This tumour type is located in the skin or subcutaneous on the trunk, mammary glands, limbs and head.

Behaviour

These tumours grow slowly, look encapsulated, can be soft to solid and invade the surrounding tissues. After their surgical elimination, a relapse is regularly seen. Metastases, if present, will spread via the blood (metastases will seldom appear in local lymph nodes).

Breed predisposition

This tumour type occurs mostly in large breeds, but in general there is no strong predisposition for certain breeds.

Symptoms

This tumour grows slowly and generally forms a non-painful mass. Depending on the location and invasion of the tumour, local symptoms will appear (e.g. a tumour that penetrates the bone or that complicates bending the toes will cause the dog to limp). Satellite lesions are regularly seen. These satellite lesions are new nearby tumour lumps that form as a result of the local pressure the tumour experiences (which in turn seeks the path of least resistance to spread).

Fine needle aspiration

It is always advised to do a punction of a tissue mass to make the distinction between a tumour and other tissue masses with a similar aspect (tissue masses such as fat lumps, accumulations of encapsulated liquids, inflammations, abscesses). On the other hand, the nature of fibrosarcomas can be deceitful and punctions false-negative. When cancer cells are found in the content of a tumour aspiration, this proves the tissue mass is a tumour/metastasis. When no cancer cells are found, this does not exclude that the tissue mass is cancerous.

Tissue sample

For a definitive diagnosis it is necessary to do a biopsy. Tissue can be obtained by taking a small sample via a big needle, by resecting a part of tumour tissue or by excising the tumour in its entirety.

Imaging

In order to exclude lung metastases, it is advised to take radiographic photos of the thorax. The radiographic photos, ultrasound, CT/MRI scan of the primary tumour location can be necessary when the tissue mass invades underlying structures and we want to know how much the underlying structures are affected. This way, the elimination of the tumour can be better planned. It is a.o. known that ultrasound will detect a need for larger margins in 19% of cases, with a CT scan this is true for 65% of cases.

Grading

Via regular histopathology it is not always evident to differentiate a fibrosarcoma from other tumour types. Therefore, it is sometimes necessary to use antibodies that bind certain markers present in the tissue of fibrosarcomas. If these markers bind the sampled tissue, this means that there is a high probability it’s a fibrosarcoma. The tissue grade is based on the degree of differentiation (how well the different elements in the tissue are recognizable), the multiplication index (how much the cancer cells are multiplying) and necrosis (tissue death). The less differentiated and the higher the multiplication index and level of necrosis, the worse the prognosis is.

Surgery

Surgery with large margins is advised for fibrosarcomas (a minimum of 3 cm and 1 to 2 tissue layers are necessary). In case of relapse of an incompletely eliminated fibrosarcoma, surgery or radiotherapy are recommended. When there are no metastases after a complete excision of the tumour, the dog is cured. If the tumour responds to chemotherapy, the effects are usually short-lived.

Chemotherapy

Chemotherapy is advised as palliative treatment for fibrosarcomas of a certain size and grade 3 fibrosarcomas. Metastases of fibrosarcomas have been treated with doxorubicin and other chemotherapeutics and have led to measurable results. However, their effect on micrometastases is unknown, as well as whether chemotherapy can prevent or delay a local relapse or metastases. If the tumour responds to chemotherapy, these effects are usually short-lived.

Radiotherapy

Large tumours are generally considered to be resistant to radiotherapy. If radiotherapy is used, it is usually as adjunctive therapy after incomplete surgical removal.

In general, it’s more likely that the primary tumour causes problems than the metastases. The size of the tumour, the tissue grade and the entirety of the tumour excision are connected with the success rate of the surgical treatment. A large tumour, with a high grade and incomplete margins, has a higher risk of relapse than a small tumour, with a low grade, that can be completely eliminated. When the tumour is not entirely removed, 20 to 35% (higher in tumours with a high grade) of tumours relapse and reappear in the same spot. It’s more difficult to control relapsing tumours than those that have not been removed in the past (those relapse quicker and have a higher risk for metastases). When there is still no relapse 12 months after the surgical excision, it is not likely this will happen.

• Grade I sarcomas have the lowest relapse risk after surgical resection. When the excision is complete, relapse is rare. When the margins are small, the tumour comes back in only 7% of dogs. Metastases are considered rare for grade I fibrosarcomas.
• Grade II sarcomas also rarely relapse after a complete excision, but in case of incomplete margins, they relapse more (35%) and more quickly than grade I fibrosarcomas.
• Grade III fibrosarcomas are the least prevalent and carry the highest potential of relapse (higher than grade II) and metastases.

With radiotherapy alone, the number of dogs (achieving complete response) for whom the disease did not reoccur after 1 year (at 50 Gy total dose) is 67% and 20%-33% after 2 years. When radiotherapy is used as adjunctive therapy, the number of dogs for whom the disease did not reoccur after 1 year was 71-95% and 60-91% after 2 years.

This tumour type and its metastases will rarely lead to a dog’s demise, as fibrosarcomas grow slowly (as do their metastases), and this tumour type is mostly seen in older dogs.