A fibrosarcoma is a malignant tumor in the subcutaneous tissue. In cats, there are several forms of fibrosarcomas. Far from being simple masses, they are indeed cancers and their management should therefore not be neglected. Any appearance of one or more masses in your cat warrants a consultation with your veterinarian. Indeed, in the event of cancer, the evolution can be rapid and serious complications can occur.

To understand what fibrosarcoma is, it is important to understand what a tumor is. By definition, a tumor is a mass of cells that have undergone a genetic mutation: they are called tumor cells. This genetic mutation can be caused by carcinogens but it can also be spontaneous. 

Distinguish benign tumors from malignant tumors

A distinction is made between benign tumors which are localized in one place of the body and whose prognosis is mainly favorable, from malignant tumors which can give rise to metastases (cancer cells which will colonize other places of the body) and whose prognosis is mainly unfavorable . Malignant tumors are more often called cancers.

A fibrosarcoma is defined as a malignant tumor of connective tissue (sarcoma). This tumor is therefore a cancer made up of fibroblasts (hence the prefix “fibro”), cells located within the connective tissue, which have undergone a mutation. In cats, we speak of a “feline fibrosarcoma complex” which groups together 3 forms of fibrosarcomas: 

• the solitary form;
• the multicentric form generated by a virus (FSV for Feline Sarcoma Virus);
• as well as the form linked to the injection site (FISS for Feline Injection-Site Sarcoma). 

The FISS is often simply called fibrosarcoma and it is the one we will be interested in here.

The origins of FISS in cats are not yet fully understood, but it seems that the mutation is induced by a local inflammatory reaction. Indeed, an injection being a trauma to the skin, it will be the cause of an inflammatory reaction at the injection level. The most probable hypothesis reveals that repeated injections in the same place, in particular in the event of vaccination or treatment of a disease by repeated injections of a drug for example, could be the cause of this cancer. However, in some more sensitive cats, a single injection can cause fibrosarcoma.

The appearance of a fairly firm and painless subcutaneous mass is noted. As the FISS is linked to repeated injections, in particular vaccines, it will therefore be found more frequently in the area between the shoulder blades. This area is now avoided to vaccinate cats. It can be one or more masses present in this place but also in other places of the body.

Fibrosarcoma is a very invasive tumor, that is to say that by enlarging it will infiltrate the underlying tissues that it will cross on its way (muscle tissue or even bone). It therefore does not form a well-defined mass. Sometimes on her way, she may come across blood or lymphatic vessels. It is through this that cancer cells can break off and find their way into the blood and lymphatic circulation to lodge in other organs. This is called metastases, new secondary foci of cancer cells. Regarding fibrosarcoma, metastases remain fairly rare but are possible (between 10 to 28% of cases), mainly in the lungs, regional lymph nodes and more rarely other organs.

If you see a mass present in your cat, the first instinct should be to make an appointment with your veterinarian. Indeed, even if a lump is not necessarily painful or bothersome, it can be cancerous and have serious repercussions on your animal. It is not possible to determine whether a tumor is benign or malignant with the naked eye, it is necessary to take samples in order to visualize the cells / tissues that the mass contains under a microscope. This will help determine the nature of the tumor.

The treatment of fibrosarcoma consists of surgical excision, that is, the removal of the mass. Before that, an extension assessment can be carried out. This involves taking a series of x-rays of the cat in order to determine or not the presence of metastases, which could darken the prognosis. Since fibrosarcoma is very invasive in the underlying tissues, a large resection is recommended. This involves removing the tumor large enough to maximize the chances of removing all cancer cells that have infiltrated neighboring tissues. The veterinarian will therefore remove not only the mass but also the neighboring tissues over at least 2 to 3 cm around the tumor or even more. It is difficult to remove all cancer cells, which is why another technique is usually associated with this surgery. Radiotherapy can be performed in addition. This involves destroying the remaining cancer cells with ionizing rays. Chemotherapy or even immunotherapy are techniques that can also be considered.

Unfortunately, recurrence of fibrosarcoma is common. This is because remaining cancer cells can multiply and form new masses. This is why the care of a cat that has one or more mass (s) must be fast. The faster the surgery is performed, the less tumor cells will be able to colonize other tissues.

In addition, vaccination being essential for the health of your cat but also for that of its congeners, it should not be neglected. Cat owners are therefore advised to carefully monitor the injection site after any vaccination and to notify their veterinarian in case of doubt.