Eye Tumors – Melanoma in Dogs
These notes are provided to help you understand the diagnosis or possible diagnosis of cancer in your pet. For general information on cancer in pets ask for our handout "What is Cancer". Your veterinarian may suggest certain tests to help confirm or eliminate diagnosis, and to help assess treatment options and likely outcomes. Because individual situations and responses vary, and because cancers often behave unpredictably, science can only give us a guide. However, information and understanding for tumors in animals is improving all the time.
What are melanocytes?
Melanocytes are cells that produce a pigment called melanin. They are found in many parts of the body where there is pigment, particularly the skin, hair and eyes. In embryonic development, melanocytes are related to cells of the nervous system. Melanogenesis (formation of melanin) is a complex process with genetic, hormonal and ultraviolet (UV) control. A suntan is the result of stimulation of these cells by sunlight (UVA and UVB). Inside the eye melanocytes are found in the iris and choroid layers (a thin layer of cells lying beneath the retina).
What are melanocytic eye tumors?
Melanocytic tumors are formed by abnormal melanocytes. Melanocytoma is a benign (non-spreading) melanoma. Malignant (spreading) tumors are sometimes called 'melanoma' or more specifically "malignant melanoma" or "melanosarcoma".
In dogs, the most common melanomas of the eye originate from the iris (anterior uvea) [see diagram]. They are usually benign. Eyelid, conjunctival, limbal and choroid tumors are all rare. Only conjunctival tumors are malignant. Limbal tumors (tumors that form at the junction between the cornea and sclera) are nodules that can be removed surgically. The anterior uveal and choroid tumors are usually benign but may cause problems by increasing the intra-ocular pressure (glaucoma) that can lead to blindness.
What do we know about the cause?
The reason why a particular pet may develop this, or any cancer, is not straightforward. Cancer is often the culmination of a series of circumstances that come together for the unfortunate individual.
In humans, environmental and host factors are important in development of melanoma. In dogs, these tumors are most common in heavily pigmented animals.
Why has my dog developed this cancer?
The genetic make up of your dog and/or heavy pigmentation is likely to be the main predisposing factor. 'Diffuse uveal melanosis' is a clinically distinct presentation in Cairn Terriers.
Are these common tumors?
Melanomas are common in dogs. Anterior uveal melanocytomas are the most common intra-ocular tumor in dogs. More rarely, melanomas are found in other parts of the eye. They include limbal tumors, arising from the line of melanocytes at the junction between cornea and sclera, choroid tumors, conjunctival melanoma and the very rare diffuse uveal melanosis.
How will these cancers affect my pet?
Primary conjunctival tumors may cause redness and weeping from the eye before the lump is visible. This is painful. Limbal tumors are protruding black nodules at the corneoscleral junction. They have a smooth surface.
"Small tumors may not be causing clinical disease at the time of diagnosis."
The benign intra-ocular tumors are slow-growing masses that eventually bulge into the vitreous fluid, the fluid within the eye. Some become large and eventually cause uveitis (inflammation), glaucoma (increased ocular pressure), retinal detachment or optic nerve compression. All these will result in blindness. Small tumors may not be causing clinical disease at the time of diagnosis.
How are these cancers diagnosed?
Clinically, these tumors are usually visible with an ophthalmoscope. Not all are pigmented and other types of tumors may also be pigmented or look dark. Therefore, accurate diagnosis of the type of tumor relies upon microscopic examination of tissue. Your veterinarian may use one or more methods of obtaining the appropriate tissue samples for diagnostic purposes, including cellular aspirates, biopsies and full excision. Iris tissues can be sampled by the vacuum technique for rapid diagnosis by cytology (the microscopic examination of cell samples). This assesses the need for surgery, after which excised tumors may undergo microscopic examination of tissue (histopathology). Your veterinarian will submit the tissues to a specialized diagnostic laboratory for examination by a veterinary pathologist. Histopathology enables accurate diagnosis and prediction of behavior (prognosis).
The histopathology report indicates the type of tumor and mitotic rate (which is important to predict life expectancy). The pathologist may need to remove (bleach) the pigment to check for malignancy with greater certainty.
What types of treatment are available?
In dogs, melanocytic tumors arising in different parts of the eye show different behavior and need different treatment. Conjunctival tumors are usually malignant so full surgical removal is the treatment of choice. Limbal tumors are protruding black nodules at the corneoscleral junction. They have a smooth peripheral growth pattern so are good candidates for surgical excision without removing the eye. Most other tumors require more extensive surgery. As in humans, there has been little progress with other treatments.
Can these cancers disappear without treatment?
Cancer very rarely disappears without treatment. Very occasionally, spontaneous loss of blood supply to the cancer can make it die but the dead tissue will still need surgical removal. The body's immune system is not effective in causing these tumors to regress.
How can I nurse my pet?
After surgery, an "Elizabethan collar" may be provided to prevent your pet rubbing his or her eye and interfering with the operation site. This needs to be kept clean. Any loss of stitches or significant swelling or bleeding should be reported to your veterinarian. Specific treatment may include eye drops and ointments with antibiotics and anti-inflammatory drugs.
If you require additional advice on post-surgical care, please ask.
How will I know how the cancer will behave?
The histopathology report will give your veterinarian the diagnosis that helps to indicate how it is likely to behave. The veterinary pathologist usually adds a prognosis that describes the probability of local recurrence or metastasis (distant spread).
When will I know if the cancer is permanently cured?
'Cured' has to be a guarded term in dealing with any cancer.
Melanomas arising from the conjunctiva are malignant and may recur at the site or metastasize (spread to the remainder of the body) even if histologically they appear benign. It is difficult to promise complete cure once your dog has developed this type of tumor.
Anterior uveal tract melanocytomas and choroidal and limbal melanocytomas are usually benign. The few malignant tumors arising from the anterior uvea can be recognized histologically. Only about 5% of tumors metastasize.
"Only about 5% of tumors metastasize."
Limbal tumors have a smooth surface so can often be cured by surgical excision without removal of the eye.
Benign intra-ocular tumors are still clinically significant as they spread within the globe and cause glaucoma and uveitis. Canine diffuse uveal melanosis causes blindness.
Are there any risks to my family or other pets?
No, these are not infectious tumors and are not transmitted from pet to pet or from pets to people.