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Insertion of T-shaped IUD may destroy precancerous lesions or trigger immune response, researchers sayGetty Images

Using an IUD for birth control cuts a woman's risk of developing cervical cancer by half, a new study shows.

That is surprising because it is well-established that cervical cancer is caused by infection with human papillomavirus.

But the researchers, led by Xavier Castellsagué of the cancer epidemiological research program at Llobregat Hospital in Spain, believe the most likely explanation is that the actual insertion of the device provides protection.

Inserting an intrauterine device – a T-shaped wire – in the uterus may destroy precancerous lesions, or it may cause an immune response that prevents HPV infection from progressing, Dr. Castellsagué said.

The new study, published online Monday in the medical journal The Lancet Oncology, was based on data from more than 20,000 women in eight countries.

The researchers found that women who had used IUDs had a 44 per cent lower risk of developing squamous-cell carcinoma and a 54 per cent lower risk of suffering from adenocarcinoma or adenosquamous carcinoma.

The duration of IUD use did not seem to alter the risk of developing cervical cancer –the risk was reduced by nearly half in the first year of use and remained fairly steady over the next 10 years.

That's the main reason researchers believe it is the insertion, and more specifically the trauma that occurs to the reproductive tract, that confers protection.

"IUD use does not modify the likelihood of prevalent HPV infection, but might affect the likelihood of HPV progression to cervical cancer," Dr. Castellsagué said.

In a commentary, also published in The Lancet Oncology, Karl Ulrich Petry of the Wolfsburg Cancer Clinic in Wolfsburg, Germany, said the surprising findings should spark new lines of research.

If cervical trauma provides protection against cancer, all kinds of medical procedures may be having an effect, including IUD insertion, biopsies and even Pap tests, he said.

Dr. Petry said that would help explain why the rates of cervical cancer are dramatically higher in parts of the developing world where there is no screening compared with rates in the developed world, where screening is commonplace. In other words, it is not finding pre-cancerous lesions that cuts the risk of cancer, but the procedures that look for them.

There are two principal types of IUDs: Hormonal IUDs release small amounts of the hormone progestin, preventing pregnancy by preventing fertilization. Copper IUDs release small amounts of copper, which seems to alter the chemistry of the uterus and prevent fertilized eggs from latching on.

Worldwide, IUDs are one of the most popular methods of female contraception, but not in North America, where birth-control pills are the method of choice.

Women who use hormonal IUDs have sharply lower rates of endometrial cancer, as do women who use birth-control pills. In fact, IUDs are now used as a treatment for women of reproductive age with endometrial cancer because it allows them to preserve fertility.

About 1,300 Canadian women will be diagnosed with cervical cancer this year and 350 will die of it, according to the Canadian Cancer Society.

About 4,500 new cases of uterine cancer will be diagnosed in Canadian women this year, with an estimated 790 deaths attributed to the disease. (About 95 per cent of those cases originate in the endometrium, the lining of the uterus.)