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 the cells are found in the iris and choroid layers [see diagram].
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.
We understand that this can be a very worrying time. We apologize for the need to use some technical language. If you have any questions please do not hesitate to ask us.
What are these 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”.
Eyelid and conjunctival tumors are usually malignant in cats. Limbal tumors are benign and can be removed surgically. The most common tumor has its origin in the iris (anterior uvea) [see diagram]. They are usually benign but cause problems by increasing the intra-ocular pressure (glaucoma), which 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 seemingly 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 cats, tumors are most frequent in heavily pigmented animals.
Why has my pet developed this cancer?
The genetic make up of your cat and/or heavy pigmentation are probably the main predisposing factors.
Are these common tumors?
Melanomas are the most common eye tumors in cats. ‘Iris diffuse melanoma’ is a presentation unique to cats and is the more common type. Conjunctival and limbal tumors are rare. Ocular melanomas occur mostly in older cats.
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 corneo-scleral junction. They have a smooth periphery.
Iris diffuse melanoma causes diffuse thickening of the iris. The cells proliferate over a period of years although the rapidity of progression is highly variable. The malignant tumor may develop slowly from plaques on the iris and take up to seven years before it causes glaucoma and blindness. At this stage, it needs removal. Early tumors may show no 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 so accurate diagnosis of the type of tumor relies upon microscopic examination of tissue. Various degrees of surgical sampling may be needed including cellular aspirates, biopsies and full excision. Irises 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 tumors may undergo microscopic examination of tissue (histopathology). This enables more accurate diagnosis and prediction of behavior (prognosis). Histopathology is done at a specialized laboratory by a veterinary pathologist.
What types of treatment are available?
In cats, 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 corneo-scleral junction so can be surgically removed without removing the eye. Most other tumors eventually 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?
Histopathology 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 eyelids and conjunctiva are malignant and both recur and metastasize (spread to the remainder of the body) even if histologically benign. It is difficult to promise complete cure once your cat has developed this type of tumor.
The prognosis of diffuse iris melanomas is controversial. It is uncertain whether all iris spots progress or not. The consensus is that although these tumors are more likely to eventually metastasize than the similar tumor of dogs this will only occur in about 20 percent of cases. The general recommendation is to watch these tumors for any sudden change in growth habit. Tumor-induced glaucoma (internal fluid build-up) may occur slowly over several years in the affected eye and may eventually require removal of that eye.
Limbal tumors have a smooth surface so can often be cured by surgical excision without removal of the eye.
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.