heart rate, blood changes such as anemia and a variety of other symptoms caused by the tumors that affect the tissue they are close to.
BREAST AND OVARIAN CANCER IN MARES
Breast Cancer, or more correctly, mammary neoplasia, in mares is uncommon in comparison to rates among women. Abbatior studies (cited by Equine Disease Quarterly July 2008) report an incidence of barely 2%, and less than a dozen published cases known to exist. But of that compilation, all but one were malignant. And the results of necropsies, conducted between 1994 and 2008 by Livestock Disease Center at the University of Kentucky on 11 mares diagnosed with malignant mammary neoplasia (cancer), have also concluded that equine mammary cancer is "much more likely" to be malignant the benign and carries a "poor prognosis" for long term survival.
PROSTATE AND TESTICULAR CANCER IN GELDINGS, STALLIONS
Tumors of the testicles are uncommon, the incidence is unknown as most male horses are castrated at an early age, but the tumors are unilateral. Testicular tumors are divided for descriptive purposes into either Germ Cell Tumors or Non-germinal tumors. A Seminoma is the most popular of the germ cell tumors. This type usually effects the aging stallion. A Teratoma is another germ cell tumor. These tumors are usually oval or round in shape and are most common in the abdominal testicles. They are rarely malignant. Non-germinal tumors are rare in horses, but can cause increased testosterone production,
Prostate glands in male horses are susceptible to the same cancers, conditions and issues as human, with not-so-rare cases of death per Dr. Kuehnle. This type of cancer is becoming more of a problem as horses are exposed to abnormal internal, and external factors.
The important thing to remember is if you see any change in your horse, his routine, his demeanor, unexplained physical changes such as lumps or sores that do not heal, seek immediate veterinary care. Please remember a horse is a prey animal, and as with any prey animal, signs of weakness or unsoundness attract predators, therefor a horse's survival instinct will cause it to hide signs of weakness.
BRIAN S. BURKS, D.V.M. DIPL., A.B.V.P. Board Certified Equine Specialist
Dr. Brian Burks grew up in the San Joaquin Valley of California, where he was active in the 4-H club, specifically canine obedience and rabbitry. In an effort to realize his dream of becoming a veterinarian, he began to work as a veterinary assistant for Dr. Michael Stabbe at Clovis Equine Clinic. During this time he attended California State University, Fresno. Brian is a graduate of Oklahoma State University School of Veterinary Medicine. In preparation for a residency program, he completed an internship at Equine Medical Associates, outside of St. Louis, Missouri. After an internal medicine residency at the University of Georgia, he came to Fox Run Equine Center in 1998. He has authored journal articles and has been a speaker for veterinary continuing education programs. In 2006, he became boarded by the American Board of Veterinary Practitioners, in equine practice. The ABVP recognizes excellence in the practice of veterinary medicine, and is an arduous endeavor involving case reports and examination.
What’s that bump? Equine Sarcoid 12/17/2012
By Brian S. Burks, D.V.M., Diplomate American Board of Practitioners (Equine)
Have you ever noticed any lumps or bumps in the skin of your horse? If so, you should have these evaluated by your veterinarian. These can range from innocuous bug bites to bacterial/fungal diseases, or skin tumors.
The sarcoid is the most common skin tumor in the horse. Their behavior is completely unpredictable. They can be ‘silent’ for many years and then suddenly begin to grow quite rapidly. Sarcoids tend to be locally aggressive, but seldom spread from one site to another; even though they do not spread, they can become quite deep, involving the underlying soft tissue and bone. They do not spread to internal organs.
The etiology of the equine sarcoid is unknown. It has been said that a bovine (cattle) papilloma virus is the cause, and their DNA has been found in the sarcoid, but no active virus has ever been found. Other types of viruses have also been proposed.
Sarcoid types include the occult, verrucous, nodular, fibroblastic forms, and mixed sarcoid, containing features of several of the tumor types. The occult form occurs mainly in thin haired areas and is fairly flat. They occasionally open and drain. They may only show small areas of hair loss initially, before becoming an overt tumor. Verrucous sarcoids are a bit larger and form more crusts. They have local skin thickening around the main area, indicative of further tissue invasion. Nodular sarcoids are large, firm masses in the skin and subcutaneous tissues. The last type, the fibroblastic sarcoid is much larger and appears similar to exuberant granulation tissue, or proud flesh. These tumors are quite vascular, and bleed easily. They may be pedunculated or have a larger base. It usually develops at the site of a wound, especially on the limb. Many times these various tumor types are mixed, with occult and verrucous and nodular forms all occurring together.
