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CONTRIBUTOR(S): Claire Speight, Anna Meredith OBE,

Fibrosarcoma

Fibrosarcomas are aggressive malignant (cancerous) tumors found in rabbits. Although they are commonly visible as a lump, they may also be internal and difficult to detect.

A fibrosarcoma on the skin © Richard Saunders

Where are they found?

Fibrosarcomas are a type of rapidly growing tumor that develops from the connective tissue, which is found in between other tissues and organs everywhere in the body. They can then go on to spread and invade surrounding tissues, such as muscle, and also metastasize (spread via the blood) to other areas of the body, such as the heart, kidneys, lungs, etc. They can develop in any part of the body.

How are they diagnosed?

Rabbits can suffer from a variety of lumps; some of these are benign and some are malignant, and it is important to know what the cause of the lump is so that the most suitable treatment can be offered such as surgery to remove the lump. Your veterinarian will usually perform a fine needle aspirate (FNA) which involves taking some of the cells from the lump, through a needle and analyzing these under a microscope. This can often be done with your rabbit conscious. Sometimes the sample will be sent away to an external laboratory, and they will be able to diagnose what type of lump your rabbit has, including if it is a fibrosarcoma. Alternatively, your vet might wish to anaesthetize your rabbit to take a biopsy, or to try and remove the lump entirely and submit a sample for analysis.

Can they be treated?

If it is a fibrosarcoma, the tumor will need to be surgically removed as soon as possible. The longer it is left the more likely it is to spread and invade other areas of the body. If it is located on a limb, then amputation is usually recommended, to make sure all the tumor is removed along with margins of skin and muscle to ensure no cancer cells are left behind. If the fibrosarcoma is not on a limb, then it can be surgically removed along with some of the surrounding tissue. This will be submitted to a laboratory so the tumor can be analyzed to make sure all of it has been removed, reducing the likelihood of it coming back.

Will the tumour come back?

These tumors do have a high incidence of reoccurring, up to 70%, which is why it is important to have any lumps checked out by your veterinarian as soon as possible and for them to try and get large enough margins around the tumor when it has removed. However, even then, as the tumors spread to surrounding tissues and within the bloodstream, reoccurrence at the same or a different site is often seen.

Can chemotherapy and radiotherapy be given?

Yes, it is possible to use both chemotherapy and radiotherapy in rabbits, but there are many considerations to think about. Treatment is unlikely to cure this type of tumor but may slow down the progression and spread and may be used after surgery if it was not possible to totally remove the fibrosarcoma. Both chemotherapy and radiotherapy cause immune suppression which can allow sub-clinical infections, such as E. cuniculi and pasteurellosis, which many rabbits have, to emerge and make your rabbit very ill. These treatments would likely need referral and would incur significant expense. Many rabbits will not tolerate the frequent handling and stress involved, and radiotherapy involves a general anesthetic, which is another risk to consider. You will need to discuss these options with your veterinarian. Sometimes these treatments may be used prior to surgery to attempt to shrink the tumor if it is particularly large and it is not possible to remove it fully at its current size.

What happens if surgery is not possible?

Recurrent fibrosarcomas, or those which are not possible to surgically remove, will continue to spread. Maintaining your rabbit’s quality of life, and ensuring they are eating, drinking, and able to move around pain free, is imperative. Your veterinarian can prescribe pain relief, and when quality of life is not acceptable, euthanasia is the kindest thing to do.

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