An eight-year-old tortoiseshell cat, she'd always been in the best of health, but it looked this time as if she'd had a tussle with someone and come off worse. But thinking perhaps she'd bumped her shoulder, we left her alone for a day or two to see what happened.

When she didn't improve, we took her to the vet, who diagnosed a hard abscess. She x-rayed it, (inconclusive), drained it, prescribed antibiotics and home we went.

The swelling went down, Lucy improved no end and it seemed that all was well. And then she started limping again. A second course of antibiotics only held the swelling at bay for a few days, then the lump suddenly and dramatically doubled in size. Back we went to the vet for an excision.

When I phoned the next day, I knew from the receptionist's voice that something was horribly wrong. And it was terrible news. Cancer. Not just cancer, but a highly aggressive one - a sarcoma, a cancer of the soft tissues. Not treatable, not operable, fatal.

She'd tried to get all of it, she said, but it was extensive, and we would still have to await the results of the biopsy, which would take a couple of weeks. "And if it isn't cancer?" I said hopefully.

"Let's just get the results first."

What is VAS?

It is always a terrible time when a vet gives your cat a death sentence, but particularly so when you're just not expecting it.

What Lucy had was vaccine-associated sarcoma (VAS), also known as injection-site sarcoma. This has been a problem known to the veterinary community since 1991 and yet as a cat owner, I had been entirely unaware of it.

So, I later found, are most owners whose animals are diagnosed with this awful disease. One reason is that VAS is very rare. Only about three in 10,000 inoculated cats will contract VAS, and the jury still remains out on the exact cause. But the other is that vaccines are implicated and vets don't want to put owners off having their cats inoculated.

Some analysts believe that VAS is caused by adjuvants - aluminium salts that are added to vaccines to make them more effective. In the US, the rabies vaccine (which is compulsory in many states) and the FelV vaccine have both been implicated in VAS, and both contain aluminium salts.

However, other research indicates that it may simply be the act of injection itself, which inflames the cat's skin and muscle tissue, triggering an unstoppable neoplasm that can take months or even years to appear.

Cats seem to be much more likely to respond to injections with inflammation than other domestic animals. A few cases of VAS are known to have occurred in ferrets, but dogs, for instance, appear to be immune.

However, VAS may be rare but it is still the biggest cause of cancer in cats, and when it strikes it is deadly. Cats can sometimes survive when the cancer is found early and dealt with promptly, preferably by a specialist oncologist. But if the tumour is already advanced, or not recognised for what it is immediately, the prognosis is very poor.

Diagnosis and treatment

The first that most owners know about VAS is when they find a lump on their cat. Often this is at an injection site, such as between the shoulder blades, or near a front shoulder joint. Sometimes, however, it's further away - a back hip joint appears to be a common area. It may be tiny - the size of a lentil - or it may already be bigger. In a few unfortunate cases (Lucy's included) the tumour does not become apparent until it becomes necrotic, when it can be mistaken for an abscess.

When an owner finds a lump, the vet often takes a fine-needle biopsy and the results are negative. But this is where the concerned owner needs to beware. A fine-needle biopsy takes too few cells and often throws up false negatives with this cancer - a surer method is a core biopsy using a Tru-Cut needle, which removes a narrow core of tissue.

Once diagnosed, the only hope for certain cure of VAS is radical excision. The entire lump needs to be removed, along with margins of clean tissue of at least 3cm in every direction. To many owners' distress, this often involves radical amputation of a limb, including the pelvis, shoulder bones or spinal processes. However, excisions in these areas have a higher success rate than those in areas such as between the shoulder blades, where it is more difficult to obtain a clean margin.

Excision or amputation should be performed by a specialist oncologist for the animal to have the best chance of survival. VAS tumours spread like fingers, projecting into sound tissue, and are very difficult to fully eradicate - ordinary vets are often reluctant to take the wide margins required.

Radiation therapy is often recommended afterwards, and sometimes chemotherapy. This is not because VAS metastasises - it generally does not, unless multiple surgeries are performed. Instead it usually recurs on the same spot, growing faster and more malignantly than before. However, chemotherapy, including treatment with basic anti-inflammatories such as prednisolone or NSAIDs, can prevent the cancer from producing inflammatory agents known as cytokines, which it uses to metabolise. Effectively, by giving even an anti-inflammatory, you are starving the cancer at source.

Some cat owners also back up conventional medical treatment with complementary therapies. These include a raw-food diet, a carbohydrate-free diet, or anti-cancer agents such as Solgar Antioxidant Complex, Essiac tea, ES Clear and omega-3 oils. Even if some of these products are medically unproven against cancer, they help to keep the animal fit and well for longer, and therefore better able to fight the disease.

Prevention

An ounce of prevention is worth a pound of cure, and in prevention lies the greatest hope for eradicating VAS. In particular, owners need to assess the value and frequency of vaccinations.

Few vets would suggest that cat owners should not have their cats vaccinated at all. Rabies and FelV are killer diseases and in any case, Britons whose pets have Pet Passes for the Continent are required to vaccinate. It is the frequency that is at issue. In the US, a debate is raging over the requirement for annual injections for rabies, particularly since this vaccine gives an immunity that lasts three years.

Most British vets also vaccinate for FelV annually as a matter of course, but it is worth discussing the real needs of your pets with your vet. It your cats never go outdoors, they are highly unlikely to contract FelV, which is passed on only by close contact, and may not need the vaccine at all. In other countries such as France, vets recommend that the FelV vaccine is given only twice in a lifetime.

Another approach to the problem of VAS is to change the injection site. Tumours that develop in the scruff region, where cats have traditionally been injected, have a very poor prognosis, so some vets are now injecting in the hind foot - the gruesome argument being that if a tumour should appear, it will at least be easier to amputate.

After injection, the vet should also rub the area strongly, to disperse the liquid into the tissues. The owner should then watch the area closely and return to the vet IMMEDIATELY if there is any swelling on the site.

Some vets are now considering injections in the tail, which a cat can lose fairly happily.

Euthanasia

Fibrosarcoma is a disease that is basically physical - in action it effectively behaves almost like a benign tumour. The tumours themselves appear to be painless (though they painfully encroach on and infiltrate other organs) and simply grow larger and larger until they become completely debilitating.

In the wild, this means the animal would be unable to hunt and care for itself and would starve to death. In a domestic setting, however, the owner has the unenviable task of deciding when the cat has so little quality of life left that euthanasia is the only option. Often this is when the cat begins to have difficulty breathing.

For Lucy, we expect to make that decision in about two weeks, which means that from her first limp to her last breath will have been about 14 weeks.

She is one of the unlucky ones and the irony is that her life has been wiped out by something we did in order to prolong it.

But if you follow the advice in this article, and consult carefully with your vet, you can hopefully prevent your cats from joining her

• VAS Support group on Yahoo - invaluable for pet owners
• VAFSTF (Vaccine-Associated Feline Sarcoma Task Force)
• VAS Awareness
• ES Clear