Cat & Dog - Ovarian Tumours

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.

What are the ovarian tumours?

The ovary contains several different cell types. These include the germ cells, which make the eggs, the supporting (stromal) and hormone-producing cells as well as epithelium (lining cells), connective tissue and blood vessels.

Any or all of these cell types may become cancerous. When germ cells become cancerous, the tumours are called dysgerminomas. Tumours of ovarian stromal cells include granulosa cell tumours, thecomas and interstitial cell tumours (luteomas). These tumour types overlap and they may occur singly or in any combination. Epithelial tumours include papillary adenoma and adenocarcinomas. Rare types of ovarian tumour include the teratoma formed by embryonic germ (primitive) cells that develop abnormally to produce many different tissues.

Some ovarian cancers are benign and others malignant. In some cases, removal of the affected ovary will be curative. Spread to other internal organs (metastasis) is possible with some types, particularly the larger tumours.

Some tumours are not cancerous but are fluid-filled cavities or cysts. In and around the ovary, cysts are common and arise in a number of different ways, often impossible to differentiate by the time they are noticed.

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.

Cancer causes include radiation, chemicals, hormones and infections. At some stages, ovarian tumours need hormones to enable them to grow. It is thought that some surface epithelial tumours can be induced by ovarian granulosa tumours.

Why has my animal developed this cancer?

We do not know precisely why some animals develop these cancers and others do not but abnormal hormone balance is one cause. The more divisions a cell undergoes, the more probable is a mutation so cancer is commoner in older animals.

It is worth considering that a spayed (neutered) pet has had her ovaries removed and therefore cannot develop these cancers.

Are these common tumours?

None of these tumours is common. Granulosa cell and epithelial tumours are the commonest ovarian tumours in bitches. They are infrequent in the queen cat. Dysgerminomas and teratomas are rare. It is difficult to generalise on occurrence of dysgerminoma because they are rare but they appear to be tumours of advanced age. Stromal tumours such as thecoma are infrequent and interstitial cell tumours (luteomas) are rare.

How will the cancer affect my pet?

Some tumours are found because of the clinical signs due to the excess hormones they are producing. These signs include abnormal seasons, persistent oestrus (season, heat), discharge from the vagina, and masculinisation. Granulosa cell and epithelial tumours produce the female hormone oestrogen so they cause enlargement of the uterus, sometimes with secondary infection (pyometra). Some sub-types have the potential to produce male hormones as well. Most interstitial cell tumours are hormonally active. It is not uncommon to find mammary (breast tissue) tumours in bitches and queens with ovarian tumours.

Stromal granulosa cell, epithelial tumours and dysgerminomas may become very large so a swollen abdomen is the main sign. Granulosa cell tumours grow up to 16 cm diameter in the bitch.

A few tumours may not be noticed until there is weight loss, illness and lethargy associated with anaemia, secondary infection of the uterus or metastatic tumours elsewhere in the body. Pieces break off the shaggy surface of epithelial tumours so these often grow on the inner wall of the abdomen. Lymph or blood-borne fragments grow in internal organs such as the spleen, liver and lungs.

How is this cancer diagnosed?

Clinically, these tumours can be difficult to diagnose because of the great variety of clinical signs they can cause. Increased blood hormone levels are not reliable for diagnosis of these tumours.

Once the tumour is removed, definitive diagnosis of the type of tumour, the stage it has reached and therefore prediction of behaviour (prognosis) relies upon microscopic examination of tissue (histopathology). This is done at a specialised laboratory by a veterinary pathologist.

What treatment is available?

The usual treatment is surgical removal of both ovaries and the uterus. Other treatments are not generally available and their effectiveness is unknown.

Can these cancers disappear without treatment?

Cancer rarely disappears without treatment but as development is a multi-step process, it may stop at some stage. Rarely, loss of blood supply will make the cancer degenerate but the dead tissue can cause toxic problems. The body's immune system is not effective in making these tumours stop growing. Theoretically, removal of hormonal stimuli will stop the growth of some tumours but in most cases this can only be achieved by removal of the ovaries with the tumour itself.

How can I nurse my pet?

After surgery, the operation site needs to be kept clean and your pet should not be allowed to interfere with it. Any loss of sutures (stitches) or significant swelling or bleeding should be reported to your veterinary surgeon. You may be asked to check that your pet passes urine and faeces. 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).

Dysgerminomas are usually single, large tumours. In the bitch, 10-20% spread locally in the body cavity or metastasise to other organs but this is thought to be in advanced stages. However, it is difficult to generalise on behaviour because they are rare. Teratomas are usually benign.

Granulosa cell tumours are usually malignant in cats and sometimes are in dogs. Secondary tumours implant on the inner wall of the abdomen and in internal organs such as the spleen, liver and lungs. Thecomas are probably curable by removal of the ovaries but reliable information is not available. Interstitial cell tumours are reported to be benign.

Epithelial tumours are difficult to differentiate into benign and malignant categories. Large tumours are more likely to be malignant and histopathology may indicate invasion.

How will I know if the cancer is permanently cured?

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

It is very difficult to promise complete cure once your pet has developed one of these tumours but knowing the type will help this assessment. Although dysgerminomas are usually unilateral (on one side), epithelial and stromal tumours may be multicentric (having more than one centre) and bilateral (both sides).

Histopathology may also indicate if there is evidence of malignant cells spreading at the time of surgery. Regrowth of the tumour may be indicated by swelling of the abdomen, hormonal effects or illness of your pet.

Most tumours are age dependent so an older animal has more risk of developing tumours. However, this is not absolute and an ovarian epithelial tumour has been recorded in association with pyometra (pus in the uterus) at one year of age. It is therefore advisable to have your pet checked at intervals.

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.