BMC Cancer 2008, 8:157 http://www.biomedcentral.com/1471-2407/8/157 While some epidemiologic studies have observed associa- tions between cigarette smoking and breast cancer, more studies have not [3,4]. Most studies of secondhand smoke exposure suggested no association with breast cancer risk [5-17], although a few have observed increased breast can- cer risk among women exposed to secondhand tobacco smoke [18-21], particularly among premenopausal women [22] and women with exposure prior to first full- term birth [23,24]. Notably, studies that measured detailed exposures to both active and secondhand tobacco smoke indicate that risk associated with active smoking may have been masked in earlier studies because women exposed to secondhand tobacco smoke were included in the reference groups [18-20,24-27]. A recent meta-analy- sis concluded that studies with more thorough assessment of exposure to tobacco smoke tended to support an asso- ciation between breast cancer and both active and second- hand tobacco smoke, whereas studies with cruder exposure assessment were less likely to observe associa- tions [28]. Here we report on a case-control study in Del- aware, incorporating detailed exposures to active and secondhand smoke assessed both at home and in the workplace during critical periods in a woman's reproduc- tive life. Methods Study population Breast cancer cases were identified through the Delaware Cancer Registry, a statewide, population-based cancer reg- istry maintained by the Delaware Division of Public Health. Cases were defined as female residents of Dela- ware, ages 40–79, diagnosed with microscopically-con- firmed incident primary invasive breast cancer (International Classification of Diseases, 10th revision (ICD-0) code 174) in 2000–2002. Cases were excluded from the study if they had a prior history of invasive breast cancer, lived in an institutional setting, or did not have a telephone. Physician approval was required prior to con- tacting cases. Potential participants were then mailed a let- ter and contacted by telephone 1–2 weeks later. Of the 1,076 potential cases identified through the Delaware Cancer Registry, physician permission was obtained to contact 617, of whom 217 could not be located or were not eligible for the study. Of the remaining 400 eligible cases, 28% refused to participate in the study, resulting in a final sample size of 287 cases. As compared to the cases who participated in the study, cases who were not con- tacted or refused to participate were somewhat less likely to be White, while no differences were observed between the groups by county or stage at diagnosis. Controls were selected using two public databases: Dela- ware motor vehicle drivers license records for women <65 years and records maintained by the Health Care Finance Administration (HCFA), currently known as the Center for Medicare and Medicaid Services, for women 65+ years. Controls were frequency-matched to cases on age within 10-year groups. Telephone numbers were obtained using reverse telephone directories, and letters were mailed to women with phone number matches, informing women of their selection into the study and the phone call that they should anticipate. Potential controls were then called 1–2 weeks after letters were mailed, and of those eligible controls contacted, 46% participated. Power calculations performed during the design phase of the study indicated that 300 breast cancer cases and 300 controls were needed in order to achieve 80% power to detect an odds ratio between active smoking and breast cancer of 1.6 at a significance level of p < 0.05, assuming the prevalence of exposure in controls was 47%, the pro- portion of women of similar ages as the proposed controls who reported ever smoking in the Delaware Behavioral Risk Factor Surveillance Survey [29]. Data collection In accordance with the study protocol, which was approved by the human subjects review boards at the Johns Hopkins Bloomberg School of Public Health, the Delaware Department of Health and Social Services, and the University of Delaware, informed consent was com- pleted by phone. Trained interviewers administered a detailed questionnaire with smoking questions modeled after the Missouri Women's Health Study [30,31]. Each woman who had ever smoked (i.e. smoked at least 100 cigarettes in her lifetime) answered questions about the age she first smoked cigarettes, how many cigarettes she smoked per day, whether she smoked cigarettes that were filter versus non-filter, menthol versus non-menthol, and regular versus low-tar, if she usually inhaled smoke into the chest, and if in addition to cigarettes, she used any other type of tobacco product on a regular basis, including pipes, cigars, cigarillos, snuff, and chewing tobacco. If any of these characteristics of her smoking habits changed since she started smoking, then the age at the time of the change was recorded, and the same characteristics were assessed for the next smoking period. Secondhand smoking in childhood (<18 years) and adult- hood was based on enumeration of smokers living in the participant's household. Then, for each smoker, the number of packs, cigars or pipefulls smoked per day were recorded, in addition to the hours/day the participant was exposed. As with active smoking, new data were collected any time any aspect of exposure changed. Occupational exposure to secondhand smoke involved enumeration of jobs with exposure and, for each job, reporting of length of employment and average daily hours of exposure. Sub- jects also provided a subjective rating of exposure inten- sity (light, moderate, heavy) for each job. Other questions Page 2 of 9 (page number not for citation purposes)