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Key takeaways

Fibrosarcomas in dogs are common malignant tumors that develop on or beneath the skin.

  • Most commonly observed on the limbs or trunk
  • Less common to develop in the mouth, nose or bone Tumors are locally invasive, slow growing, and can drastically reduce lifespan
  • Typically the only sign is the presence of a lump on the dog’s body
  • In the case of a fibrosarcoma in the oral cavity, they may experience drooling or inappetence
  • Diagnosis of a fibrosarcoma is done by cytology and biopsy
  • Surgery is required for treatment (often aggressive and complicated)
  • Due to the invasive nature of fibrosarcomas, recurrence is common
  • Chemotherapy and radiation therapy may be beneficial along with surgery or if surgery cannot be performed
  • The cause is unknown and prognosis is dependent on several factors and varies from good to poor
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A closer look: Fibrosarcomas in Dogs

While not an emergency, any new or growing lumps or masses found on a dog must be assessed by a veterinarian for further diagnostics.

Fibrosarcomas are a common tumor found in dogs. They are a form of soft tissue sarcoma, and are considered a type of cancer. Like most cancers, these have the risk of metastasizing (spreading) to other areas of the body.

Fibrosarcomas that originate in the bone are considered a type of bone cancer, and unlike cutaneous (skin) fibrosarcoma, can grow quickly and aggressively with a higher risk of metastasis.

As with many types of cancer, prognosis depends on several factors, including size, grade and location of the tumor, and whether metastasis has occurred. Even with prompt diagnosis, these tumors can dramatically decrease a dog’s life expectancy.

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Risk factors

Fibrosarcomas usually occur at the average age of 10 years in dogs. Breeds such as Irish Wolfhounds, Doberman Pinschers, Brittany Spaniels and Gordon Setters appear to be at a greater risk of developing them, also suggesting a potential genetic component.

Some fibrosarcomas may cause discomfort to a pet if they become ulcerated or raw. Dogs may be observed licking and chewing the affected area, thereby causing further irritation.

A dog with a fibrosarcoma in its mouth or nose may experience sneezing, drooling, pawing at the area, or lack of appetite.

A dog with a fibrosarcoma in their bone (usually occurring in a limb) may present with lameness.

If a fibrosarcoma has metastasised, symptoms can vary depending on the location.

Left untreated, symptoms can continue and often worsen as the tumor grows.

Possible causes

The cause of fibrosarcomas in dogs is unknown, however it is believed that it develops similar to most other cancers which is when there is damage to the cells. These tumors are formed when fibroblasts (a type of cell found in connective tissue) experience an overgrowth within the body. This may be associated with injury, radiation, chemicals, or infection.

Main symptoms

Dogs with a fibrosarcoma may not display any immediate clinical signs, it is usually the presence of a new lump that initiates investigation.

Fibrosarcomas usually appear as a small bump on the skin and are outwardly difficult to distinguish from other cutaneous and subcutaneous lumps. These masses tend to grow slowly and occasionally can bleed or ulcerate, which leaves them prone to infection.

Testing and diagnosis

Diagnosis of a fibrosarcoma begins with a thorough physical examination.

Fibrosarcomas cannot be diagnosed with visual inspection alone - a microscopic examination is required. This can be done through either cytology (using a needle to obtain cells) or biopsy (removing a small piece of the mass). A biopsy is the preferred method as more information is able to be obtained by this sample such as whether it is a low or high grade tumor (if it is rapidly producing cancerous cells or not) and if the excision was complete.

Once a diagnosis has been determined, it is important to determine if the tumor has spread, or metastasized. This is done with the following tests:

  • Diagnostic imaging (X-Rays, ultrasound, CT, and/or MRI)
  • Bloodwork
  • Urinalysis

Steps to Recovery

A diagnosis of fibrosarcoma almost always warrants removal due to the invasive nature of the tumor. These tumors have a tendency to re-grow, as they are locally invasive to the surrounding tissue and muscle. Surgery often needs to be aggressive with large margins, affecting deep muscle, and in the case of bone fibrosarcomas, may mean amputation. If the tumor cannot be removed (in cases where there is not enough tissue to close the incision, such as in the mouth or nose), radiation therapy, chemotherapy and supportive care is warranted.

If left untreated, fibrosarcomas continue to slowly grow. As they grow they begin to locally spread deeper into tissue and potentially into bone. The chance of metastasis into other areas of the body increases the longer the tumor remains. The mass may become ulcerated or infected, and in the case of fibrosarcoma in the bone, it can weaken the affected limb and predispose dogs to fractures.

With treatment, fibrosarcomas are at a high risk of recurrence due to their invasive nature and difficulty in completely excising the tumor. If originating in a limb bone, amputation is curative if no metastasis is present.

Prognosis depends on the treatment pursued, complete removal, recurrence, and whether or not there is metastasis. Many dogs diagnosed with fibrosarcoma have relatively normal life spans if caught early, removed completely, and have not spread. However, prognosis is poorer in dogs that did not have tumors removed completely, or with metastasis


There is no known way to prevent the formation of fibrosarcomas. If a dog has had a fibrosarcoma removed, it is imperative to closely monitor the affected area for recurrence.

Fibrosarcomas are not contagious to other animals or people.

Is Fibrosarcomas in Dogs common?

Fibrosarcomas are common skin tumors found in dogs.

Typical Treatment

  • Surgical Removal of mass
  • Amputation of affected limb
  • Chemotherapy
  • Radiation Therapy


Malcolm Weir, DVM, MSc, MPH; Debbie Stoewen DVM, MSW, RSW, PhD - Writing for VCA Animal Hospitals
Alice E. Villalobos, DVM - Writing for Merck Veterinary Manual
No Author - Writing for Pet Health Network
M Martano, S Iussich, E Morello, P Buracco - Writing for Veterinary Journal
Melissa Boldan, DVM - Writing for PetMD

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