Affiliation of authors: Department of Epidemiology and Surveillance Research, American Cancer Society, Atlanta, GA.
Correspondence to:Eric J. Jacobs, PhD, Department of Epidemiology and Surveillance Research, American Cancer Society, National Home Office, 1599 Clifton Rd. NE, Atlanta, GA 30329 (e-mail: eric.jacobs{at}cancer.org)
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ABSTRACT |
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INTRODUCTION |
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A protective effect of aspirin use on pancreatic cancer is biologically plausible. Several different nonsteroidal anti-inflammatory drugs (NSAIDs) have been shown to inhibit pancreatic cancer in a hamster model (5,6), and both sodium salicylate (7) and other NSAIDs (8) have been shown to inhibit the growth of human pancreatic cancer cell lines. To our knowledge, five epidemiologic studies (913) have specifically examined the association between aspirin use and pancreatic cancer risk. In the Iowa Women's Health Study cohort (including 80 cases of pancreatic cancer), frequent aspirin use was associated with substantially reduced risk (rate ratio [RR] = 0.40, 95% confidence interval [CI] = 0.20 to 0.82, for use 6 times per week compared with no use) (9). However, use of nonaspirin NSAIDs was not associated with pancreatic cancer risk (RR = 1.28, 95% CI = 0.68 to 2.43, for use
6 times per week compared with no use). In contrast, a recent analysis from the Nurse's Health Study cohort (which included 161 cases) found that regular aspirin use (
2 tablets per week) for 20 or more years was associated with increased risk of pancreatic cancer (RR = 1.58, 95% CI = 1.03 to 2.43) (10). No association between aspirin use and pancreatic cancer risk was found in a U.S. hospital-based case-control study (11) that included 194 cases or in a small U.S. cohort (12) that included 30 cases. A Danish study (13) that used pharmacy database information found no association between low-dose aspirin prescriptions and pancreatic cancer risk but could not examine standard aspirin dosages.
Three additional studies (1416) have examined the association between overall NSAID use and pancreatic cancer but did not examine aspirin use specifically. In a U.S. hospital-based case-control study (14), there was a suggestion of decreased risk with regular continuing NSAID use for 5 or more years (odds ratio [OR] = 0.6, 95% CI = 0.4 to 1.1). In a Danish study (16) that used pharmacy database information, receipt of 10 or more nonaspirin NSAID prescriptions was not associated with pancreatic cancer risk (RR = 0.9, 95% CI = 0.6 to 1.3). In a case-control study (15) in the United Kingdom that used pharmacy database information, receipt of seven or more NSAID prescriptions during the 13- to 36-month interval before the date of diagnosis was associated with an increased risk of pancreatic cancer (RR = 1.49, 95% CI = 1.02 to 2.18). However, NSAID use before this interval was not examined, and the increased risk associated with recent NSAID use may have been a result of NSAID use to relieve pain from undiagnosed cancer.
We examined the association between aspirin use and pancreatic cancer mortality in the Cancer Prevention Study II (CPS-II), a large cohort of U.S. adults. In this cohort, aspirin use has been previously demonstrated to be associated with reduced risk of colon, stomach, and esophageal cancer mortality (17,18).
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SUBJECTS ANDMETHODS |
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Subjects in this analysis were drawn from the 1 184 588 participants (508 318 men and 676 270 women) in the CPS-II. Participants were enrolled in 1982 by American Cancer Society (ACS) volunteers in all 50 U.S. states, the District of Columbia, and Puerto Rico, as previously described (19). Participants completed a four-page baseline self-administered questionnaire in 1982 that included information on demographic characteristics and various behavioral, environmental, occupational, and dietary factors.
