There are a number of reasons why an individual may develop more than one primary cancer

Professor Henrik Moller

A research team searched the Thames Cancer Registry database covering a population of 14 million people in the South East and containing over 1.5 million cases of cancer.

Around 5% of the patients on the registry had been diagnosed with more than one cancer.

The researchers found over 127,000 patients who had been diagnosed with bowel cancer between 1961 and 1995.

They then compared the numbers of new cancers developing after the original diagnosis with expected rates in the general population.

The results showed that the risk of developing cancer of the small intestine was higher in men diagnosed with bowel cancer before the age of 60, and more than twice as high in women after the age of 65.

Conversely, bowel cancer was more likely to develop in those whose first cancer had arisen in the small intestine.

Eye cancer

Cancer of the eye was also significantly more common in men diagnosed with bowel cancer before the age of 60.

Women whose first bowel cancer was diagnosed when they were under 65 were also significantly more likely to develop a new bowel cancer, and they were also at significantly increased risk of developing cervical, womb, and ovarian cancers.

But the increased risk of ovarian cancer was confined to the first five years after a diagnosis of bowel cancer, raising the possibility of spread from the original tumour.

Helen Evans, of the Thames Cancer Registry, told BBC News Online: "There are a number of reasons why an individual may develop more than one primary cancer and these are not unique to bowel cancer.

"There may be a genetic predisposition; a common environmental exposure such as smoking; an immunological deficiency or therapy for one cancer may induce a second cancer."

Genetic syndrome

Dr Shirley Hodgson, of Guy's Hospital, London, said it was likely that a small number of people who developed bowel cancer while young, or who had a family history of the disease, might have a genetic susceptibility to developing other forms of cancer.

These people, who have a form of the disease known as hereditary nonpolyposis colon cancer (HNPCC), are thought to carry a gene mutation that puts them at slightly increased risk of tumours in other organs, including the uterus, ovary, stomach, and small bowel.

Dr Hodgson said: "These people need to take genetic advice, and to be slightly more careful about monitoring."

Only about 5% of people with bowel cancer have the HNPCC version. The rest develop one-off tumours not likely to be caused by any genetic susceptibility.

Dr Rob Glynne-Jones, medical director for Colon Cancer Concern, told BBC News Online: "These findings further reinforce our advice that anyone concerned about their bowel health should take into consideration their family history and, if worried, act quickly in alerting their GP to ensure that early diagnosis is possible."

The research is published in the journal Gut.