Study cohort: The JACC study
A detailed description of the Japan Collaborative Cohort Study (JACC Study) has been published elsewhere . Initiated in 1988, the JACC Study is one of the representative cohort studies in Japan. At baseline (1988–1990), 110,585 people (46,395 men and 64,190 women), aged 40–79 years, were enrolled from 45 areas throughout Japan. Informed consent for participation was obtained from each participant in the majority of study areas. However, it was obtained at the group level in a few areas because the concept of informed consent was not popularized during the 1980s in Japan. In that case, the municipality head gave the consent to participation representing the participants living in that area. The ethics committee at the Aichi Medical University School of Medicine approved the JACC Study, including study design, informed consent procedure, and data collection and analysis.
At baseline, all participants completed a self-administered questionnaire that solicited information on demographic characteristics, family history of cancer, medical history, occupation, and lifestyle factors. Regarding occupation, we asked the participants to choose from pre-coded categories: employed, working a part-time job, self-employed, housewife, no occupation, or other.
We followed the study subjects until December 31, 2009 in 35 areas. Because of logistical problems, we discontinued follow-ups prior to December 31, 2009 in 10 areas. Approximately 6 % of the cohort participants moved out of the study area. During the follow-up period, we checked the vital status using resident registry data obtained from the municipalities. We ascertained information on mortality based on the causes of death recorded on death certificates. Biliary tract cancer was classified according to the 10th revision of the International Classification of Disease (ICD), in which C23 represents malignant neoplasm of the gallbladder and C24 represents malignant neoplasm of other (or unspecified) parts of the biliary tract. C24 comprises malignant neoplasms of the extrahepatic bile duct (C24.0), ampulla of Vater (C24.1), overlapping sites of the biliary tract (C24.8), and unspecified parts of the biliary tract (C24.9). Malignant neoplasms of other (or unspecified) parts of the biliary tract can be generally referred to as malignant neoplasms of the extrahepatic bile duct because the majority of malignant neoplasms of other (or unspecified) parts of the biliary tract are malignant neoplasms of the extrahepatic bile duct.
Men who were 40–65 years of age and who reported working full-time or were self-employed at baseline were included in the present analysis. Men were excluded if they had missing data on occupation or a history of cancer at baseline, which left 22,224 men eligible for the present analysis. We examined the characteristics such as sex, age, body mass index, and cigarette smoking between those subjects and 46,935 men at baseline. Overall there were no significant differences between the two groups.
Information on shift work was collected on the basis of the question: “Which form of work schedule have you engaged in for your longest occupation?” Men were asked to indicate the most regular schedule they had undertaken among three work schedules: daytime work, permanent nighttime work, or rotating shift work. The rotating shift work may or may not involve night work.
We also collected information on covariates, including age, height, weight, medical history, family history of cancer, cigarette smoking (current, former, never), alcohol consumption (current, former, never), job type (office work, manual work, or other), physical activity at work (sitting, alternate sitting and standing, or standing with/without moving), workplace (indoor, outdoor, or both), level of perceived stress (low, moderate, high, or, very high), educational level, and marriage status. Body mass index (BMI) was calculated from self-reported height and weight.
Person-years of follow-up were calculated for each cohort participant from baseline to December 31, 2009, or to the date of pancreatic cancer death or any other cause, or to the time of moving out of the study area, whichever occurred first. Subjects who died from causes other than pancreatic cancer or who moved out of the study areas were treated as censored.
Cox proportional hazards models were used to estimate HRs and 95 % CIs for the association between shift work and the risk of death from biliary tract cancer. The results are presented in the forms of both age-adjusted HRs and multivariable-adjusted HRs. Potential confounding factors added into the models included age (continuous), BMI (<20, 20–22.4, 22.5-24.9, ≧25.0), history of cholelithiasis (yes, no), history of diabetes (yes, no), alcohol drinking (never, past, current), cigarette smoking (never, past, current), sleep time (continuous), and perceived stress (low, moderate, high). Individuals with missing covariate data were treated as an additional group. We included time-dependent covariates in the Cox models to test the proportional hazard assumption, and we found that the data met the proportionality assumption. All analyses were performed using SAS 9.1 software (SAS Institute, Cary, NC). P values for the statistical tests were two-tailed and were considered to be statistically significant if they were <0.05.