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 the cells continue to grow instead of stopping when they should. This causes the cells to reproduce in an uncontrollable manner, producing a lump of 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 there are a number of significant risk factors for stomach cancer.
These are discussed below.
Your risk of getting stomach cancer increases with age. Most cases occur in people over the age of 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 this is the case.
Smokers may be twice as likely to develop stomach cancer compared with non-smokers. This is because some tobacco smoke will always be swallowed when you inhale and end up in your stomach and tobacco contains many harmful substances that can damage cells.
The more you smoke and the longer you have been smoking the bigger on risk. On averages smokers are 1 ½ times more likely to get stomach cancer than non-smokers.
H. pylori infection
H. pylori is a common type of bacteria that is thought to be present in up to half of the world’s population.
In most people they are harmless but in some people an H pylori infection can cause conditions such stomach ulcers, recurring bouts of indigestion or chronic inflammation of the stomach lining (atrophic gastritis).
Research has found that people with severe atrophic gastritis have the highest risk of developing stomach cancer, although this risk is still very 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 have much higher rates of stomach cancer than would normally be expected.
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. One study estimated that if one of these family members develops stomach cancer this increases your risk by around 1 ½ .
It is not fully understood why this is, but it is possible that it is due to family members having certain risk factors in common.
In around 1 in 50 cases of stomach cancer, testing has found that people share a genetic mutation (where the instructions contained in all living cells become scrambled in some way) in a gene known as the E-cadherin gene.
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
There is also a condition that runs in families called familial adenomatous polyposis (FAP), which may make developing stomach cancer more likely. FAP causes small growths, called polyps, to form in your digestive system, and it 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 cancer of the prostate, bladder, bowel or testicle. If you are a woman, your risk of developing stomach cancer increases if you have had cancer of the ovary, breast or cervix.
Certain medical conditions
Having certain medical conditions can also increase your risk of developing stomach cancer.
These are outlined below.
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 does stomach cancer spread
There are three ways that stomach cancer can spread:
- directly – the cancer can spread out of the stomach and into nearby tissues and organs, such as the pancreas, colon and small intestine
- via the lymphatic system - lymphatic system is a series of glands (or nodes) that are located throughout your body, similar to the blood circulatory system; glands produce many specialised cells that are needed by your immune system to fight infection
- via the blood – which in many cases will see the cancer spread from the stomach into the liver
Stomach cancer that spreads to another part of the body is known as metastatic stomach cancer.