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Paper 3 of the thesis is not available in Munin: 3. Bjerkaas E, Parajuli R, Engeland A, Maskarinec G, Weiderpass E, Gram IT: 'Social inequalities and smoking-associated breast cancer – results from a prospective cohort study', submitted manuscript. Summary: According to the WHO, breast cancer is by far the most frequently diagnosed cancer, and the most frequent cause of cancer death among women in the world. Tobacco smoking is the single largest cause of cancer worldwide and has been linked to cancer in most organ systems. The aim of this thesis was to illuminate the association between smoking and breast cancer incidence, mortality, and to study the socioeconomic consequences of smoking-associated breast cancer in a large Norwegian cohort with a high number of female smokers. In Paper I we investigated the association between active smoking and breast cancer incidence. We found an increased risk of 15% for ever smokers overall, as well as an increased breast cancer risk with increasing number of cigarettes smoked per day, smoking duration, number of pack-years smoked, and lower age at smoking initiation. We found an increasing risk with longer smoking duration before first childbirth, and no increased risk among those who started to smoke after first childbirth. In Paper II we investigated the association between smoking and breast cancer mortality, which revealed a 15% increased risk of breast cancer mortality for ever smokers. Most of the results for the different smoking exposures considered were not statistically significant. In Paper III, we used level of education as a validated measure of socioeconomic status, and investigated whether level of education had an impact on the risk of smoking-associated breast cancer. We did not find an increased risk of smoking-associated breast cancer in women with high level of education, but we were able to confirm that smoking before first childbirth remains a risk factor for breast cancer, regardless of educational achievement. Smoking-associated breast cancer does not seem to have an important impact on social inequalities in health. Active smoking, in particular active smoking before first childbirth, emerges as a risk factor for breast cancer incidence.

This papers is part of Ranjan Parajuli's doctoral thesis, available in Munin at http://hdl.handle.net/10037/6610 Background: Smoking has recently been established as a risk factor for rectal cancer. We examined whether the smoking-related increase in rectal cancer differed by gender. Methods: We followed 602,242 participants (49% men), aged 19 to 67 years at enrollment from four Norwegian health surveys carried out between 1972 and 2003, by linkage to Norwegian national registries through December 2007. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated by fitting Cox proportional hazard models and adjusting for relevant confounders. Heterogeneity by gender in the effect of smoking and risk of rectal cancer was tested with Wald χ2. Results: During a mean follow-up of 14 years, 1,336 men and 840 women developed invasive rectal cancer. Ever smokers had a significantly increased risk of rectal cancer of more than 25% for both men (HR = 1.27, 95% CI = 1.11-1.45) and women (HR = 1.28, 95% CI = 1.11-1.48) compared with gender-specific never smokers. Men smoking ≥20 pack-years had a significantly increased risk of rectal cancer of 35% (HR = 1.35, 95% CI = 1.14-1.58), whereas for women, it was 47% (HR = 1.47, 95% CI = 1.13-1.91) compared with gender-specific never smokers. For both men and women, we observed significant dose–response associations between the risk of rectal cancer for four variables [Age at smoking initiation in years (both ptrend

Recent cohort studies suggest that increased breast cancer risks were associated with longer smoking duration, higher pack-years and a dose-response relationship with increasing pack-years of smoking between menarche and first full-term pregnancy (FFTP). Studies with comprehensive quantitative life-time measures of passive smoking suggest an association between passive smoking dose and breast cancer risk. We conducted a study within the European Prospective Investigation into Cancer and Nutrition to examine the association between passive and active smoking and risk of invasive breast cancer and possible effect modification by known breast cancer risk factors. Among the 322,988 women eligible for the study, 9,822 developed breast cancer (183,608 women with passive smoking information including 6,264 cases). When compared to women who never smoked and were not being exposed to passive smoking at home or work at the time of study registration, current, former and currently exposed passive smokers were at increased risk of breast cancer (hazard ratios (HR) [95% confidence interval (CI)] 1.16 [1.05-1.28], 1.14 [1.04-1.25] and 1.10 [1.01-1.20], respectively). Analyses exploring associations in different periods of life showed the most important increase in risk with pack-years from menarche to FFTP (1.73 [1.29-2.32] for every increase of 20 pack-years) while pack-years smoked after menopause were associated with a significant decrease in breast cancer risk (HR = 0.53, 95% CI: 0.34-0.82 for every increase of 20 pack-years). Our results provide an important replication, in the largest cohort to date, that smoking (passively or actively) increases breast cancer risk and that smoking between menarche and FFTP is particularly deleterious.

Recent cohort studies suggest that increased breast cancer risks were associated with longer smoking duration, higher pack-years and a dose-response relationship with increasing pack-years of smoking between menarche and first full-term pregnancy (FFTP). Studies with comprehensive quantitative life-time measures of passive smoking suggest an association between passive smoking dose and breast cancer risk. We conducted a study within the European Prospective Investigation into Cancer and Nutrition to examine the association between passive and active smoking and risk of invasive breast cancer and possible effect modification by known breast cancer risk factors. Among the 322,988 women eligible for the study, 9,822 developed breast cancer (183,608 women with passive smoking information including 6,264 cases). When compared to women who never smoked and were not being exposed to passive smoking at home or work at the time of study registration, current, former and currently exposed passive smokers were at increased risk of breast cancer (hazard ratios (HR) [95% confidence interval (CI)] 1.16 [1.05-1.28], 1.14 [1.04-1.25] and 1.10 [1.01-1.20], respectively). Analyses exploring associations in different periods of life showed the most important increase in risk with pack-years from menarche to FFTP (1.73 [1.29-2.32] for every increase of 20 pack-years) while pack-years smoked after menopause were associated with a significant decrease in breast cancer risk (HR = 0.53, 95% CI: 0.34-0.82 for every increase of 20 pack-years). Our results provide an important replication, in the largest cohort to date, that smoking (passively or actively) increases breast cancer risk and that smoking between menarche and FFTP is particularly deleterious.