Soft Tissue Sarcomas

By Christopher Pinard, DVM

What is a soft tissue sarcoma?

Soft tissue sarcomas are a broad category of tumors including those that arise from the connective, muscle, or nervous tissues in dogs or cats. These tumors are the result of abnormal production of these cell types in an uncontrolled manner. Connective, muscle, and nervous tissues are present throughout the entire body; therefore, these tumors can develop over the chest, back, side, legs, and facial tissues of your pet. Soft tissue sarcomas make up about 15% of cancers of the skin affecting dogs, and about 7% of those affecting cats. Fibrosarcomas are common in dogs and are a type of soft tissue sarcoma (see handout "Fibrosarcoma in Dogs" for more information on fibrosarcomas).

"Soft tissue sarcomas can in arise connective, muscle, and nervous tissue anywhere in the body."

Even though soft tissue tumors arise from many different types of cells, they all behave in a similar manner and their treatment is typically the same.

What causes soft tissue sarcomas?

The reason why a particular pet may develop this, or any tumor or cancer, is not straightforward. Very few tumors and cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary.

"For most cases of soft tissue sarcomas, no direct cause has been determined for their development."

For most cases of soft tissue sarcomas, no direct cause has been determined for their development. Sarcomas at injection sites occur in cats but are rare in dogs (see handout “Post-Vaccination Sarcoma in Cats” for further information on this type of sarcoma). In cats exposed to a form of the feline leukemia virus (called feline sarcoma virus), development of sarcomas on the head and neck sometimes occurs.

What are the clinical signs of soft tissue sarcomas?

The clinical signs your pet may have depend on where the tumor is located. Because these tumors arise virtually in any part of the body, signs vary greatly depending on the tissues that are affected. Often, pets have a noticeable mass that is growing in size.

Signs related to soft tissue sarcomas include the following:

  • Pets that have tumors arising from muscle tissue may show signs of pain in the affected region and may have a distinct firm and growing mass (tumor).
  • Tumors that are located on the limbs may cause changes in your pets’ ability to walk and the limbs may have obvious swelling.
  • Pets that have tumors arising from nervous tissue may be unable to use the affected limb or may show other neurological signs.
  • Pets with intestinal tumors may have signs of an intestinal blockage such as vomiting, diarrhea, lack of appetite, weight loss, and abdominal pain.
  • Pets with soft tissue sarcomas in the mouth often have bad breath (halitosis), difficulty eating, loss of appetite, bleeding in the mouth, or obvious tumors in the mouth.
  • Signs of a soft tissue sarcoma affecting the reproductive system depend on the location of the tumor. For example, if the prostate is affected, difficult urinating or defecating may be noted.

How are soft tissue sarcomas diagnosed?

In some cases, a fine needle aspiration (FNA) may be performed. FNA involves taking a small needle with a syringe and suctioning a sample of cells directly from the tumor and placing them on a microscope slide. A veterinary pathologist then examines the slide under a microscope.

However, if a diagnosis is not confirmed by this method, a biopsy may need to be performed. A biopsy is a surgical excision of a piece of the tumor. Pieces of the tumor are then examined under the microscope. This is called histopathology. Biopsy is beneficial because it gives an indication as to how aggressive the tumor is, as well as how treatment should be approached.

Staging (searching for potential spread to other locations in the body) may be recommended. This may include bloodwork, urinalysis, X-rays of the lungs, and possibly an abdominal ultrasound. If any lymph nodes are enlarged or feel abnormal, further sampling may be pursued to determine if spread is present.

How do these tumors typically progress?

This is entirely dependent on location and the grade of the tumor. Typically, the higher the grade (these tumors are graded from I to III) the more likely that spread is possible. However, one of the biggest concerns with soft tissue sarcomas is their ability to invade the local surrounding tissues. They can almost be described as an ’octopus’ where the bulk of the tumor is the head, and the microscopic cells that invade the surrounding tissue are like small tentacles. These ‘tentacles’ become challenging to treat when managing your pet’s tumor, either by surgery or radiation therapy.

What are the treatments for these types of tumors?

The most commonly pursued treatment for all soft tissue sarcomas is surgery. As mentioned, because these tumors typically produce ‘tentacles’ of abnormal cells, wide-margins (the amount of tissue that needs to be removed) must be obtained for the best control of the tumor. If microscopic cells are left behind after surgery, recurrence of the tumor is much more likely. If cells are left behind (determined through histopathology), either a second surgery, or a combination of surgery and radiation therapy may be pursued.

Chemotherapy is not usually pursued as a primary treatment unless surgery or radiation are not options for your pet based on the tumor size or location. Chemotherapy may be an option after surgery.

"Chemotherapy is not usually pursued as a primary treatment unless surgery or radiation are not options for your pet based on the tumor size or location."

Metronomic chemotherapy (daily administration of lower doses of chemotherapy rather than traditional schedules) may be recommended. These therapies will be discussed with you if they are relevant to your pet’s particular type of sarcoma.

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