Affiliations of authors: Dipartimento di Area Critica Medico Chirurgica, Università degli Studi di Firenze, Firenze, Italy (GFG, AAC, RA); Fondazione Don Carlo Gnocchi, IRCCS, Firenze (GFG, AAC)
Correspondence to: Andrea A. Conti, MD, MPH, Dipartimento di Area Critica Medico Chirurgica, Università degli Studi di Firenze, Viale Morgagni 85, Firenze, Italy I-50134 (e-mail: aa.conti{at}dac.unifi.it)
In a recent paper published in the Journal, Schernhammer et al. (1) wrote that, after evaluation of the data from the Nurses Health Study, "extended periods of regular aspirin use appear to be associated with a statistically significantly increased risk of pancreatic cancer among women." They reached this conclusion after having documented 161 cases of pancreatic cancer during 18 years of follow-up among 88 378 women. These 161 cases translate to about 182 cases in 100 000 women.
United States epidemiologic data indicate that the annual incidence rate of pancreatic cancer in the adult female population, though it changes according to different age decades, could be estimated in a conservative way as approximately 4 cases per 100 000 women (2). The cumulative incidence rate over a period of 18 years would therefore be 72 cases. If we calculate the number needed to harm (NNH, an indicator of the harm of health care interventions expressing the number of patients that need to be treated to have one that develops an additional negative outcome, i.e., harm) for the association of aspirin use and risk of pancreatic cancer, a hypothetical NNH of approximately 910 emerges from the data of Schernhammer et al. Such an NNH is hypothetical because the incidence of pancreatic cancer in the Nurses Health Study is compared with the incidence of this neoplasm in the general population, not with that in a control group that did not take aspirin. An NNH greater than 250300 is usually considered not clinically significant; in general, the higher the NNH, the safer the intervention under consideration. On the basis of our calculation, aspirin use appears to be relatively safe, regarding its association with the risk of pancreatic cancer, contrary to the assertions of Schernhammer et al. (1).
Overall, no statistically significant difference in risk of pancreatic cancer between aspirin users and nonusers was shown in the article by Schernhammer et al. (1), and the relative risks reported are only modestly increased (3). Among women reporting aspirin use on at least two of three consecutive biennial questionnaires, the risk increased with increased use and achieved a statistically significant value only when at least 14 tablets per week (and no fewer) were used. These data are of course confined to women.
In conclusion, an NNH of 9001000, spread over a time period of almost two decades, is high, even for cancer, and is susceptible to further increases resulting from higher estimations of the annual incidence rate of pancreatic cancer in the adult female population. In contrast, there remains a consolidated number needed to treat (NNT, an indicator of the benefits of health care interventions expressing the number of patients who need to be treated with the experimental intervention to create one additional improved outcome in comparison with the control intervention) ranging from 20 to 90 for the use of aspirin in the secondary prevention of cardiovascular diseases (4,5).
In interpreting and balancing such numbers (a neoplastic NNH of 9001000 versus a cardiovascular NNT of 2090), we therefore ask authors so that others can always provide the complete data to assess established benefits and putative risks.
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;6:228.
2 National Cancer Institute. Surveillance, Epidemiology, and End Results (SEER) Program. Available at: http://seer.cancer.gov/publications/ethnicity/pancreas.pdf. [Last accessed: March 13, 2004.]
3 Baron JA. What now for aspirin and cancer prevention? J Natl Cancer Inst 2004;96:45.
4 Gensini GF, Conti AA. Evidence-based evaluation of benefits in therapeutic interventions: methodologically controlled and non-randomly assigned reflections on the number needed to treat. Ital Heart J 2003;4:803.[Medline]
5 Weisman SM, Graham DY. Evaluation of the benefits and risks of low-dose aspirin in the secondary prevention of cardiovascular and cerebrovascular events. Arch Intern Med 2002;162:2197202.
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