A Case-control Study On Shift Work, Light-at-night, And The Risk Factors Of Breast Cancer

ObjectivesBreast cancer is one of the most frequent malignancies in women worldwide. However,the risk factors of breast cancer are still not known at present. The recognized risk factors ofbreast cancer only account for47%breast cancer. Maybe50%or above breast cancerattributes to the unrecognized risk factors. The International Agency for Research onCancer/World Health Organization in2007concluded, that “shift work that involvescircadian disruption is probably carcinogenic to humans, Group2A”. Light-at-night is animportant risk factor that affects people’s circadian rhythm. Therefore, we conducted acase-control study to explore the relationships between shift work, light-at-night and therisks of breast cancer, in order to provide the theoretical basis for cause exploration andprevention of breast cancer.MethodsThe cases were firstly histologically-confirmed, incident women breast cancer patientsat the abdominal surgery of Cancer Hospital, Chinese Academy of Medical Sciences, fromJuly2011to December2011. The controls were histologically-confirmed women breastbenign disease patients from the same time, the same department and the same hospital.There are three screening conditions for all of the cases and controls: Beijing, the Hannationality and30-80years. Finally, there were182cases and183controls in our study. Weused the self-designed questionnaire to survey the related information of cases and controls,mainly containing some socioeconomic information, some factors about the way of life,shift work information, light-at-night, history of disease, menstrual and reproductive history and so on. We used the Statistical Analysis System(SAS)9.2software for analyzingdata.The Chi-square Test and Fisher’s Exact Test to compare the relevant informationbetween the two groups. And Multi-factor Unconditional Logistic Regression to explore therelationship between shift work, light-at-night, and breast cancer.ResultsThe mean of cases’age was53.09.9years. The cases were older than controls (mean:48.2±9.7years) significantly. There were23.6%cases that annual income per person infamily were less7500yuan in cases,44.5%cases less than20000yuan. However, therewere18.0%controls that annual income per person in family were less7500yuan incontrols,38.8%controls less than20000yuan. It is diverse significantly about thedistribution of annual income per person in family between the two groups (P<0.05). Inaddition, We found they were not markedly difference in the distributions of Body MassIndex, marriage status, education level, smoking and drinking between cases and controls(P>0.05).The distributions medical history and menstrual and reproductive history were notdifferent significantly between cases and controls (P>0.05). After adjusting the age andincome, the results of Unconditional Logistic Regression indicated that there were notrelationships significantly between breast cancer and medical history and menstrual andreproductive history.Among these population,25.8%subjects were shift works. Thereinto,26.9%shiftworkers in cases and24.6%shift workers in controls. After adjusting the age and income,the results of Unconditional Logistic Regression indicated that there was no significantrelationship between shift work, light-at-night and the risk of breast cancer.According to the postmenopausal state, we had a stratified analysis. After adjusting theage and income, the result of stratified analysis indicated shift work could increase the riskof breast cancer among the pre-menopause women (Odd Ratio(OR)=2.6,95%ConfidenceInterval (CI)=1.3-5.3). And the result of trend test pointed out the later age of beginning tobe engaged in shift work, the more years of shift work, the more times of shift work perweek, the more times of shift work in total, then the risk of breast cancer would increase (Ptrend0.05) among the pre-menopause women. However, we found no increased risk of breast cancer associated with shift work among post-menopause women. In addition, wefound neither pre-menopause women nor post-menopause women was significantassociation between light-at-night and risk of breast cancer, after adjusting the age andincome.ConclusionsShift work could increase the risk of breast cancer among the pre-menopause women.Moreover, the later age of beginning to be engaged in shift work, the more years of shiftwork, the more times of shift work per week, the more times of shift work in total, then therisk of breast cancer would increase among the pre-menopause women. In addition, Neitherpre-menopause women nor post-menopause were significant association betweenlight-at-night and risk of breast cancer.