Differential diagnoses include papillomatosis (warts), chronic blistering, hyperkeratosis (thickened skin from something like sweet itch), equine sarcoidosis (granuloma) exuberant granulations tissue, and squamous cell carcinoma. Sarcoids most closely resemble exuberant granulation tissue.
All skin tumors should be sampled and sent for histopathology to determine the type of tumor and distinguish sarcoids from granulation tissue or other infections. Histopathology also allows assessment of the margins, to determine if the entire tumor was removed. Simple removal tends to make sarcoid tumors much more aggressive, so that they will enlarge rapidly. This is because the entire tumor has not been removed due to its invasion of surrounding skin.
Treatment modalities are many, and only very rarely do these tumors resolve on their own. Cryotherapy (freezing with liquid nitrogen) immunologic treatment with BCG or Eqstim, and repeated intra-lesional injection with cisplatin, bleomycin, and 5- fluorouracil (5-FU) have all been used. Cisplatin and 5-FU are the most successful. Currently, sarcoids may be implanted with cisplatin beads, to allow a longer exposure time of the tumor to the cisplatin. Another way to administer cisplatin is via electro-chemotherapeutic poration. This equipment opens the cell membranes to cisplatin, allowing more of this chemotherapeutic agent to get inside the cell to effect cell death. Unfortunately, the expense of the equipment severely restricts its use. Bleomycin is another such agent, but it has not shown much success.
Other topical anti-tumor medications have also been used, with varying success, including Aldara and XXTERA. The latter, in some cases, may transform the tumor, making it much more aggressive. Also transplanting some of the tumor to a distant site in the horse has been used, with the thought of immunizing the patient to the tumor; this has enjoyed only limited success.
I personally have the best result using a surgical laser combined with cisplatin and sometimes 5-fluorouracil. This removes most of the tumor, destroys cells several millimeters beyond what the eye can see, and leaves medication locally to kill any tumor cells that may remain. If the tumor is able to be completely removed via excision (usually only small lesions) this is best, and is considered curative.
Refractory cases can be treated with local radiation therapy with Iridium 192, known as interstitial brachytherapy, by implanting radioactive beads in the tissue. The beads are left in until a pre-determined dose is reached. Patients must be quarantined while the radioactive implants are in place, restricting this modality to a few referral centers. The success rate approaches 100%. These tumors are less ably treated by Teletherapy or Tomotherapy (this combines a CT with Teletherapy) an external radiation beam, which requires specialized equipment and repeated general anesthesia and enjoys a success rate of less than 30%.
Thus, the equine sarcoid is the most common skin tumor in the horse. Fortunately, although it can be locally aggressive, it normally does not spread to distant sites (metastasize). There are several different types of sarcoid that vary in appearance, and may be confused with other types of lesions, including other tumor types. Simple surgical removal or biopsy is not recommended because this tends to make sarcoid tumors extremely aggressive, growing very quickly, and spreading through the local tissue. Removal with an Nd:YAG laser, combined with local anti-tumor medication gives the best success rates, although very occasionally some tumors may persist. Brachytherapy is the most successful treatment modality for refractory tumors, but requires specialized centers. You and your horse should seek treatment for any mass that is actively growing.
Treatment of Neoplastic Disease with Radiotherapy 1/14/2013
By Brian S. Burks, DVM, Dipl. ABVP (Equine)
If your horse is diagnosed with cancer, the veterinarian’s role is now to make the owner aware of the further diagnostic steps and treatment, and then to execute that treatment. That treatment may be surgical, medical, or both.
Diagnostic modalities include radiography, ultrasound, endoscopy, CT, MRI, and nuclear scintigraphy. The goal is to define the extent or the spread of the tumor, if any. Histopathology is used to determine the type of tumor, and to determine whether the neoplastic cells have been completely removed. Knowing the type of tumor helps to determine therapy and the likelihood of it becoming malignant.
Many tumors can be removed in horses, particularly external tumors, which are the most common. Removal is often curative, if complete excision can be achieved. Sometimes a tumor must be debulked, and then treated with some adjunctive treatment.
Medical treatments include anti-neoplastic chemotherapeutic drugs, biologic agents, or ionizing radiation. The former include cisplatin, 5-fluorouracil, bleomycin, doxirubin, cyclophosphamide, etc. Biologic drugs include BCG and other ‘vaccines’.
