Cat & Dog - Connective Tissue and Nerve Tumours - Visceral

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 these tumours?

All tissues and organs of the body may develop cancer (an abnormal overgrowth of their constituent cells). Every organ (liver, lungs, spleen, kidneys and so on) contains a supporting framework of fibrous connective tissue as well as nerves to relay information to and from the brain. Most organs also contain muscle tissue and special cells (histiocytes or macrophages) whose function is to clear debris. Cancers of these tissues can therefore occur anywhere in the body.

There are a multitude of names for these tumours including Schwannoma, peripheral nerve sheath tumour and neurofibroma for those of nerves and fibroma, fibrosarcoma and spindle cell tumour for fibrous tissue tumours. Muscle tumours are rhabdomyomas, rhabdomyosarcomas, leiomyomas, leiomyosarcomas and granular cell tumours (myoblastomas). The tissue of origin is not always clear and granular cell tumours may be of nerve (Schwann cell) origin. Fibrous histiocytoma is a subtype of these cancers, possibly originating from a primitive cell, which can develop to either fibrous tissue or histiocytes.

What do we know about the causes?

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.

Little is known about the cause of these tumours in dogs. Some fibrous tumours in young cats and multiple ones in older cats may be initiated by a certain feline virus (recombinant forms of feline leukaemia virus - FeLV). The virus disrupts the cell DNA genome and causes chromosomal changes. Most older cats have immunity to the virus so, in older cats, solitary tumours are not FeLV virus associated.

Are they common tumours?

All these are unusual tumours in internal organs although fibrous tumours are common in the skin and mouth.

Rhabdomyosarcoma may occur in the urinary bladder of young animals.

Granular cell tumours (myoblastomas) are most commonly found in the tongue of dogs. One has been recorded in the tonsil of a cat.

Fibrous histiocytoma and malignant fibrous histiocytoma (giant cell tumour of soft tissue) are uncommon in dogs and rare in cats. It is doubtful whether any are truly benign. They usually occur in older animals but a malignant one has been recorded in a 4-month old dog. Multiple nodular lesions have also been described in young dogs.

How will the cancer affect my animal?

The most obvious effect of most cancers is a lump. Depending on its site, this may cause abdominal swelling and be palpable (can be felt) or have physical effects on the surrounding structures, blocking the intestine or preventing urination for example. Others may cause other clinical signs such as difficulty in breathing if they are in the chest. Malignant cancers may spread through the body by seeding in body cavities and invading the blood, lymph and nerve transport systems.

Weight loss due to loss of body fat and muscle is common in malignant cancer. The immune system is often damaged which allows cancers to develop and infections to persist. A few tumours induce signs that are not readily explained by local or distant spread of the tumours. These are known as 'paraneoplastic syndromes'. Some are due to abnormal hormone production by the cancer. Examples include low blood sugar, which can be associated with some muscle tumours.

How is this cancer diagnosed?

Clinically, cancer may be suspected.  Radiograph (x-rays) and ultrasound scans can indicate lumps but the techniques are incapable of distinguishing non-cancerous nodules and true cancers.

In order to identify the tumour type, it is necessary to obtain a sample of the tumour itself. Needle biopsies through the skin are rarely diagnostic even when guided by ultrasound and exploratory surgery is usually required. The tissue samples are submitted for microscopic examination by histopathology, the microscopic examination of specially prepared and stained tissue sections. This is done at a specialised laboratory where the slides are examined by a veterinary pathologist.

The histopathology report typically includes the name of the tumour and a prognosis (what will probably happen) that includes an indication of the probability of local recurrence.

What treatment is available?

The treatment for all these tumours is surgical removal, usually of the lump but occasionally more radical such as removal of a whole kidney or liver lobe.

These tumours do not respond to chemotherapy. The tumours are sensitive to radiotherapy but this is unsuitable for tumours in internal sites.

Can this cancer disappear without treatment?

Cancer rarely disappears without treatment but as development is a multi-step process, it may stop at some stages. The body's own immune system can kill cancer cells but it is rarely 100% effective. Rarely, loss of blood supply to a cancer will make it die but the dead tissue will probably need surgical removal. The body's immune system is not effective in causing this type of tumour to regress.

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 the site. Any loss of sutures or significant swelling or bleeding should be reported to your veterinary surgeon. If you require additional advice on post-surgical care, please ask.

How will I know how this 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).

How and when will I know if the cancer is permanently cured?

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

The prognosis of fibrosarcomas depends on the location (for ease and margin of excision) and grade as determined by cell divisions seen through the microscope (mitotic index). Although it is unusual for these cancers to spread to distant parts of the body (metastasise), many are locally invasive.

Rhabdomyosarcoma is a highly malignant tumour capable of widespread metastasis. Granular cell tumours (myoblastoma) do not recur after full surgical removal nor do they metastasise. Leiomyomas and leiomyosarcoma are a continuum from benign to malignant so prediction of behaviour is not always easy. However, although they may spread (metastasise), this is late in the disease and often only to the local lymph nodes (glands). The outlook is usually good following surgery.

Fibrous histiocytoma and malignant fibrous histiocytoma (giant cell tumour of soft tissue) exhibit a broad range of morphology (shape) and behaviour. Most recur and a few metastasise. They are usually locally invasive so difficult to remove. It has been suggested that the histiocytic ones are more malignant than the fibrocytic.

Are there any risks to my family or other pets?

No, these tumours are not transmissible from pet to pet or to people by contact or through body fluids.