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Wednesday, 2 January, 2002, 00:20 GMT
Asthmatics 'at higher risk of lung cancer'
Women are at greater risk than men
Women are at greater risk than men
A major study has shown people with asthma are more likely to develop lung cancer.

The Swedish study looked at more than 100,000 patients over 30 years.

Unlike previous studies, which have looked back at cancer patients' health histories, this research followed people after they had been diagnosed in hospital with asthma.

The researchers found asthmatics were affected by lung cancer more frequently than the general population - the cancers registered over the course of the study exceeded the anticipated level by 58%.

There could be one mechanism causing both asthma and cancer

Dr Paulo Boffetta, Study author
Women have an even higher risk. Their excess risk is 78%, against 51% for men.

Scientists say smoking could be a factor in the link between asthma and lung cancer.

In-depth study

Previous retrospective research findings have been mixed, with some finding a link between asthma and future lung cancer - in some studies only for men - and others finding no such connection.

In this study, an international team of scientists decided looking back was unsatisfactory because the cancer was already diagnosed, and because patient memory was too uncertain to be scientifically reliable.

They used the Swedish national health registers, which allowed them to monitor patients' medical history and identify those hospitalised and diagnosed with asthma over a 30-year period.

Only those who were alive one year after discharge from hospital, and who showed no signs of cancer at that stage, were included, meaning just 92,986 of the 118,000 patients originally identified were studied.

On average, patients were monitored for eight and a half years - a total of almost 700,000 monitored person-years.

Over the course of the study, which involved the World Health Organization's International Agency for Research on Cancer, the Karolinska Institute of Stockholm and the University of Uppsala 713 people were registered as having cancer - 58% more than the anticipated level.

Search for cause

What the researchers now how to look at is why the link exists.

Dr Paolo Boffetta, of the Unit of Environmental Cancer Epidemiology of the International Agency for Research on Cancer in Lyon, France, who worked on the study, said: "We do not really know if asthma as such causes the increased risk".

"There could be one mechanism causing both asthma and cancer, for example chronic inflammation leading to an excess of free radicals."

Free radicals are highly reactive molecules which have been linked to both heart disease and cancer. Antioxidants such as vitamins A, C and E offer protection against them.

Bronchial inflammation generates free radicals, and there is evidence that the antioxidant levels in the respiratory tract lining of asthmatics are reduced.

As free radicals have the potential to cause genetic damage, they may be linked to the development of tumours, the scientists suggest.

Dr Boffetta added: "There could quite possibly be a susceptibility factor common to asthma and cancer, or that an external factor is involved and plays a role both in the activation or progression of asthma and the contracting of lung cancer.

"Such a role could be played, for example, by environmental factors, above all tobacco smoking.

He added: "The prospective monitoring of young patients with known smoking habits over a longer period could provide useful information on the timing of a possible carcinogenic effect of asthma."

The researchers found the types of cancers for which asthmatics had an increased risk were those commonly found in smokers (such as squamous-cell carcinomas).

These cancers were no less frequent in women than man.

"It would be interesting to look at lifestyle factors. But smoking jumps off the page.", she said.

The results are published in the European Respiratory Journal.

See also:

25 Sep 01 | Health
Passive smoking 'causes asthma'
18 Oct 01 | Health
Asthma and obesity 'link'
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