Radiotherapy is another medical treatment that may be curative or palliative. It may be used in areas not amenable to surgery or when surgery has failed. It may also be used adjunctively with other medical or surgical treatments. Definitive therapy is aggressive and intense, requiring daily treatments to achieve complete and lasting tumor control. Palliative therapy is less aggressive and is used to control pain of bone tumors, shrink tumors that limit function, and to relieve obstructions.
There are several methods to administer radiation. It may be injected, as in Iodine-131 for thyroid cancer in cats; applied to the surface, as in Strontium-90 probes used on superficial and small tumors such as squamous cell carcinoma; left in situ (brachytherapy) such as sarcoid tumors or hemangiosarcoma for a pre-defined period of time (radiation dose); or, lastly, by external beam application of gamma or x-rays, or electrons. (Note x-radiation is used to make a radiograph; you cannot see x-rays). LASER therapy is another form of radiation used to ablate tumors.
Teletherapy (external beam radiotherapy) requires specialized equipment, and is performed in only a few specialized animalian facilities. Teletherapy is delivered usually with a linear accelerator, which produces X-rays of high energy in a beam form. The dose is fractionated, requiring multiple general anesthetics as movement is not acceptable during treatment, and the entire dose cannot be given at one time without destroying a large volume of normal tissue. This modality allows the dose to be placed exactly, including in depth, minimizing damage to surrounding normal tissue. Teletherapy has few case reports for solid tumors in the horse, largely due to expense and paucity of availability; however, it has been successfully used in the treatment of lymphoma and squamous cell carcinoma.
Brachytherapy, or localized, implanted, high dose radiation therapy has also been used in human sarcomas. The use of 192iridium for squamous cell carcinoma, equine sarcoid, and other soft tissue sarcomas is well established and interstitial brachytherapy is preferred to teletherapy in large animals. The radioisotope is sealed in a metallic container known as a ‘seed’ or ‘wire’. For ocular and periocular squamous cell carcinoma, emitting 90strontium implants have also been used successfully.
192Iridium is the most commonly used radioisotope in veterinary brachytherapy, emitting gamma radiation. It also emits minimal beta energy, which is largely absorbed by the nylon ribbon encasing the seeds. Specialized catheters are surgically placed using guide needles which allow the introduction of plastic catheters which hold the seeds. Ribbons containing the radioactive sources are placed into the catheters after the implant has been constructed (afterloading) thereby reducing the exposure time of surgical personnel to radiation.
Tumor debulking may be necessary prior to implantation. This reduces the gross tumor volume, allowing decreased doses of radiation, providing additional safety for the patient and the surgical team. Tumors with high recurrence rates, and those that are difficult to surgically excise, are excellent choices for brachytherapy because regional function can be maintained, the cure rate for localized neoplastic disease is high, and the surrounding tissue receives little irradiation.
Ionizing radiation kills cells primarily by interaction with DNA, resulting in cleavage of the DNA or by free radical formation. Rapidly dividing cells will die faster than more slowly dividing cells.
Ionizing radiation may have several side effects, including alopecia, skin depigmentation, dry skin, and ulceration. Localized edema and infection can occur, necessitating anti-inflammatory and antimicrobial therapy. These complications usually resolve following implant removal. In horses, self-mutilation, and subsequent implant removal may occur often resulting in loss of the iridium seeds or ribbons, therefore care should be taken to protect the implant by covering it with gauze rolls or utilizing methods of restraint.
Radiotherapy can be used following diagnosis and tumor staging to treat nearly any solid tumor. It can be tailored to affect the tumor, whilst sparing surrounding normal tissue.
Equine Melanoma
By Brian S. Burks, DVM, Dipl. ABVP, Board Certified Equine Expert
Did you ever wonder about differing skin colors in animals and people? Skin color can change, depending upon the number of special cells called melanocytes. These cells produce melanin, a black pigment, and may be found in the skin, hair, and the iris of the eye. It can be protective from the effects of solar radiation. These cells are found deeper in the skin, associated with hair follicles, and are not found in the epidermal (outer) layers of skin. Thus, tattooing of skin is not considered protective against other forms of cancer associated with UV radiation/light.
Melanomas (melanocytomas, melanocytic nevi, melanosarcoma, melanomatosis) are tumors arising from melanocytes. The nomenclature is confusing and may not indicate truly indicate benign versus malignancy. With newer information, it is best to refer to Melanocytoma as benign tumors and Melanoma as malignant cell types.
