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A woman's age and the type of treatment she gets may play a big role in the risk that abnormal cells on the cervix will return or develop into cervical cancer, U.S. researchers said on Tuesday.

The said older women and women treated with a freezing procedure known as cryotherapy have the highest risks of having the abnormal cells come back or progress to cervical cancer.

How severe the abnormal changes in cells were in the first place also plays a role.

"We now have a much more clear idea of the risks of recurrent abnormal cells and invasive cervical cancer over time after treatment of these cells," said Joy Melnikow of the University of California Davis, whose study appears in the Journal of the National Cancer Institute.

The condition, known as cervical intraepithelial neoplasia (CIN), can progress to cancer. The main cause is the human papillomavirus or HPV, a common sexually transmitted disease.

Pap smears, which women in most developed countries undergo regularly, sample the cervix for these abnormal cells.

CIN is grouped into three stages, ranging from mild or grade 1, with just a few abnormal cells, to severe or grade 3, in which there are precancerous cells on the top layer of the cervix.

Melnikow and colleagues studied more than 37,000 women from the British Columbia Cancer Agency in Canada who were treated for CIN from 1986 through 2000. They compared them to a group of more than 71,000 women who had no history of abnormal cervical cells.

Both groups were followed through 2004.

BIOPSY TREATMENT LOWER RISK

The researchers found the risk of cervical cancer and recurrence of grade 2 or grade 3 abnormal cells was highest for women who were over 40, who were previously treated for grade 3 or for those who whose cells were frozen using cryotherapy.

Women treated with cone biopsy, a way of surgically removing the cells, were the least likely to have CIN again later.

In general, they found most recurrences of CIN happened in the first six years after treatment, Melnikow said in an e-mail.

She said women who have had treatment for the condition in the past have "a low, but higher-than-average risk of invasive cancer, so they need regular screening over an extended period of time."

Melnikow said women treated surgically have higher risks of bleeding and preterm labor if they become pregnant after treatment. "Younger women planning later pregnancies may prefer cryotherapy; their risk of recurrence is lower and a recurrence can be treated again," she said.

Melnikow said it was not possible to tell from the study whether women who have a recurrence had HPV infections that caused their original abnormal cells to come back.

Cervical cancer is the second most common cancer in women, with 500 000 new cases each year worldwide. More than half of women with cervical cancer will die, according to the World Health Organization.