Cat - Eye Tumours - Melanomas

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. If not already performed your veterinary surgeon may suggest certain tests to help confirm or eliminate the 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 of tumours in animals is improving all the time.

We understand that this can be a very worrying time and we apologise for the need to use some technical language. If you do not understand anything, please do not hesitate to ask.

Structure of the eye and melanocyte biology

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.

What are these tumours?

Melanocytic tumours are formed by abnormal melanocytes. Melanocytoma is a benign (non-spreading) melanoma. Malignant (spreading) tumours are sometimes called 'melanoma' or more specifically 'malignant melanoma' or 'melanosarcoma'.

Eyelid and conjunctival tumours are usually malignant in cats. Limbal tumours are benign and can be removed surgically. The commonest tumour has its origin in the iris (anterior uvea). 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?

In humans, environmental and host factors are important in development of melanoma. In cats, tumours are most frequent in heavily pigmented animals.

Why has my animal developed this cancer?

The genetic make up of your cat and/or heavy pigmentation are probably the main predisposing factors.

Are these common tumours?

Melanomas are the commonest eye tumours in cats. 'Iris diffuse melanoma'  is a presentation unique to cats and is the commonest type. Conjunctival and limbal tumours are rare. Ocular melanomas occur mostly in older cats.

How will these cancers affect my pet?

Primary conjunctival tumours may cause redness and weeping from the eye before the lump is visible. This is painful. Limbal tumours are protruding black nodules at the corneo-scleral (at the edge of the white portion of the eye ball) junction. They have a smooth periphery.

Iris diffuse melanoma causes diffuse thickening of the iris. The cells proliferate (grow/divide) over a period of years although the rapidity of progression is highly variable. The malignant tumour 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 tumours may show no clinical disease at the time of diagnosis.

How are these cancers diagnosed?

Clinically, these tumours are usually visible with an ophthalmoscope. Not all are pigmented and other types of tumours may also be pigmented or look dark so accurate diagnosis of the type of tumour 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 tumours may undergo microscopic examination of tissue (histopathology). This enables more accurate diagnosis and prediction of behaviour (prognosis). Histopathology is done at a specialised laboratory by a veterinary pathologist.

What types of treatment are available?

In cats, melanocytic tumours arising in different parts of the eye show different behaviour and need different treatment. Conjunctival tumours are usually malignant so full surgical removal is the treatment of choice. Limbal tumours are protruding black nodules at the corneo-scleral junction so can be surgically removed without removing the eye. Most other tumours 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 tumours 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 veterinary surgeon.

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 veterinary surgeon 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?

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 tumour.

The prognosis of diffuse iris melanomas is controversial. It is uncertain whether all iris spots progress or not. The consensus is that although these tumours are more likely to eventually metastasize than the similar tumour of dogs this will only occur in about 20 percent of cases. The general recommendation is to watch these tumours for any sudden change in growth habit. Tumour-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 tumours 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 tumours and are not transmitted from pet to pet or from pets to people.