Cigarette smoking, physical activity, and alcohol consumption as predictors of cancer incidence among women at high risk of breast cancer in the NSABP P-1 trial.

BACKGROUND

NSABP P-1 provides an opportunity to examine the association of behavioral factors with prospectively monitored cancer incidence and interactions with tamoxifen.

METHODS

From 1992 to 1997, 13,388 women with estimated 5-year breast cancer risk greater than 1.66% or a history of lobular carcinoma in situ (87% younger than age 65; 67% postmenopausal) were randomly assigned to tamoxifen versus placebo. Invasive breast cancer, lung cancer, colon cancer, and endometrial cancer were analyzed with Cox regression. Predictors were baseline cigarette smoking, leisure-time physical activity, alcohol consumption, and established risk factors.

RESULTS

At median 7 years follow-up, we observed 395, 66, 35, and 74 breast cancer, lung cancer, colon cancer, and endometrial cancer, respectively. Women who had smoked were at increased risk of breast cancer (P = 0.007; HR = 1.3 for 15-35 years smoking, HR = 1.6 for ≥ 35 years), lung cancer (P < 0.001; HR = 3.9 for 15-35 years, HR = 18.4 for ≥ 35 years), and colon cancer (P < 0.001; HR = 5.1 for ≥ 35 years) versus never-smokers. Low activity predicted increased breast cancer risk only among women assigned to placebo (P = 0.021 activity main effect, P = 0.013 activity-treatment interaction; HR = 1.4 for the placebo group) and endometrial cancer among all women (P = 0.026, HR = 1.7). Moderate alcohol (>0-1 drink/day) was associated with decreased risk of colon cancer (P = 0.019; HR = 0.35) versus no alcohol. There were no other significant associations between these behaviors and cancer risk.

CONCLUSION

Among women with elevated risk of breast cancer, smoking has an even greater impact on breast cancer risk than observed in past studies in the general population.

IMPACT

Women who smoke or are inactive should be informed of the increased risk of multiple types of cancer.