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A low-dose aspirin taken every other day was associated with a reduced risk of colon cancer in women, researchers reported.

But the benefit, seen in extended follow-up of a large randomized trial, took years to appear and may be counterbalanced by an increase in gastrointestinal bleeding, according to Nancy Cook, ScD, of Brigham and Women's Hospital in Boston, and colleagues.

In addition, there was no reduction in the overall risk of cancer, or of lung and breast cancer, Cook and colleagues reported in the July 16 issue of Annals of Internal Medicine.

Currently, the U.S. Preventive Services Task Force recommends aspirin in women 55 through 79 only if potential benefits are greater than harms.

"With more evidence for long-term effects in carcinogenesis, recommendations might be reconsidered," Cook and colleagues argued.

Indeed, their report is “an additional study in the long line of investigations that have shown that aspirin can reduce the risk of colorectal cancer,” commented Randall Burt, MD, of the Huntsman Cancer Institute in Salt Lake City.

But, he told 口袋女优, “The effect is not strong enough for us to prescribe aspirin in women for colon cancer prevention.”

The researchers were reporting on a total of 18 years of follow-up in the Women's Health Study, a decade-long randomized trial investigating the effects of aspirin and vitamin E on cardiovascular disease and cancer risk.

During the trial, 39,876 women 45 or older with no cardiovascular disease or history of cancer were randomly assigned to take 100 mg of aspirin every other day for 10 years.

At the end of the study, 33,682 women agreed to take part in extended follow-up; data for this analysis are through March 14, 2012 and the median total follow-up was 17.5 years.

All told, Cook and colleagues found 5,071 confirmed cancer cases -- 2,070 breast, 451 colorectal, and 431 lung -- as well as 1,391 cancer deaths.

Over the full follow-up period, aspirin had no association with overall risk of any cancer, lung cancer, or breast cancer.

But there was a 20% reduction in the risk of colorectal cancer (HR 0.80, 95% CI 0.67 -0.97, P=0.021).

The reduction was mainly for proximal colon cancer, where the hazard ratio was 0.73 (95% CI 0.55-0.95, P=0.022).

The difference was not seen until after a decade -- the post-trial reduction was 42% (P<0.001) -- but there was no extended effect on cancer deaths or colorectal polyps.

The researchers observed a greater risk of gastrointestinal bleeding in the women who had been randomized to aspirin (HR 1.14 95% CI 1.06-1.22, P<0.001).

Peptic ulcers were also more common in the aspirin group HR 1.17 95% CI 1.09-1.27, P<0.001).

Cook and colleagues cautioned that not all participants in the randomized trial took part in the extended follow-up, so there is a possibility of ascertainment bias. Also, gastrointestinal bleeding, peptic ulcers, and polyps were self-reported during the extended follow-up.

The findings "reinforce the need to consider the risks and benefits of aspirin separately in men and women," commented Peter Rothwell, MD, PhD, of the University of Oxford.

In an accompanying editorial, Rothwell argued that a sustained risk for bleeding, coupled with no reduction in all-cause mortality or risk of cancer, "should temper any recommendations for widespread use of aspirin in healthy middle-aged women."

Among other things, he noted that the benefits, even of daily aspirin, are likely to be small among women, since the colon, esophagus, and stomach, taken together, accounted for only about 8% of all cancer cases in the study.

In contrast, he noted, they accounted for about 23% of all cancer deaths in aspirin trials among men.