The vital status of study participants was determined through December 31, 2000, using two approaches. ACS volunteers made personal inquiries in September 1984, September 1986, and September 1988 to determine whether the participants they had enrolled were alive or dead and to record the date and place of all deaths. Reported deaths were then verified by obtaining death certificates (20). At completion of the 1988 follow-up obtained from ACS volunteer reports, vital status was known for 98.2% of the cohort (20). Automated linkage using the National Death Index then extended follow-up of the entire cohort from September 1988 through December 31, 2000, and also identified deaths among the 21 704 participants lost to follow-up between 1982 and 1988 (20). At the completion of follow-up in December 2000, 332 673 participants (28.1%) had died, 849 019 (71.7%) were alive, and 2896 (0.2%) had follow-up truncated on September 1, 1988, because of insufficient data to perform linkage with the National Death Index. Death certificates or codes for cause of death have been obtained for 98.7% of all known deaths. The underlying cause of death was coded according to the International Classification of Diseases, 9th and 10th revisions (ICD-9 and ICD-10) (21). Pancreatic cancer deaths were defined as ICD-9 codes 157.0157.9 and ICD-10 codes C25.0C25.9. All aspects of the CPS-II study have been approved by the Emory University School of Medicine Human Investigations Committee. Informed consent to participate was implied by the return of a completed self-administered questionnaire in 1982.
All analyses excluded participants who at enrollment reported a history of cancer other than nonmelanoma skin cancer (n = 82 341) or who had incomplete or uninterpretable data on aspirin use (n = 64 930) or cigarette smoking status (n = 49 727). A total of 987 590 participants (438 144 men and 549 446 women) remained for analysis, of whom 4577 died from pancreatic cancer.
We also performed a subgroup analysis in the CPS-II Nutrition Cohort, a subset of the larger CPS-II cohort. The CPS-II Nutrition Cohort (hereafter referred to simply as the Nutrition Cohort) consists of CPS-II participants living in 21 states who in 19921993 returned a mailed self-administered enrollment questionnaire providing updated or new information about nutrition, aspirin use, cigarette smoking status, and other health-related factors (22). In the Nutrition Cohort subgroup analysis, we were able to examine the association between consistent aspirin use at two time points (1982 and 19921993) and pancreatic cancer mortality. Of the 987 590 CPS-II participants included in the main analyses, 162 519 were participants in the Nutrition Cohort. Of these, 18 871 were excluded because they provided incomplete aspirin use information or reported a history of cancer on the 19921993 questionnaire. The Nutrition Cohort subgroup analysis included the remaining 143 648 participants (68 466 men and 75 182 women), of whom 311 died from pancreatic cancer.
Ascertainment of Aspirin Use
Aspirin use at enrollment in the CPS-II cohort was ascertained from the 1982 questionnaire, which included a section about use of medications and vitamin supplements. Participants were asked to fill in two boxes, the first box reporting the number of times in the last month they used aspirin and the second box reporting the number of years of use. Participants were instructed to write "" in the times per month box if they used aspirin only occasionally. No information was collected on aspirin dose or on aspirin use that had stopped before study enrollment. No information was collected on use of NSAIDs other than aspirin. For the Nutrition Cohort subgroup analysis, aspirin use was also ascertained from the 19921993 Nutrition Cohort enrollment questionnaire, which asked participants if they took aspirin regularly and, if so, for how many days per month. The Nutrition Cohort enrollment questionnaire also included similar questions on the use of ibuprofen and "other" NSAIDs.
Statistical Analysis
We used Cox proportional hazards modeling (23) to calculate rate ratios for pancreatic cancer mortality associated with aspirin use, while adjusting for other potential risk factors. The time axis used was follow-up time since enrollment in 1982. We found no evidence that the proportional hazards assumption was violated when we modeled interaction terms between measures of aspirin use and follow-up time. We categorized aspirin use as unquantified "occasional," 114 times per month, 1529 times per month, or 30 or more times per month. We selected these categories because use 30 or more times per month could include daily use of aspirin, while use 15 or more times per month could include use every other day. All Cox models were adjusted for age, sex, and several additional factors associated with risk of pancreatic cancer in the CPS-II cohort and other study populations (race [white, black, other/unknown], cigarette smoking status [never, former, current], body mass index [weight in kg/height in m2], and diabetes [yes, no]). All covariates except age were modeled as dummy variables using the categories shown in Table 1. We adjusted for age by stratifying on exact year of age at enrollment within each Cox model (24). For the Nutrition Cohort subgroup analysis, updated information reported in 19921993 was used to adjust for body mass index, cigarette smoking status, and diabetes. Body mass index and cigarette smoking status were defined using information reported in 1982 when information on these variables was incomplete or uninterpretable on the 19921993 questionnaire.
