© 2004 by Oxford University Press
© 2004 Oxford University Press
CORRESPONDENCE |
RESPONSE: Re: A Prospective Study of Aspirin Use and the Risk of Pancreatic Cancer in Women
Affiliations of authors: Channing Laboratory, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA
Correspondence to: Eva Schernhammer, MD, DrPH, Channing Laboratory, 181 Longwood Ave., Boston, MA 02115 (e-mail: eva.schernhammer{at}channing.harvard.edu)
We thank Dr. Gensini et al. for their interesting contribution. They suggest that we, as well as other authors of observational studies, should always provide number needed to treat (NNT) and number needed to harm (NNH) to assess risk–benefit profiles for public health recommendations. Our analyses, based on data from the Nurses’ Health Study cohort, revealed an elevated pancreatic cancer risk among women who reported extended periods of regular aspirin use (1).
Aspirin is widely used, both in therapeutic and preventive health care settings. In addition to its well-established beneficial effects on cardiovascular risk, there is good evidence for a protective effect of aspirin on colon carcinogenesis (2). There are also risks associated with the use of aspirin, such as higher risks for gastrointestinal bleeding and cerebral hemorrhage. Thus, a full risk–benefit analysis of regular aspirin use actually goes well beyond the concept of NNH and NNT for pancreatic cancer only.
Ultimately, the purpose of our analysis was to examine the relation between aspirin use and the risk of pancreatic cancer. We believe that such an analysis not only offers important insights into cancer prevention but also provides vital clues to pancreatic cancer biology. Although our results in a large prospective cohort study have merit, other large studies are needed to confirm our findings before we can accurately conduct a risk–benefit analysis of aspirin use that incorporates pancreatic cancer risk.
With respect to the particular analysis proposed in the letter by Gensini et al., there are a number of important factual problems. Specifically, the authors provide an inaccurate estimate of pancreatic cancer for women in the United States. Surveillance, Epidemiology, and End Results (SEER)1 Program data indicate an annual incidence rate of 9.8 per 100 000 women (3), not four per 100 000. Furthermore, the calculations provided by the authors would vary greatly depending on age. Consistent with data on pancreatic cancer in the SEER database, the incidence rate of pancreatic cancer in our cohort is very low in younger women but much higher among older women. Thus, the NNH varies greatly with the age distribution of the population being studied. Gensini et al. also incorrectly identify the comparison group for our analysis. Rather than the general population, the referent group in our cohort was women of approximately the same age as those in the study group who avoided regular aspirin use.
Notwithstanding, if we accept the calculations of Gensini et al. and the notion that the association between aspirin use and pancreatic cancer is truly causal, we submit that an NNH of 900 is not trivial. The Environmental Protection Agency regulates exposures at levels with far smaller adverse effects, often as low as one in 20 000. Any excess death due to pancreatic cancer would be a cause for concern and should be weighed in making decisions.
NOTES
1 Editor’s note: SEER is a set of geographically defined, population-based, central cancer registries in the United States, operated by local nonprofit organizations under contract to the National Cancer Institute (NCI). Registry data are submitted electronically without personal identifiers to the NCI on a biannual basis, and the NCI makes the data available to the public for scientific research. 
REFERENCES
1 Schernhammer ES, Kang JH, Chan AT, Michaud DS, Skinner HG, Giovannucci E, et al. A prospective study of aspirin use and the risk of pancreatic cancer in women. J Natl Cancer Inst 2004;96:22–8.
2 Thun MJ, Henley SJ, Patrono C. Nonsteroidal anti-inflammatory drugs as anticancer agents: mechanistic, pharmacologic, and clinical issues. J Natl Cancer Inst 2002;94:252–66.
3 National Cancer Institute. SEER Cancer Statistics Review 1975–2000. Available at: http://seer.cancer.gov/csr/1975_2000/results_merged/topic_race_ethnicity.pdf. [Last accessed: March 13, 2004.]
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