Fibrosarcoma and Spindle Cell Tumors in Dogs

 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 about tumors and their treatment in animals is improving all the time.

This can be a very worrying time. If you have any questions please do not hesitate to ask your veterinarian.

What are these tumors?

Fibrosarcomas and spindle cell tumors originate from the connective tissue of, or beneath, the skin. The tumor is diverse in appearance and several different cell lines produce tumors of similar appearance. Some pathologists try to classify this group of tumors by their cells of origin, but this is not always possible. Therefore, these connective tissue tumors may have several different names, including fibrosarcoma, schwannoma, neurofibroma, peripheral nerve sheath tumor, and hemangiopericytoma. Most are slow-growing. The tumors often recur locally after surgical removal but spread to other parts of the body (metastasis) is rare.

What do we know about the cause?

Little is known about the cause of these tumors. Cancer is often the culmination of a series of circumstances that come together for the unfortunate individual. It is the result of non-lethal genetic damage to cells, with "external" contributory factors such as radiation, chemicals, hormones and infections. The mutated cells upset the normal regulation of cell death and replacement. They do this by activating growth-promoting oncogenes (cancer genes), inactivating suppressor genes and altering the genes that regulate normal, programmed cell death (apoptosis).

Why has my pet developed this cancer?

Some breeds have far more cancers than others, often of specific types. This tumor is more common in larger breeds of dog.

Is this a common tumor?

This is a common group of tumors in dogs, mainly in middle- aged to older animals and often on the limbs. However, incidence of the individual tumors that make up the group is uncertain because differentiation between them requires specialised and expensive marker studies. All the cell lineages within this group have the same prognosis, based on grading.

"Incidence of the individual tumors that make up the group is uncertain because differentiation between them requires specialised and expensive marker studies."

The average age of occurrence is 10 years but an aggressive form of fibrosarcoma (with a poor prognosis) is occasionally reported in dogs less than one year of age.

Fibrosarcomas comprise variable percentages of the tumors diagnosed depending on an individual pathologist's criteria for inclusion. Some pathologists reserve the term "fibrosarcoma" for the most aggressive tumors of high grade. Whereas approximately 7% of tumors are diagnosed as spindle cell tumors, only about 1.5% are fibrosarcomas.

How will this cancer affect my pet?

The tumor is nodular, usually on the limbs. It may ulcerate, bleed and become infected. The tumor often invades between adjacent structures so it can affect their function.

 

How is this cancer diagnosed?

Clinically, this tumor often has a typical appearance, but accurate diagnosis relies upon microscopic examination of the tumor. To obtain the appropriate samples, your veterinarian may recommend one or more of various sampling techniques such as fine needle aspiration, punch biopsy and full excision. With these tumors, a full excision should include wide surgical margins (removal of apparently healthy tissue surrounding the lump). The sample will be prepared and examined by either cytology or histopathology. Cytology is the microscopic examination of cell samples obtained by aspiration techniques. Cytology is rarely diagnostic for this tumor type because few cells can be aspirated from the tumor, and distinction from normal cells of the same type is difficult except in highly malignant tumors. Sometimes cytology can help to rule out other tumors and plan surgery. Histopathology is the microscopic examination of tissue samples that have been specially prepared and stained. Histopathology will give a more accurate diagnosis and prognosis, and will rule out other diseases including more serious cancers. Your veterinarian will submit the samples, either a small part of the mass (biopsy) or the whole lump, to a specialized laboratory for examination and diagnosis by a veterinary pathologist.

"For this tumor, there are two grades, low and high, dependent upon the mitotic rate."

The histopathology report typically includes the name of the tumor and a grade. For this tumor, there are two grades, low and high, dependent upon the mitotic rate (number of cells dividing). Most are low grade. From this, the veterinary pathologist adds the prognosis (what will probably happen), which includes an indication of the probability of local recurrence.

Some pathologists reserve the term "fibrosarcoma" for the most aggressive tumors of high grade.

The pathologist is also able to indicate whether surgical removal of the tumor was complete. Many tumors appear well circumscribed (have defined edges), but under the microscope this impression is shown to be false because outside the tumor there may be further tumor cells infiltrating local structures. Since many of these tumors form on the limbs, where large margins are surgically impossible, recurrence is the norm.

What treatments are available?

The treatment for this tumor is surgical removal, usually of the lump, but occasionally more radical surgery such as amputation of the limb will be necessary.

These tumors do not respond to chemotherapy. The tumors are sensitive to radiotherapy, including interstitial brachytherapy. Direct beam radiotherapy is no longer used for this tumor because of side effects and disappointing results.

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.

Lymphocytes of the body's immune system are often seen in this tumor but they are not effective in causing this type of tumor to regress.

How can I nurse my pet? 

Preventing your pet from rubbing, scratching, licking or biting the tumor will reduce itching, inflammation, ulceration, infection and bleeding. Any ulcerated area needs to be kept clean.

After surgery, you need to keep the surgical site clean and dry and prevent your pet from interfering with the incision. Report any loss of sutures or significant swelling or bleeding to your veterinarian. If you require additional advice on post-surgical care, please ask.

How will I know if the cancer is permanently cured?

'Cured' has to be a guarded term in dealing with any cancer. This tends to be a recurrent tumor because it is not possible to get adequate surgical margins.

"This tends to be a recurrent tumor because it is not possible to get adequate surgical margins."

The histopathology report 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).

Low grade (slow growing) tumors have a recurrence rate of 25% after surgical excision. Median survival time is 2-4 years. A 2% metastatic rate has been quoted. High grade tumors (which are rare) have 62% local recurrence and median survival time of 49 weeks. Metastatic rate is quoted at 15% but this is higher than commonly experienced, probably because it is based on difficult cases referred to specialized centers.

Are there any risks to my family or other pets?

No, this is not an infectious tumor and it is not transmitted from pet to pet or from pet to people.

Contributors: Joan Rest, BVSc, PhD, MRCPath, MRCVS © Copyright 2009 Lifelearn Inc. Used and/or modified with permission under license.