Fibrosarcoma in dogs: symptoms, diagnosis and treatment

Overview

Fibroblasts are normal connective tissue cells. Connective tissue supports and links tissues and organs. Fibrosarcoma belongs to the sarcomas, tumours originating in connective and supporting tissues. The soft tissue sarcoma group also includes nerve sheath tumours and myxosarcomas, among others.

Fibrosarcomas account for about 18–23% of canine soft tissue sarcomas. They occur particularly on the trunk and limbs. They can also affect the mouth or nasal cavity. Rarely, the tumour originates in bone. An oral fibrosarcoma can also invade the jawbone locally.

Even a slow-growing lump may be a locally invasive cancer. Tumour extensions beneath the skin can exceed the area you can feel.

Causes

Disrupted connective tissue cell growth

Fibrosarcoma arises from uncontrolled fibroblast multiplication. For most dogs, no single precise cause can be identified. A tumour may develop without a recognisable preceding injury. In some cases, connective tissue sarcomas have been associated with previous radiation exposure, trauma, a foreign body or a surgical implant.

Prolonged local inflammation may be associated with sarcoma development. Fibrosarcomas developing at injection sites have also been described in dogs. Inflammatory cells may accumulate in surrounding tissue in these tumours.

A tumour can extend along tissues

Fibrosarcoma cells can spread along fascial planes into muscles. A soft tissue sarcoma may have a pseudocapsule invaded by tumour cells. The apparent boundary is therefore not necessarily the true edge. Cell differentiation and division activity influence predictions of tumour behaviour.

Risk factors

Fibrosarcoma is most commonly diagnosed in middle-aged and older dogs. The average age is about 10 years. Occasionally, a tumour is diagnosed before 1 year of age. Such an early-onset form may be aggressive with a poor outlook.

Large breeds have shown increased susceptibility. Breeds considered predisposed include Irish Wolfhounds, Golden Retrievers and Dobermanns. Increased susceptibility has also been described in Rottweilers. Evidence about sex hormone effects on canine soft tissue sarcoma risk remains insufficient.

Gordon Setters and Brittanys are also among breeds with described increased fibrosarcoma susceptibility.

Typical symptoms

Initially, your dog may have only a subcutaneous lump without other signs. Growth can be slow over months or years. In some dogs, however, a soft tissue sarcoma grows within a few weeks.

• A firm nodule is felt in or beneath the skin.
• The lump may seem attached to deeper structures.
• The affected area swells.
• Skin over the lump ulcerates or bleeds.
• Your dog avoids touch and withdraws from company.
• A limb tumour can cause lameness or difficulty rising.
• Pain may reduce appetite.

If several fibrosarcomas are present, they usually occupy the same body region. A soft tissue sarcoma can also feel soft and movable. Softness alone therefore cannot establish that a lump is benign.

Complications and consequences

Local damage and metastases

An ulcerated tumour can become infected. A soft tissue sarcoma pressing on a nerve can hurt. A tumour around a joint can hinder movement. Bone invasion can weaken bone and predispose it to fracture.

Estimated fibrosarcoma metastatic rates range from about 2% to 41%. Risk relates to tumour grade. Metastases can affect the lungs or nearby lymph nodes. Across all soft tissue sarcomas, metastases occur in under 10% of low-grade cases. For intermediate-grade tumours, the corresponding proportion is 10–20%. For high-grade tumours, it is 40–50%.

Recurrence risk and outlook

The outlook depends on location, grade and completeness of removal. Among 350 soft tissue sarcoma patients treated in primary practice, local recurrence occurred in 20.8%. This population, weighted towards less severe tumours, had a 5-year survival rate of 70%.

If removal margins contain tumour, the local recurrence rate for soft tissue sarcoma is about 20–35%. High-grade tumours generally have a greater recurrence risk. Recurrent soft tissue sarcoma is often harder to control.

Tumours over 5 cm have been associated with shorter disease-free survival than smaller soft tissue sarcomas. Large size can also make complete removal harder.

Diagnosis at the vet

Fine-needle sampling and biopsy

Your vet reviews medical history and symptoms caused by the lump. Investigation before surgery helps plan sufficiently wide removal. Fine-needle sampling collects cells for microscopy. However, fibrosarcoma may release few cells because they adhere tightly to each other.

An inconclusive cell sample therefore does not remove the need for biopsy. Biopsy removes a tissue piece for a pathologist to examine. Complete removal of a small, freely movable tumour can also serve as a biopsy. A large or fixed tumour is often sampled partially first.

Grade and spread

Fibrosarcomas are usually graded I–III. Tissue assessment examines differentiation, dividing cell numbers and areas of necrosis. A grade assigned from a small biopsy may be refined after the whole tumour is examined. Special stains are used in selected cases to investigate other tumour possibilities.

Size, mobility and attachment are assessed at the practice. CT can define boundaries and help plan surgery. Suspected bone invasion may need X-rays or CT.

Staging usually includes chest X-rays from 3 directions. A regional lymph node can be sampled for metastases. Even a normal-sized node may contain tumour cells. Blood and urine tests assess general health.

