What is it?
Cancer, or more properly neoplasia, is an abnormal growth of body cells. It occurs due to damage to the DNA (the "blueprints" or "instructions" that tell the cell how to behave), and especially to those genes which tell a cell when to stop multiplying itself. However, it's important to realise that cancer isn't a single disease - there are thousands of different types. Some are benign (grow but don't spread); some are malignant (invade tissues and spread around the body); and some are intermediate, being invasive but not prone to spreading.
What causes it?
Ultimately, all cancers are due to a mistake when copying the DNA, and is usually due to a random mistake. However, most mutations do not cause cancer - the damaged cell may repair itself (there are enzymes called dismutases whose only job is to fix errors in DNA!); if not, it will usually self-destruct, and if it doesn't, the immune system is very, very good at finding and destroying any mutated cells. However, with so many billions of cells in the body, every now and again, one slips through the net, and grows into a cancer, or tumour. That said, although there is a constant background level of random mutations occurring, some environmental factors can increase the risk - such as radiation, certain chemicals (carcinogens), or because they've inherited defective genes that mean the body doesn't shut down a cancerous growth so efficiently.
What dogs are at risk?
In general, cancer is a disease of older dogs - because they've had many more cell divisions, and therefore more chance of a mutation occurring somewhere along the way. There are a few tumours that are predominantly seen in younger animals, but they are relatively uncommon. In addition, there are a few breeds that are at increased risk of cancer (such as Flat Coat Retrievers) because of their genetics. Finally, some animals are immune to some types of cancer - for example, a castrated dog cannot develop testicular cancer because he doesn't have testicles to become cancerous!
What are the symptoms?
Ultimately, it depends on what the cancer is, where it is, and how big it is. For example, a brain tumour may cause seizures or wobbliness, whereas a bone tumour in the leg might cause lameness. In general though, in most cancers you would expect to find a lump or swelling somewhere in the body; weight loss; lethargy and perhaps depression; swelling of the dog's lymph nodes (as the immune system tries to fight it); and occasionally vomiting or diarrhoea.
How is it diagnosed?
As usual, it depends on the type of tumour! There are, however, a range of different techniques we can use. In general, blood tests or the symptoms will make us suspicious of a tumour somewhere, and then we go on a "tumour hunt", using our eyes and hands, plus X-rays and ultrasound scans (and sometimes CT or MRI scans) to look for it. Once we've found the lump, we'll often take part of it out and send it to a pathologist to find out exactly what it is (you can't always tell just by looking!) and how best to treat it.
How can it be treated or managed?
There are four major approaches. (1) a benign, slow growing tumour (such as a lipoma), may simply be left and monitored, especially if the dog is very old and sick and treatment might be more dangerous than it's worth. If it changes behaviour or starts causing a problem, we would, however, reassess it. (2) Surgical removal of the tumour tissue - this is usually the mainstay of treatment, and we'd aim, to remove every single tumour cell. Sadly however, this isn't always possible. (3) Chemotherapy is increasingly available, to prevent regrowth, mop up any stray cells, or slow down the growth of inoperable tumours. Unlike in humans, we use lower doses because keeping the dog comfortable and happy is more important than killing every last tumour cell. (4) Radiation therapy is available at some specialist hospitals to kill the tumour tissue.
Can it be prevented?
No, full stop. Cancers will occur - probably the vast majority of dogs will develop one in their lifetime if they live long enough (although in most cases, it'll be so small no-one will ever know).