Affiliations of authors: A. Jemal, R. E. Tarone (Division of Cancer Epidemiology and Genetics), K. C. Chu (Office of Special Populations Research), National Cancer Institute, Bethesda, MD.
Correspondence to: Ahmedin Jemal, Ph.D., D.V.M., National Institutes of Health, 6120 Executive Blvd., EPS 8049, Bethesda, MD 20892 (e-mail: Jemala{at}exchange.nih.gov).
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ABSTRACT |
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INTRODUCTION |
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For men and women born after these cohorts with peak risk, the birth-cohort risk of lung cancer declined continuously up until about 1950 (7). The age-adjusted lung cancer mortality rate has declined among males since 1990 (10) and, while the rate among females is still increasing, the rate of increase has diminished considerably (10,11). Thus, overall lung cancer trends are favorable. However, the lung cancer mortality for birth cohorts after 1950 has not been evaluated. Smoking trends in teenagers have not consistently shown the decreases evident in overall smoking trends (12), and the implications of unfavorable smoking trends in teenagers (13) with regard to trends in lung cancer risk are unknown.
We present the results of ageperiodcohort analyses of lung cancer mortality data from 1970 through 1997 to evaluate recent lung cancer trends.
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SUBJECTS AND METHODS |
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To adjust for age, period, and cohort effects simultaneously, ageperiodcohort models were fitted by Poisson regression to the lung cancer mortality data by use of 2-year age and calendar-period intervals as described previously (15). For whites, there were 30 age intervals (ranging from 2425 years of age to 8283 years of age), 14 calendar-period intervals (ranging from 19701971 to 19961997), and 43 four-year birth-cohort intervals (ranging from 18861889 to 19701973). Each birth cohort will be identified in the text by the third year in the interval. For example, the 1952 birth cohort will refer to people born from 1950 through 1953, and 75% of people in this cohort will have been born in the middle 2 years, 1951 or 1952. Lung cancer mortality rates for blacks under 30 years of age were unstable because of small numbers of lung cancer deaths. For blacks, therefore, there were 27 age intervals (ranging from 3031 to 8283 years of age), 14 calendar periods (ranging from 19701971 to 19961997), and 40 birth-cohort intervals (ranging from 18861889 to 19641967).
Changes in the slope of the long-term trend in birth cohort or calendar-period effects were examined by use of identifiable parameters defined as differences between two linear contrasts (16). The coefficients for each linear contrast were selected to be those defining the first-degree polynomial in the appropriate set of orthogonal polynomials (16,17). For each identifiable parameter evaluated in this study, dividing the parameter by the sum of the squared coefficients of each linear contrast provides an estimate of the change in the slope of the birth-cohort or calendar-period trend. Each parameter evaluated was selected after examination of the estimated birth cohort and calendar-period effects, but the same parameter was applied to all four race/sex groups. Consistency of the estimates across these groups suggests that the reported findings are not false-positive results. A change in the birth-cohort trend usually indicates changes in an etiologic factor, resulting in increasing or decreasing risk, e.g., changes in the prevalence of smoking. A change in the calendar-period trend can indicate the impact of newly introduced or improved medical interventions, a change in ascertainment or coding of cause of death, or possibly, in the case of lung cancer, increased smoking cessation or a change in composition of cigarettes (6,18). Standard errors of the linear contrasts were adjusted for possible overdispersion when the deviance for the full ageperiodcohort model exceeded the number of residual degrees of freedom (19). Two-sided P values are reported and are considered to indicate statistical significance when they are less than .05.
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RESULTS |
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where h denotes the birth cohort effect for the cohort identified by the year h. This parameter is the difference between two linear contrasts, the first contrast characterizing the slope of the birth-cohort curve from 1950 through 1960 and the second contrast characterizing the slope from 1940 through 1950. A positive value for this parameter denotes an increase in the birth-cohort slope in 1950, indicating a worsening of the birth-cohort trend in lung cancer mortality after 1950. The magnitude of the increase in slope can be estimated by dividing the parameter by 70. The estimate of the change in cohort slope (per year) is 0.037 (P = .0001; 95% confidence interval [CI] = 0.018 to 0.056) for white males, 0.054 (P<.0001; 95% CI = 0.032 to 0.076) for white females, 0.027 (P = .11; 95% CI = -0.006 to 0.060) for black males, and 0.027 (P = .14; 95% CI = -0.008 to 0.062) for black females. The 2-year age-specific rates for white males and females are plotted by year of birth in Fig. 2
. The increased birth-cohort slope in 1950 can be seen in a slowing of the decline in the rates after 1950 in almost all age intervals, demonstrating that the increased birth-cohort slope is not an artifact of the model fitting (e.g., certain interactions, such as increases or decreases in the calendar-period slope with increasing age intervals, can give the appearance of a change in the birth-cohort slope in log-linear modeling). There are indications of a short period of declining birth-cohort risk after 1960 for whites.