In horses, the grey horse seems to be most affected, though other colors may also be affected. Up to 80% of grey horses may have multiple melanomas. They are thought to arise from perturbed melanin metabolism, leading to new cell formation or overproduction of melanin. Over time, the cells undergo malignant transformation. In humans, melanoma is thought to be caused by solar radiation, but the vast majority of equine tumors are protected from the sun, being under the tail or, sometimes, even under the skin. Chronic exposure to insecticides may also be a risk factor in humans.
Clinically, melanomas occur in horses over 5 years of age, with no sex predilection. They may be found in many breeds, but are over represented in Percherons, Arabians, and Lipizzaners. The tumors are often found under the tail or peri-anally, but may be found in or around the eye, on the external genitalia, at the ear base, on the lips and even in the guttural pouch. I have removed them from the parotid salivary gland and have seen one very large melanoma under the scapula (shoulder blade). Occasionally, there may be a black, tarry discharge, but most commonly the tumors are solid. They are most often coalescing nodules, but can be a single nodule.
There are three growth patterns for melanomas. They can grow very slowly for years without metastasis, they can grow slowly for years and suddenly become metastatic, or they can be metastatic from the beginning. Any tumor that is large enough to press upon sensitive structures can cause other clinical signs such as lameness and neurologic signs.
Diagnosis of equine melanoma is based upon history, physical findings, and histopathology. There are some new techniques available to determine the metastatic potential of this tumor. Equine melanoma differs from other species in beginning in or around the hair follicles and sweat glands rather than the dermo-epidermal junction. Often, this tumor does not require definitive diagnosis via histopathology, and clinical suspicion is enough.
Treatment can be of several forms used alone or concurrently. Wide surgical excision is curative. Some areas make surgery very difficult or impossible without damaging other structures, such as the anus. Laser surgery, using an Nd:YAG surgical laser, can be used stand alone or in combination with sharp removal. The laser will ablate remaining tumor cells following sharp dissection. The laser beam can also be defocused to treat iris melanoma.
Cimetidine, an antihistaminic drug, has been used with varying success. At best, the tumor may reduce by 50%, but most often it is thought to prevent further growth. Cisplatin, a chemotherapeutic drug most often used for sarcoids, may be effective on small tumors. Beta-radiation may also be useful for very small tumors.
There are some newer options available for treatment. The first is a canine melanoma vaccine called Oncept. The vaccine was developed for canine oral melanoma, which is very aggressive and fatal. This is not yet approved in horses, but has shown some promise. The vaccine targets tyrosinase, an enzyme expressed by the tumor; by targeting this enzyme, the tumor growth is slowed or reversed. The vaccine is administered biweekly for four treatments, then again at three and six month intervals. This vaccine has been shown to be safe and to shrink equine melanomas. Sections of the tumor have also been taken to be made into an autologous (from self) vaccine. The efficacy has been questionable.
The second is gene therapy where genes that code for Interleukins (proteins that cause inflammation) are injected into the tumor. The inflammatory response kills the tumor cells.
A final treatment involves the use of Frankincense mixed with DMSO used either topically, for small tumors, or injected directly into larger tumors. Injection of Frankincense may continue weekly until the tumor has shrunk to an acceptable size. This has shown great promise in both human and equine melanoma, and is a less invasive option compared to other treatments such as surgery, which is sometime associated with complications from other nearby anatomic structures.
Equine melanoma is similar to, but also different from, other species such as dogs, cats, and humans. It is most often considered to be a benign disease, but it can spread both externally and to internal structures. The definitive cause is unknown. There are multiple treatments available, depending upon the aggressiveness of the tumor and its location.
What’s That Bump? Equine Squamous Cell Carcinoma
By Brian S. Burks, DVM, Dipl. ABVP (Equine) Board Certified in Equine Practice
Squamous cell carcinoma (SCC) is a malignant tumor arising from cells called keratinocytes, which are skin cells. It is the second most common tumor of the horse, but the most common tumor of the eyelid and external genitalia. It is locally aggressive, but slow to metastasize.
The cause is unknown, but thought to be from solar radiation, especially in light pigmented horses (white, grey, palomino) or those with sparse hair. Sometimes this neoplasm occurs in areas that get little, if any, exposure to sunlight. It may, rarely, arise from chronically infected wound sites. Preputial SCC may be due to damage to the penile skin from smegma (a good reason to clean your horse’s sheath). In humans and small animals, a papilloma virus has been implicated in causing SCC, but this has not been consistently shown in horses.
