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Division of Epidemiology, American Health Foundation, Valhalla, New York 10595 [S. D. S., J. E. M., E. L. W.]; Division of Epidemiology, Aichi Cancer Research Institute, Nagoya, Japan [T. T., K. T.]; Harvard School of Public Health, Boston, Massachusetts 02115 [L. W.]; Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, New York 10032 [S. D. S., Y. C., A. I. N.]; ProHealth, Inc., Lake Success, New York, 11042 [M. L. C.]; Tobacco Control Research Branch, Division of Cancer Control and Population Sciences, National Cancer Institute, Bethesda, Maryland 20852 [M. V. D.]; New York University Medical School, New York, New York 10016 [S. H.]; Department of Human Sciences, Aichi Mizuho College, Toyota, Japan [H. O.]; and Aichi Cancer Center, Nagoya, Japan [K. A.]
| Abstract |
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| Introduction |
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The rates of lung cancer in Japanese migrants and their offspring in the United States are similar to United States-born whites, which strongly suggests that most of the international variation in lung cancer rates is not attributable to ethnic differences in susceptibility (6) . In a comparison of several prospective studies of lung cancer mortality rates, smoking- specific rates were consistently lower in Japanese than in British, Norwegian, Swedish, or American smokers (7 , 8) . Although there are other risk factors for lung cancer including exogenous factors such as diet (9 , 10) , occupation (11) , and inborn differences in metabolizing enzymes (12) , cigarette smoking causes >90% of cases (8) . Therefore, attempts to explain this anomaly must begin with a comparison of smoking-specific factors such as the duration of smoking habit, cpd,4 or age at onset of smoking.
This study took advantage of long-standing collaborations between the American Health Foundation and Japanese researchers. In 1977, Wynder and Hirayama (1) summarized the trends and differences in United States and Japanese cancer rates in relation to lifestyle characteristics. Subsequently, they found that lung cancer rates were rapidly increasing among Japanese men as a consequence of increased numbers of heavy cigarette smokers (2) . The United States-Japan lung cancer anomaly has persisted in more recent comparisons (13) . Because these ecological comparisons of population data cannot directly establish the risk of smoking, we conducted a multicentric case-control study in the United States and Japan to compare smoking behaviors as risk factors in both countries.
| Materials and Methods |
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Eligibility.
Cases were patients between the ages of 20 and 81 years newly diagnosed with primary lung cancer within 1 year of the interview date and who had no previous history of another tobacco-related cancer. All of the cases were histologically or cytologically confirmed through hospital pathology reports or discharge summaries. United States lung cancer cases were patients admitted to participating hospitals between March 1992 and February 1997; Japanese cases were enrolled between June 1993 and May 1998. Hospital controls were patients ages 2081 years admitted for a nontobacco related condition and having no history of the following tobacco related diseases: cancer of the esophagus, larynx, oral cavity, nasal sinus, pancreas, liver, kidney, or bladder. Patients hospitalized for other tobacco-related cardiovascular conditions (i.e., myocardial infarction, stroke, or coronary artery disease) and respiratory disorders (i.e., emphysema, chronic bronchitis, or chronic obstructive pulmonary disease) were also excluded as hospital controls. Hospital controls in both countries were frequency matched on age (±5 years), hospital, and date of interview (initially±2 months, extended in September 1995 to± 4 months in Japan because of the requirement of obtaining individual physician consents). Hospital controls in both countries included large percentages of patients with digestive, genitourinary, and musculoskeletal diseases, and injury or poisoning. However, a higher percentage of United States controls were admitted for neoplasms (29.0% versus 1.2% in Japan), of which 38.9% were cases of malignant neoplasm of prostate, and 25.9% were cases of leukemia and lymphoma; 56.3% of Japan hospital controls were diagnosed with digestive and genitourinary diseases compared with 32.1% of United States hospital controls.
Hospital controls were included in the United States component of the study because we previously showed no differences in smoking prevalence between hospital and community-based controls (14) . Because it had not been determined previously whether smoking prevalence was similar in Japanese hospital patients and community members, the Japanese study design included both hospital and community-based controls. Controls were frequency matched by age (±5 years), date of interview (±2 months;±4 months after September 1995), and neighborhood of residence to cases. Community controls were chosen from residents who lived in the same or neighboring district of the hospital in which matching cases were chosen in a random selection process from a list of names with corresponding addresses and dates of birth, kept as part of the electoral records system in Nagoya and Okazaki City. The United States portion of the study comprised 371 white male cases and 373 white male hospital controls. The total number of male Japanese subjects available for analysis included 410 lung cancer cases, 252 hospital controls, and 411 community controls. The overall response rate among United States patients was 82.8% of those approached and was nearly identical among cases and controls. Response rates in Japanese cases and controls were similar and also exceeded 80%.