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where the js denote the calendar-period effects. A negative value for this parameter denotes that the slope of the calendar-period risk curve decreased around 1990, indicating an improvement in the calendar-period trend for lung cancer mortality after 1990. The magnitude of the decrease in slope can be estimated by dividing the parameter by 20. The estimates of the change in calendar-period slope (per year) are -0.012 (95% CI = -0.014 to -0.010) for white males, -0.013 (95% CI = -0.016 to -0.010) for white females, -0.021 (95% CI = -0.029 to -0.013) for black males, and -0.019 (95% CI = -0.028 to -0.010) for black females (P<.0001 for all four contrasts). Examination of age-specific rates (data not shown) indicates that the calendar-period decrease in slope is apparent primarily in people 55 years of age and older, and ageperiodcohort analyses of Surveillance, Epidemiology, and End Results (SEER)1 incidence rates (data not shown) indicate a similar decrease in calendar-period slope around 1990 for white and black men and women.
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DISCUSSION |
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It is possible that marijuana smoking contributed to the birth-cohort pattern of risk after 1950. Marijuana smoke contains many of the same carcinogens found in cigarettes, and marijuana smoke condensate is carcinogenic in experimental animal systems (27,28). Accumulating evidence from studies of histopathologic and molecular changes in lung tissue of smokers suggests that marijuana smoking could increase the risk of lung cancer in humans (2931), although an epidemiologic study (32) demonstrated no increased lung cancer risk for smoking marijuana, despite a statistically significant increase in lung cancer risk for smoking cigarettes. If marijuana smoking causes lung cancer, it is possible that increased smoking of marijuana by teenagers and young adults in the 1960s and 1970s contributed to the birth-cohort increase in lung cancer mortality around 1950. Trends in first use of marijuana for ages 1217 years were quite similar to those for cigarette smoking (Fig. 3). Regardless of the relative contributions of cigarette smoking and marijuana smoking to lung cancer risk, the sharp increase in use of both cigarettes and marijuana since 1991 among teenagers will likely be reflected by an increase in the birth-cohort slope for lung cancer risk for people born around 1975.
Although the peaks of the birth-cohort effect curves for men and women appear to have occurred at approximately the same time (Fig. 1), peaks of birth-cohort effect curves are not identifiable (i.e., the location of a peak is not determined uniquely) in ageperiodcohort analyses (see legend to Fig. 1
) (16). Agecohort analyses show that the peak birth-cohort risk occurred in 1923 for men and in 1931 for women (data not shown), consistent with the earlier use of cigarettes by men (57,12).
There were no dramatic improvements in lung cancer treatment that could explain the calendar-period decrease in mortality rates around 1990. Five-year relative survival rates from 19741976 to 19891996 increased in whites, but only from 12.5% to 14.4%, and decreased in blacks from 11.5 % to 11.3% [Table XV-7 in (33)]. Moreover, decreases in the slopes of incidence rate curves were observed in white and black men and women around 1990 [Fig. XV-5 in (33)], which suggests that the mortality decrease is a result of a decrease in lung cancer risk, not of an improvement in survival. Although changes in the prevalence of risk factors usually alter the pattern of risk seen among birth cohorts, a substantial decrease in a relatively common carcinogenic exposure could cause a calendar-period decrease in risk after a sufficient latency period. Cigarette smoking affects both early and late stages of the carcinogenic process (3437). The effect of reducing tobacco carcinogen exposure on the late stage will be seen soon after the change in exposure. Thus, the generally convex shape of the calendar-period effect curves from 1970 through 1990 (Fig. 4) likely reflects the impact of the steadily improving trends in both carcinogen exposure from cigarettes (evidenced by the sharp decline in tar and nicotine yield) and smoking cessation over the study period (Fig. 5
) on the late-stage event. In contrast, the effect of reducing tobacco carcinogen exposure on the initiation event will not be observed for decades. The largest decreases in tar and nicotine yield and increases in smoking cessation rates occurred in the 1960s and 1970s. The sharp decline in calendar-period risk around 1990 may reflect the impact on the initiation stage of the decrease in tobacco carcinogen exposure and the increase in smoking cessation beginning around 1960; if so, the impact became manifest only after a latency period of approximately 30 years. The decrease in rates was seen primarily in people old enough (i.e., at least 55 years of age in 1990) to have smoked the high-tar cigarettes for a sufficient duration to show markedly increased lung cancer risk (36). If the 1990 calendar-period decrease reflects the rapid decreases in cigarette carcinogen exposure and increases in smoking cessation between 1960 and 1980, then this calendar-period decrease should continue unabated until at least 2010.
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Our study suggests that there may be a rather rapid effect of teenage smoking on lung cancer risk in some people under the age of 45 years and demonstrates that accurate survey data on smoking in children are essential to understanding trends in smoking-related diseases. Thus, continued support should be provided to ongoing national surveys that provide data on teenage smoking. The current trend of teenage smoking is disturbing (13,21,41). Our results suggest that increases in teenage smoking prevalence will lead to increased lung cancer risk for people in their 20s and 30s. This could provide additional incentive to prevent teenage smoking by providing evidence that the harmful effects of smoking occur earlier in life than most teenagers may realize.
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NOTES |
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Manuscript received July 20, 2000; revised November 30, 2000; accepted December 12, 2000.
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