The COX-2 (cyclooxygenase-2, inhibited by bute, Banamine, and firocoxib) enzyme may aid the neoplasm growth and invasion by increasing blood vessels (angiogenesis), invasiveness, and immunosuppression. COX-2 inhibitors have antineoplastic properties, preventing the aforementioned problems.
CLINICAL FINDINGS
The prevalence or equine SCC increases with age. No genetic or sex predilection has been shown, other than to have light colored skin. Draft type horses, Halflingers, Appaloosas and Paint horses are over represented for the tumor incidence.
The tumor may be found on any muco-cutaneous junction, including the eye, vulva, penis, stomach, in the nasal passageway or the oral cavity. For the eye, it is found on the eyelids or the nictitating membrane most commonly, but conjunctival or corneal tumors may occur.
The tumors are often solitary and poorly circumscribed. They can be mistaken for granulation tissue. As the tumor grows it may become cauliflower-like. The lesion may be painful, causing tearing, excessive blinking, and eyelid swelling. There may be discharge and a foul odor associated with the tumor.
DIAGNOSIS
Diagnosis is made on the basis of clinical findings and also histopathology (looking at the tissue under a microscope). It must be differentiated from a variety of other tumors, including sarcoid, lymphoma, and hemangiosarcoma.
Radiography and ultrasonography may be useful to look for tumor invasion into bone or tissue; or metastasis. This tumor typically will migrate to the local lymph nodes and then the lungs before other areas of the body, so chest radiographs may be necessary.
CLINCAL MANAGEMENT
Removal of the mass is often curative, but complete removal is often not possible. Thus treatment may be a combination of excision, laser surgery, radiation, and chemotherapy. Smaller tumors have a better prognosis. For tumors around the eye, it is critical to maintain eyelid integrity.
Wide surgical excision is the treatment of choice, where possible. The recurrence rate is at least 30% for periocular and preputial/penile SCC. Whatever the method, surgical reduction of the tumor is recommended prior to using adjunctive treatments such as chemotherapy, unless the mass is very small.
Laser therapy with Nd:YAG, diode, or CO2 lasers is a great adjunctive treatment to ablate tumor cells that cannot be seen with the naked eye. Caution around the eye is necessary, and use of the correct laser for the tumor site is paramount. Fox Run Equine Center has two different types of lasers, so that CO2 laser can be used on the ocular surface, but an Nd:YAG laser may be used elsewhere.
Radiotherapy is very useful, though not widely available. It is available at our hospital. The cure rate approaches 100%. Two types of radiotherapy are used: teletherapy, where a beam of radiation is used, and brachytherapy, where radioactive seeds are placed in the tissue. Typically, Iridium-192 or Strontium-90 is used to treat tumors. The latter is useful for small tumors, especially around the eye, and is used as a probe applied daily. When radioactive seeds are implanted, the horse must be quarantined from the general public. Once the seeds are removed, the horse is no longer radioactive. Complications include tissue necrosis, loss of hair, and loss of skin pigment; damage to normal structures; corneal opacity and cataracts.
Chemotherapy includes the use of cisplatin, 5-Fluorouracil, bleomycin, mitomycin-C, and COX-2 inhibitors such as prioxicam. Cisplatin is a broad-spectrum anti-neoplastic agent that causes DNA to have inter- and intra-strand cross-linking. It is mixed with medical grade sesame oil and injected into the tumor every two weeks for 4-6 treatments. The treatment is well tolerated and has minimal local side effects.
5-Fluorouracil interferes with DNA synthesis. It is a topical cream that can be applied to the tumor bed. It is best to debulk the tumor prior to application. It can also be injected intra-tumorally. The area may become red, swollen, or blistered.
Bleomycin causes breaks in DNA, while mitomycin-C is a potent DNA cross-linking medication. Both can be used by injection into the tumor.
Since SCC has COX-2 receptors, drugs that block this enzyme are useful for adjunctive treatment, following surgical debridement and/or other therapy.
Immunotherapy is of questionable benefit to treat SCC. Medications include BCG, Regressin-V, and Equi-Stim.
PREVENTION
Prevention of SCC is by avoidance of UV-radiation and by cleansing of the sheath and vulva. Avoidance of UVL includes night-time turn out, UV protective blankets and masks, and sun screens with at least 50 SPF.
CONCLUSION
Squamous cell carcinoma is a locally aggressive tumor that is slow to metastasize. Its origin is linked to UVL, but there may be other factors that affect tumor growth. Treatment is by surgical excision and anti-tumor medications. With early recognition, the outcome is often very good, but larger tumors tend to have a poorer prognosis.
