Subjects were breast cancer patients who participated in a population-based case–control study among women aged 40 years or younger (Bernstein et al, 1994
). Briefly, eligible patients were white or Hispanic women consecutively diagnosed with their first primary in situ
or invasive breast cancer from 1 July 1983 through 31 December 1989, identified by the University of Southern California Cancer Surveillance Program (CSP), the population-based cancer registry for Los Angeles County.
The University of Southern California institutional Review Board (Los Angeles, CA, USA) reviewed and approved the study that complied with all applicable federal regulations governing the protection of human subjects. Study subjects were given a detailed explanation of the study including the risk and benefits. All participants provided signed informed consent to participate in the study.
In face-to-face interviews, we obtained detailed information about demographics and known or suspected breast cancer risk factors, up to each patient's reference date, defined as 12 months before diagnosis.
We asked patients whether they had ever participated at least twice weekly on a competitive athletic team or in dance or exercise classes, or if they jogged or ran one mile at least twice weekly. For those who responded ‘yes’, we recorded for each activity the age started and stopped, the type, and the average number of hours per week of participation. We recorded each episode when activities were started and stopped more than once or when the amount of time spent in the activity changed. We computed the number of hours per week each patient participated in all recreational exercise activities beginning with the year of each woman's first menstrual period and ending at the reference date.
Details were collected of height and weight at the reference date and weight at age 18 years. Body mass index (BMI) was computed as weight (kg) divided by the square of height (m2) and weight gain as the difference of weight (kg) at the reference date and weight (kg) at age 18 years. Women with zero or negative weight gain were defined as having a zero weight gain.
Details of stage at diagnosis (in situ, localised, regional, distant) and lymph node involvement (number positive and number examined) were abstracted from the records at the CSP and at the hospital of diagnosis.
Follow-up began at interview and ended at the date of death, the date of last follow-up, or 30 June 2000, the end of the study period, whichever was earliest. Vital status and cause of death information was obtained routinely from CSP records, from computerised records of the California Death Certificate Master File and the National Death Index, and by contacting the patients or their relatives by telephone.
Of 969 eligible patients, we interviewed 744 (76.8%) but then excluded 11 postmenopausal patients, 11 with unknown cancer stage, and five deceased patients with unknown cause of death so that a total of 717 were available for analysis of BMI, weight and height. The first 199 patients enrolled in the case–control study did not receive the detailed assessment about lifetime exercise habits leaving 525 patients available for the exercise analyses. Of the 199 patients without detailed lifetime exercise data, 192 were determined to be otherwise eligible and were included in the analyses of body size and breast cancer survival; for these patients, we created a separate exercise activity code.
-values were used to assess the relationship of patient characteristics at diagnosis with breast cancer survival. Hazard rate ratios (HR) and 95% confidence intervals (95% Cl) were calculated using Cox's proportional hazards methods (Breslow and Day, 1987
). Events were deaths due to breast cancer; patients who died from other causes or were alive at the last follow-up were censored at the last follow-up date.