Questionnaires and Interview Procedures.
The Japanese version of the questionnaire was developed by translating the English version and then back-translating it into English by a different translator to insure comparability. Trained interviewers were given the same instructions in both countries. To assure procedural uniformity in the two countries, three of the Japanese investigators, who are all fluent in English, received intensive training in New York, whereas one of the United States investigators (S. D. S.) made frequent visits to the Japanese sites. The Japanese questionnaire was printed in bilingual form to facilitate data entry by the American keypunchers. All of the subjects signed a consent form, which was approved by the Institutional Review Boards of the collaborating institutions. The questionnaire included detailed items on smoking history such as age at onset, smoking status, years and amount, cigarette brand, history, and years since quitting.
Smoking Categories.
Subjects were classified as nonsmokers (never smoked regularly; i.e., at least once a day for 1 year), current smokers (smoked within the past year), or ex-smokers (did not smoke within the past year). Persons who smoked cigars or pipes only were excluded from the analyses except for the descriptions of mean age and years of education. Among United States men, "cigar/pipe only" smokers made up 5.4% and 0.85% of cases and controls, respectively; there were no Japanese subjects who reported smoking either cigars or pipes.
Statistical Methods.
OR and 95% CI were calculated by logistic regression, and adjusted for confounding variables such as age at diagnosis, education (three categories: <12, 12, >12 years), and hospital (for analyses using hospital controls). Dummy variables were introduced to code categories of smoking status (non-, current, or ex-smoker), total years of smoking (
40 or 41+ years), average number of cpd (<20, 2029,
30 cpd), age at onset of smoking (014, 1517, 1820, >20 years old), and number of years since quit smoking (19, 1015, 16+ years). Walds
2 statistic was used to test statistical significance against the null hypothesis of no association, and 95% CIs were obtained. Differences in mean values were tested for significance with Students t. The
2 test for trend was conducted by the Mantel-Haenszel extension method with data stratified into age groups by decade.
| Results |
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Table 3
also shows that the average age of smoking onset was 2.5 years later for Japanese cases than American cases (P < 0.001) and 2.4 to 2.7 years later for Japanese compared with American controls (P < 0.01). Whereas one-fifth of the currently smoking controls in the United States had started smoking before age 15, there were almost no Japanese who reported starting smoking so young. Statistically significant trends of decreasing lung cancer risk with later age of onset of smoking were observed in both countries (P < 0.01 in United States and P < 0.001 in Japan). The estimated effect of starting to smoke at a later age was more apparent using community controls as a reference than hospital controls.
Table 4
displays the pattern of the reduction in lung cancer risk in ex-smokers according to years since quitting, relative to current smokers. Among United States subjects, the risk was reduced as much as 50% for those who had quit within the last 10 years, whereas among Japanese former smokers, the corresponding reduction was only 1020%. The relative risk estimates were examined by the two most common histological types of lung cancer: AC and SCC (Table 5)
. OR estimates for SCC are not shown for the United States study population, because there were no SCC cases among nonsmokers and only 32 among current smokers. ORs for AC in the United States population and for both histological types in Japan increased both with duration and with cpd. ORs for AC in the United States population were all significantly >1 and were greater than those in Japan. Among Japanese current smokers, a steeper increase in ORs was observed for SCC compared with ORs for AC, and ORs calculated from community controls were larger than those obtained by using hospital controls.
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| Discussion |
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An important difference between United States and Japanese smokers was the lower average age at onset of smoking in American men (<15 years), which was associated with an elevated risk of lung cancer. Almost no Japanese smokers began smoking before the age of 15. Nevertheless, earlier age of smoking onset was also a risk factor in Japanese men as shown by the lower ORs associated with smoking onset at age of 18 or older compared with an earlier age at onset of smoking (between 15 and 17 years of age). Sobue et al. (17) have noted that smoking has been illegal in Japan for persons under age 20 since 1900. However, this prohibition is clearly flouted by younger Japanese, according to surveys in Japanese high schools that showed rapidly increasing smoking rates during the 1980s (24) .
Although some epidemiological studies have suggested that age at onset of smoking is an independent risk factor for lung cancer (25) , this relationship remains controversial, since early age at onset of smoking is associated with heavy subsequent cigarette consumption (26) . A recent study also showed that the effect of age at onset of smoking is not strong enough to explain the causal relationship between smoking and damage in lung tissues among current smokers, but was a significant factor among ex-smokers (27) . Hence, although it is possible that the higher risk of lung cancer in the United States might partly result from earlier age at onset of smoking among current smokers, it is unlikely to account for the large difference in OR between United States and Japan.
