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Aspirin and pancreatic cancer

Study
Results
Comment

Friday is "health-scare-in-the-media" day. Just recently Bandolier was regaled over its low-salt muesli and fruit by a story that use of aspirin caused pancreatic cancer in women. It seemed serious. One experienced presenter was quizzing the (good) media doc about whether women should just stop taking aspirin altogether.

Serious stuff this. So a quick look at the evidence, and a read of the paper [1] suggested that things might not be quite so bad. Actually all the increased risk was in women taking more than 14 aspirin tablets (325 mg equivalents) a week for more than 20 years. And even then the increased risk only just reached statistical significance.

Study

This was part of the Nurses' Health Study that enrolled 122,000 female nurses in 1976 and has been following them up ever since. Information on aspirin use was first collected at baseline and then regularly thereafter, as well as information about other risk factors. Pancreatic cancer and deaths were included in biennial questionnaires, and confirmed by review of the medical records.

Excluded were nurses who either failed to respond to the baseline questionnaire, did not provide information on aspirin use, or with a baseline history of cancer. There are many details in what was an unquestionably fine study.

Results

Information was available for 88,378 women, who reported 161 cases of pancreatic cancer over 18 years. In women who were not regular users of aspirin there were 96 cases, with a crude rate of 10 cases of pancreatic cancer per 100,000 women years. In women who did use aspirin regularly (more than two 325 mg aspirin tablets a week) there were 65 cases, 13 per 100,000 women years, a non significant increase.

Regular use of aspirin for longer periods was associated with higher rates of pancreatic cancer, but this only became significant in women who used aspirin regularly for more than 20 years (Figure 1). There were 34 cases in these women, and relative risk adjusted for a number of variables just reached statistical significance, with a relative risk of 1.6, and a 95% confidence interval of 1.03 to 2.4.



Figure 1: Pancreatic cancer according to duration of aspirin use (≥2 tablets a week): darkest bar is statistically significant















Regular use of more aspirin was also associated with higher rates of pancreatic cancer, but this only became significant in women who regularly used more than 14 aspirin tablets a week for at least 6-8 years (Figure 2). There were 20 cases in these women, and relative risk adjusted for a number of variables just reached statistical significance, with a relative risk of 2.0, and a 95% confidence interval of 1.03 to 3.4.



Figure 2: Pancreatic cancer according to amount of regular aspirin use: darkest bar is statistically significant

















Comment

This present research was done to investigate a theory that aspirin might prevent pancreatic cancer, as suggested by laboratory experiments and previous small epidemiological study. It found no such thing. The results actually indicate that unless women take relatively large amounts of aspirin (more than 14 tablets a week) for a long time (up to 18 years), there is no significantly increased risk. Even then, what increased risk there is barely achieves statistical significance, with only a few tens of women in this category actually developing pancreatic cancer.

So put the muesli aside and think for a minute. Could this just be a chance finding? Over the 18 years of the study, could a few cases be lost somewhere in the system that might overturn the result? Heavy aspirin users might be taking the aspirin for some reason, and that reason might be linked to pancreating cancer. What might be called confounding by indication.

There is certainly no evidence here to support a protective effect of aspirin. But the evidence that aspirin can cause pancreatic cancer is also weak. It is possible that if 1,000 women took more than 14 aspirin a week for 20 years, pancreatic cancer would develop in two of them, rather than in 1 in 1,000 women who took no aspirin. But unlikely.

Reference:

  1. ES Schernhammer et al. A prospective study of aspirin use and the risk of pancreatic cancer in women. Journal of the National Cancer Institute 2004 96: 22-28.

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