Stomach cancer is caused by changes in the cells of the stomach, although it is not clear what causes these changes.
Cancer begins with a change in the structure of DNA. DNA provides our cells with a basic set of instructions, such as when to grow and reproduce.
A change in DNA structure is known as a mutation, and it can alter the instructions that control cell growth. This means that cells continue to grow instead of stopping when they should. Cells reproduce in an uncontrollable manner producing tissue called a tumour.
Exactly what triggers the changes in DNA that lead to stomach cancer and why only a small number of people develop stomach cancer is still uncertain.
However, there is evidence that a number of things can affect your chances of developing stomach cancer. These are discussed below.
Your risk of getting stomach cancer increases with age. Most cases occur in people aged over 55, with the average age at diagnosis being around 70.
Men are twice as likely as women to develop stomach cancer. It is unclear why.
People who smoke may be twice as likely to develop stomach cancer compared with people who don't. This is because some tobacco smoke will always be swallowed when you inhale and end up in your stomach. The many harmful substances in tobacco may then damage the cells in your stomach.
The more you smoke and the longer you have been smoking, the bigger the risk. On average, people who smoke are about twice as likely to get stomach cancer as people who don't. Around one in every five stomach cancers in the UK is thought to be related to smoking.
H. pylori infection
Helicobacter pylori (H. pylori) is a common type of bacteria. In most people these bacteria are harmless, but in some people a H. pylori infection can cause conditions such as stomach ulcers, recurring bouts of indigestion or long-term inflammation of the stomach lining (chronic atrophic gastritis).
Research has found people with severe chronic atrophic gastritis have an increased risk of developing stomach cancer, although this risk is still small.
A diet rich in pickled vegetables (such as pickled onions or piccalilli), salted fish, salt in general and smoked meats (such as pastrami or smoked beef) increases your risk of stomach cancer.
Countries where this type of diet is popular, such as Japan, tend to have much higher rates of stomach cancer than the UK.
You are more likely to develop stomach cancer if you have a close relative with the condition, such as one of your parents or your brother or sister. In such cases, it may be appropriate for your doctor to arrange genetic counselling.
It is not fully understood why this is. It may be because of shared risk factors (such as having similar diets or having a H. pylori infection), or because of certain genes you inherit from your parents.
In around 1 in 50 cases of stomach cancer, testing has found that people share a mutation in a gene known as E-cadherin.
Research into stomach cancer has also shown that you may be more at risk of getting the condition if you have the blood type A. Your blood type is passed on from your parents, so this could be another way in which family history may increase your risk of developing stomach cancer.
There is also a condition that runs in families called familial adenomatous polyposis (FAP), which may increase your risk of stomach cancer. FAP causes small growths, called polyps, to form in your digestive system, and is known to increase your risk of developing bowel cancer.
Having another type of cancer
If you have had another type of cancer, such as cancer of the oesophagus (gullet) or non-Hodgkin's lymphoma (cancer that develops in the white blood cells in your immune system), you have an increased risk of developing stomach cancer.
There are also other cancers that can make developing stomach cancer more likely, for both men and women. If you are a man, your risk of stomach cancer is increased if you have had prostate cancer, bladder cancer, bowel cancer or testicular cancer. If you are a woman, your risk of developing stomach cancer increases if you have had ovarian cancer, breast cancer or cervical cancer.
Certain medical conditions
Having certain medical conditions can also increase your risk of developing stomach cancer, such as pernicious anaemia (a vitamin B12 deficiency, which occurs when your body cannot absorb it properly) and peptic stomach ulcers (an ulcer in your stomach lining, often caused by H. pylori infection).
Surgery affecting the stomach
If you have had surgery to your stomach, or to a part of your body that affects your stomach, you may be more likely to develop stomach cancer.
This can include surgery to remove part of your stomach (known as a partial gastrectomy), surgery to remove part of your vagus nerve (the nerve that carries information from your brain to organs such as your heart, lungs, and digestive system) or surgery to repair a stomach ulcer.
How stomach cancer spreads
There are three ways stomach cancer can spread:
directly - the cancer can spread from the stomach into nearby tissues and organs, such as the pancreas, colon, small intestine and peritoneum (the lining of the inside of the abdominal cavity)
through the lymphatic system - the lymphatic system is a series of glands (nodes) located throughout your body, similar to the blood circulatory system; glands produce many specialised cells needed by your immune system to fight infection
through the blood - which can cause the cancer to spread from the stomach to other parts of the body, most commonly the liver
Stomach cancer that spreads to another part of the body is known as metastatic stomach cancer.