Dog - 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 melanocytes 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'.

The commonest tumours originate from the iris (anterior uvea). They are usually benign. Eyelid, conjunctival, limbal and choroid tumours are all rare. Only conjunctival tumours are malignant. Limbal tumours are nodules that can be removed surgically. The anterior uveal and choroid tumours are usually benign but cause problems by increasing the intra-ocular pressure (glaucoma) that leads 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 which come together for the unfortunate individual.

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

Why has my animal developed this cancer?

The genetic make up of your dog 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 tumours?

Melanomas are common in dogs. Anterior uveal melanocytomas are the commonest intra-ocular tumour in dogs. More rarely, the tumours are found in other parts of the eye. They include limbal tumours, arising from the line of melanocytes at the junction between cornea and sclera, choroid tumours, conjunctival melanoma and the very rare diffuse uveal melanosis.

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 junction. They have a smooth periphery.

The benign intra-ocular tumours are slow-growing masses that eventually bulge into the vitreous (liquid filling the eye ball). 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 tumours may not be causing 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 accurate diagnosis and prediction of behaviour (prognosis). Histopathology is done at a specialised laboratory by a veterinary pathologist.

The histopathology report indicates the type of tumour 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 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. They have a smooth peripheral growth pattern so are good candidates for surgical excision without removing the eye. Most other tumours 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?

'Cure' has to be a guarded term in dealing with any cancer.

Melanomas arising from the conjunctiva are malignant and both recur at the site and metastasise even if histologically they appear benign. It is difficult to promise complete cure once your dog has developed this type of tumour.

Anterior uveal tract melanocytomas and choroidal and limbal melanocytomas are usually benign. The few malignant tumours arising from the anterior uvea can be recognised histologically. Only about 5% of tumours metastasise.

Limbal tumours have a smooth surface so can often be cured by surgical excision without removal of the eye.

Benign intra-ocular tumours 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 tumours and are not transmitted from pet to pet or from pets to people.