NEW YORK (Reuters) -- Increasing numbers of elderly women in the U.S. are being screened for breast and cervical cancer, a new report indicates. However, no rigorous studies have shown a clear benefit for screening in very old subjects, and in fact, most studies investigating cancer screening have excluded such subjects, according to a report published in Annals of Family Medicine.

In a statement, lead author Dr. Truls Ostbye, from Duke University in Durham, North Carolina, notes that although many cancers become more common with age, so does the risk of dying from other causes. As life expectancy drops with age, the number of years that can be saved with screening falls, he adds.

In the current study, Ostbye's team assessed the frequency of screening mammography--to detect breast cancer-- and Pap tests--to detect cervical cancer -- among older American women between 1996 and 2000. The subjects included 5942 women who were 50 to 61 years of age and 4543 women who were at least 70 years of age.

During the study period, screening rates for both mammography and Pap tests increased for all age groups, the researchers note. Women with higher levels of education, married women, those with higher incomes, non-smokers and those who had vigorous exercise habits were more likely to undergo screening.

For mammography, screening rates held steady at 70 percent to 80 percent until about 70 years of age when they began to drop to a low of 40 percent among women 85 to 90 years of age. For Pap tests, rates fell from about 75 percent at age 50 to 54 years to 25 percent at age 85 to 90 years.

In one estimate, "240 very healthy 80-year-old women would need to be screened with mammography to prevent one death from breast cancer," Ostbye noted.

"The argument is even stronger regarding Pap smears," he added. While deaths from cervical cancer are low among elderly women and there is no proven benefit for screening this population, doctors continue to order Pap smears for these women, he said.

"By examining screening behaviors, we can try and adjust recommendations in low-risk populations and increase incentives in high-risk populations to possibly use our healthcare dollars more wisely and deliver better healthcare to all populations," Ostbye said.