Preoperative biopsy grade has differed from whole-tumour assessment in 12–29% of cases. Grade describes microscopic structure. Stage instead describes local extent and metastases.

Cytology results are often available within 0–2 days of starting examination. Tissue examination usually takes about 3–7 days before the final report is ready.

Treatment

Careful planning of the first operation

Surgery is first-line treatment for cutaneous and subcutaneous fibrosarcoma. Your vet selects treatment using tumour sampling and staging where needed. Healthy surrounding tissue is removed too, to include microscopic extensions. Margins are planned according to grade and your dog's circumstances.

For a small, low-grade soft tissue sarcoma under 3 cm, a 2 cm lateral margin is recommended. An intact fascial plane beneath it is also removed. Intermediate-grade tumours have a recommended lateral margin of 3 cm. For high-grade tumours, the recommendation is 5 cm and 2 deep fascial planes.

If adequate removal of a limb tumour is otherwise impossible, amputation may be needed. Wound closure may require a skin flap or another reconstructive technique. The whole tumour is submitted to a pathologist for margin assessment. If margins contain tumour, further surgery on the scar area may be considered.

Radiotherapy and medication

Radiotherapy can help prevent local recurrence after incomplete removal. It may also be an option for an inoperable tumour. After radiotherapy for incompletely removed soft tissue sarcoma, local control is about 75% at 1 year. The median interval to local recurrence is then about 2 years.

Palliative radiotherapy may aim to reduce pain and stabilise growth. Chemotherapy is considered particularly for high-grade or metastatic sarcomas. Chemotherapy alone has shown no clear benefit for visibly present soft tissue sarcoma. Evidence of a survival benefit is inconsistent. Metronomic chemotherapy may prolong local control.

Treatment adverse effects and supportive care

Surgery carries risks of bleeding and wound infection. Anaesthesia can also involve complications. Radiotherapy may irritate skin in the treatment field and interfere with healing. Dogs usually need general anaesthesia for radiotherapy.

Postoperative radiotherapy can usually begin after 2–4 weeks, once healing is adequate. Veterinary-planned chemotherapy can cause temporary vomiting or diarrhoea. White blood cell counts can also fall temporarily. Appetite loss and lethargy may occur during treatment. Supportive medicines for nausea or diarrhoea can be prescribed for home use.

Electrochemotherapy

Electrochemotherapy combines local electrical pulses with a tumour-targeting chemotherapy medicine. It may be considered for selected patients as an alternative to radiotherapy.

Recovery and follow-up

Postoperative care includes prescribed pain relief. Exercise is restricted according to surgical extent and healing. Home care includes monitoring the wound and general condition. Follow-ups assess local recurrence and changes suggesting metastases. Examinations and chest imaging where needed may take place every 3 months, for example. A new symptom can also result from treatment adverse effects or an unrelated disease.

Prevention

There is no reliable known way to prevent fibrosarcoma. In practice, attention focuses on early investigation of new or growing lumps. A small tumour is generally easier to remove than a large one. However, a normal-looking surface does not automatically make a lump harmless. Clinical veterinary examination forms part of assessing new skin lesions. A previous fibrosarcoma site is monitored for a new lump or growth.

Skin lumps can be mapped to allow systematic monitoring.

Important for dog owners

Record when a new lump was noticed, its growth and any pain for the appointment. Mention avoidance of touch, lameness or reduced appetite. Keep the pathology report showing grade and margin assessment.

Ask whether surgery aims for complete removal or improved quality of life. Follow your dog's wound-care and exercise instructions. Report a new lump in the same area even if the original wound has healed. Monitoring continues after a cancer diagnosis even when your dog has no symptoms.

Frequently asked questions

Is fibrosarcoma the same as a fatty lump?

Fibrosarcoma is a malignant tumour arising from fibroblasts. A fatty lump, or lipoma, is a different, benign tumour type. Appearance and palpation cannot determine the nature of a subcutaneous lump, so sampling is needed.

Why is a fine-needle sample sometimes inconclusive?

Fibrosarcoma cells adhere to each other, so few may enter the needle. A tissue biopsy is therefore often more informative. It helps identify the tumour and assess its behaviour. The whole removed tumour is also examined later to assess surgical margins.

Why remove healthy tissue around the lump?

Microscopic sarcoma extensions can reach beyond the visible or palpable lump. Adequate margins aim to remove these cells too. If location prevents sufficiently wide removal, options may include more extensive surgery or radiotherapy.

Can fibrosarcoma recur despite clear margins?

Local recurrence has occurred in 7–17% of high-grade soft tissue sarcomas despite wide, clear margins. Grade also influences the outlook. Examining the surgical area and monitoring for metastases therefore remain part of follow-up after successful removal.

How long can a dog live with fibrosarcoma?

The outlook depends on size, grade, location and metastases. Many dogs may have a near-normal lifespan if the tumour is found early, removed completely and has not spread. Incomplete removal or metastases worsen the outlook.