Numerous studies have found that lung cancer risk decreases with increasing years since quitting in the United States (8 , 28 , 29) and Japan (17 , 30) . In the present study, it was observed that American former smokers experienced a more dramatic reduction in lung cancer risk during the first decade after cessation than the Japanese ex-smokers did. After 16 years of cessation, lung cancer risks for ex-smokers in both countries were slightly lower for the United States group. Given the higher overall risk among United States current cigarette smokers, this additionally emphasizes the importance of smoking cessation as an essential risk reduction step for all smokers.
The dose-response for SCC was considerably steeper than for AC in the Japanese population whether duration or cpd was the dosage variable (Table 5). This disparity in histology-specific risks is consistent with reports from case-control studies reported from both the United States and Japan over the past 2 decades (Refs. 17
, 31
, 32
; Table 6
). The risk for SCC in the United States group would undoubtedly have been much greater than for AC had there been sufficient cases to permit computation; however, the steady decline in the prevalence of current smokers among American men in the lung cancer age group along with the continuing clinical shift from SCC to AC (33)
has greatly reduced the available number of cases with SCC that are either current smokers or nonsmokers. The relative proportions of SCC and AC were similar in both countries.
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1.2 mg nicotine) inhale more than twice as much nicotine, "tar," and NNK as predicted by the United States Federal Trade Commission protocol (35)
; such measurements have yet to be made for Japanese smokers. Both genetic and lifestyle factors may modify smoking-related lung cancer risk. A higher prevalence has been reported in Japanese of genetic polymorphisms in some P450 enzymes that catalyze activation of carcinogenic polycyclic aromatic hydrocarbons such as those found in cigarette smoke (39 , 40) . Tyndale et al. (41) recently found the prevalence of the (protective) *2 and *4 null alleles of CYP2A6 to be 21.2% in Japanese compared with 2.6% in Caucasians; this enzyme is one of several that metabolically activate N-alkylnitrosamines such as NNK (42) . Polymorphisms such as these may be associated with as much as a 2-fold risk of lung cancer in both white and Japanese populations (12) , but many other factors may also be needed to explain the 10-fold differential in relative risk observed by us. Marmot and Smith (43) have pointed out that Japanese in general have a longer life expectancy than people in England, and possible explanations for the lower mortality in Japan could be the effects of different aspects of Japanese lifestyle. Wynder et al. (13) have suggested previously that differences in diet, particularly dietary fat, may also contribute to the differences in lung cancer rates. Gao et al. (4) , using data obtained earlier from a Nagoya hospital population, reported a protective effect of fruit and vegetable consumption on risk of lung cancer. Ohno et al. (20) also reported a protective effect of tea consumption against lung cancer in residents of Okinawa. All of these factors may be considered as candidate effect modifiers of smoking-related lung cancer risks in future studies.
Potential biases from misclassification of ex-smokers as nonsmokers in some Japanese case-control studies could result in an underestimate of OR (17) . However, it is unlikely that a substantial nonrandom misclassification would occur in only one of the two countries unless Japanese ex-smokers and United States ex-smokers differ greatly in the variety of self-reports of smoking status. Comparison of the distribution of smoking prevalence from this study with population statistics suggests that this is not the case. Because lung cancer in young patients differs from lung cancer in older patients (44) , all of the OR calculations were adjusted for age in this study. Also, because smoking behaviors vary by social class (45 , 46) , education level was controlled in the analyses.
Lung cancer cases from hospitals may not be representative of all of the lung cancer cases in the underlying population. The smoking prevalence in the control groups was reduced after excluding hospital patients with diseases related to smoking. However, the exclusion of smoking related diseases from hospital controls was an important element of the study design and has been shown to be a successful strategy for reducing selection bias in case-control studies of tobacco-related diseases in United States studies (14) . In the Japanese study population, all of the ORs calculated using community controls were higher than those estimated using hospital controls, but these differences were small in comparison with United States-Japan differences for all of the major dosage variables.
| Acknowledgments |
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| Footnotes |
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1 Supported by USPHS Grants CA-68384, CA-32617, and CA-17613 from the National Cancer Institute and a grant from the Verum Foundation. ![]()
2 To whom requests for reprints should be addressed, at Department of Epidemiology, Mailman School of Public Health, Columbia University, 630 West 168th Street PH-18, New York, NY 10032. Phone: (212) 305-4911; Fax (212) 305-9413; E-mail: sds91{at}columbia.edu ![]()
4 The abbreviations used are: cpd, cigarettes per day; AC, adenocarcinoma of the lung; CI, confidence interval; NNK, 4-(methylnitrosamino)-1-(3-pyridyl)-1- butanone; SCC, squamous cell carcinoma of the lung; OR, odds ratio. ![]()
Received 10/28/99; revised 8/13/01; accepted 8/21/01.
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