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We examined whether the association between aspirin use and pancreatic cancer mortality varied by cigarette smoking status, body mass index, and diabetes by modeling multiplicative interaction terms between frequency of aspirin use (continuous) and variables for cigarette smoking status (never, former, current), diabetes (yes, no), and body mass index (<25 kg/m2, 25 kg/m2). Two-sided P values for interaction were calculated using the likelihood ratio statistic (25).
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RESULTS |
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Aspirin use, even frequent use, was not associated with pancreatic cancer mortality (for use 30 times per month compared with no use, RR = 0.97, 95% CI = 0.86 to 1.09) (Table 2). Results were similar for men and for women, and all further analyses are presented for men and women combined. We found no apparent differences in the association between frequency of aspirin use and pancreatic cancer mortality by body mass index, diabetes, or cigarette smoking status (results not shown). There was no association between aspirin use and pancreatic cancer mortality, even for participants who reported both frequent aspirin use (
30 times per month) and use for 20 or more years (RR = 0.96, 95% CI = 0.69 to 1.33) (Table 3).
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Table 5 shows the association between pancreatic cancer mortality and aspirin use among participants in the CPS-II Nutrition Cohort. We observed no association between aspirin use and pancreatic cancer mortality, even among participants who appeared to be consistent long-term aspirin users, that is, those who reported aspirin use 15 or more times per month in both 1982 and in 19921993 (RR = 1.27, 95% CI = 0.78 to 2.08). There was also no association between frequency of nonaspirin NSAID use and pancreatic cancer mortality in the Nutrition Cohort (RR = 1.08, 95% CI = 0.75 to 1.55 for use of nonaspirin NSAIDs 15 times per month compared with no use) (data not shown in table).
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DISCUSSION |
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A limitation of this study is that we could examine only frequency of aspirin use, rather than daily dose, because we did not have information on the dose or number of aspirin tablets taken each time aspirin was used. In addition, the amount of measurement error for self-reported aspirin use is not known. Measurement error would be expected to bias the results toward the null, potentially obscuring a true increase or decrease in risk of mortality from pancreatic cancer associated with aspirin use. However, frequency of aspirin use has been shown to be associated with substantially decreased risk of mortality from colon cancer, esophageal cancer, and stomach cancer in this cohort (17,18), consistent with results from several other studies (4). These results suggest that any similarly important effect of aspirin use on pancreatic cancer mortality could have been detected in this cohort.
An additional limitation of our study is that information on aspirin use was not updated during the 18-year follow-up period in the main CPS-II cohort, and a substantial proportion of the cohort may have either started or stopped regular aspirin use at some point during follow-up. However, there was no suggestion of an effect of aspirin in analyses restricted to the first 6 years of follow-up. In addition, we observed no association between consistent use of aspirin 15 or more times per month and pancreatic cancer mortality in analyses that incorporated updated aspirin use information in the Nutrition Cohort. However, this subgroup was too small to allow us to examine the risk associated with consistent daily use of aspirin.
It should be noted that this analysis focused on aspirin use and does not rule out effects of nonaspirin NSAIDs, although no association with nonaspirin NSAIDs was seen in the Nutrition Cohort. In addition, this analysis examined pancreatic cancer mortality rather than pancreatic cancer incidence. However, our results are likely to be generalizable to pancreatic cancer incidence because pancreatic cancer is usually fatal (5-year survival = 5%) (26).
An important strength of this analysis is its unusually large size, resulting in risk estimates with narrow confidence intervals that excluded a strong association between aspirin use and pancreatic cancer mortality. Our results therefore provide evidence against an important effect of aspirin use on risk of pancreatic cancer.
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Manuscript received September 11, 2003; revised January 30, 2004; accepted February 9, 2004.
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