Fibrosarcomas in Dogs

By Debbie Stoewen DVM, MSW, RSW, PhD

Tumors, Pet Services

What is a fibrosarcoma?

A fibrosarcoma is a malignant tumor that develops from the uncontrolled overgrowth of cells called fibroblasts. Fibroblasts are the most common cells of the connective tissue in the body (the tissue that connects, supports, and binds or separates tissues and organs). Fibrosarcomas occur most often in the connective tissue of the skin and beneath the skin.

A fibrosarcoma is a type of soft tissue sarcoma (see handout "Soft Tissue Sarcomas"). They are most commonly found on the limbs (often the extremities) and the trunk of the body. They can also be found in the nasal cavity or mouth (sometimes invading the jaw bones). In rare cases, fibrosarcomas will originate within the jaw bones, or leg bones, causing a primary form of bone cancer. Fibrosarcomas are usually slow growing, except for those of the leg bones, which can grow very rapidly. Fibrosarcomas are common tumors in dogs.

What causes this cancer?

The reason why a particular dog 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. In the case of fibrosarcomas, although no specific risk factors or cause have been identified, sarcomas in general (tumors of the connective tissue) have been associated with radiation, trauma, foreign bodies, and orthopedic implants. Sarcomas have long been associated with injection sites in cats (e.g., feline post-vaccinal fibrosarcomas; see handout "Post-Vaccination Sarcoma in Cats" for more information), but there is growing evidence that tumors may be associated with injection sites in dogs too.

"Certain breeds of dogs, especially large breeds, are at greater risk of developing fibrosarcoma."

Certain breeds of dogs, especially large breeds, are at greater risk of developing fibrosarcoma. These include Gordon Setters, Irish Wolfhounds, Brittany Spaniels, Golden Retrievers, and Doberman Pinschers. It is more common in dogs who are middle-aged or older, with the average age of occurrence being 10 years. Occasionally dogs less than 1 year of age will develop fibrosarcoma. In these cases, the cancer tends to be an aggressive form of fibrosarcoma with a poor prognosis. Fibrosarcomas of the nasal cavity and mouth are more common in male dogs.

What are the signs of this type of tumor?

The signs of fibrosarcoma vary depending on the location, size, and the extent of the tumor.

Fibrosarcomas usually appear as a single, sometimes nodular, firm lump or bump on or under the skin, which at times may open (ulcerate), bleed, and become infected. When there is more than one, they are usually in the same area. Whether single or multiple, there may be swelling of the affected area and pain. The pain may cause your dog to withdraw (become less sociable), refuse to be touched, or lose his appetite.

"Fibrosarcomas usually appear as a single, sometimes nodular, firm lump or bump on or under the skin, which at times may ulcerate, bleed, and become infected"

If the leg is affected, there may be lameness, or difficulty getting up or lying down, or an inability to walk. When the tumor is in the bone, fractures may occur even if there has not been a physical trauma.

If the nasal cavity is affected, there may be mucus discharge from the nose (or eyes, sometimes with excessive tearing), bleeding from the nose, sneezing, snoring and snuffling sounds, and pawing at the muzzle. Likewise, if the mouth is affected, there may be difficulty picking up food or eating or swallowing, reluctance to eat, excessive salivation (drooling), and bleeding from the mouth. With nasal and oral tumors, there may be halitosis (bad breath), loose teeth, and facial deformity (especially around the muzzle).

How is this cancer diagnosed?

This cancer is often first spotted during a full physical examination. If the mass is concerning, your veterinarian will review your dog’s medical history and ask questions about any symptoms. If your veterinarian is suspicious that the mass may involve the bone, radiographs (X-rays), and/or a CT scan will often be recommended. Although the appearance of the tumor, clinical signs, and diagnostic imaging may be suggestive of fibrosarcoma, to ensure an accurate diagnosis, microscopic examination of the tumor cells is required.

There are different methods to collect a sample of tumor cells. The two most common methods are fine needle aspiration (FNA) and tissue biopsy.

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. This is called cytology. FNA with cytology is generally not the most useful method for diagnosing fibrosarcomas (as compared to other kinds of tumors), as the cells adhere to one another making it difficult to obtain a sample. A tissue biopsy is often preferred.

A tissue biopsy is the surgical excision of one or more pieces of tumor. The pieces are then examined by a veterinary pathologist under the microscope. This is called histopathology. Biopsy with histopathology is not only helpful to make a diagnosis but can indicate how the tumor is likely to behave. Fibrosarcomas are usually classified as either “high” or “low” grade. The grade can help to determine various issues, such as whether the fibrosarcoma is likely to recur in the future.

How does this cancer typically progress?

Without treatment, fibrosarcomas will continue to grow; usually very slowly. They often become ulcerated and, if they do, are prone to infection. Fibrosarcomas tend to be locally invasive, extending into the surrounding tissues or bone. If they invade the bone (or originate within the bone), they will destroy the bony tissue, weakening the bones which leads to the risk of a fracture. Usually only tumors that start within the leg bones will grow rapidly.

About 10% of fibrosarcomas in dogs will metastasize (spread) to other parts of the body, including the nearby lymph nodes and the lungs. Staging (searching for potential spread to other locations in the body) is highly recommended if there is suspicion of spread. 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.

What are the treatments for this type of tumor?

Surgery is the treatment of choice for fibrosarcomas. While surgical removal is recommended, it is important to keep in mind that complete removal is usually not possible, often due to the tumor’s location as well as its invasive nature. Wide and deep surgical margins are needed, and even with this, recurrence is still the rule. For tumors of the nasal cavity or mouth, surgery may involve removing part of the jaw bone. For tumors on the legs, especially if large or involving the bone, amputation may be the best (or only) approach. Tumor recurrence is less likely with amputation.

"Surgery is the treatment of choice for fibrosarcomas."

After surgery, the tissues are sent for histopathology, to predict, as best as possible the probability of local recurrence or metastasis. If the entire tumor is submitted for examination, the pathologist will be able to assess if the tumor was completely removed or if additional therapies (a second surgery, radiation, or chemotherapy) are necessary. Radiation treatment, alone or in addition to surgery (which is more often the case), can be of benefit. If radiation is recommended, the duration of treatment will be individualized based on your dog’s specific needs. The same goes for chemotherapy. While chemotherapy is generally less effective with fibrosarcomas, it has been used in combination with surgery and radiation, and has been recommended in cases when surgery is not an option.

Is there anything else I should know?

Fibrosarcomas are quite common, but with proper and prompt treatment, favorable outcomes are possible – even if the treatment is not curative. As fibrosarcomas tend to recur, usually in the same area, it is important to stay proactive and closely monitor your dog for signs of recurrence and report these to your veterinarian. Keep in mind that although this tumor is malignant, unlike many other cancers, it is uncommon for it to metastasize.

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