The participation rate of 77.8 % in the French-speaking region of Switzerland lies within the range of other European countries’ screening uptake of 55 to 90 % . The much lower mammography attendance in the German-speaking region of Switzerland (34.9 %) confirms previous observations that women are more likely to adhere to BC screening in countries with screening programs than in those with opportunistic screening [15, 21]. Factors associated with non-attendance to BC screening might differ between opportunistic and organized BC screening programs [12, 14, 15] and the question arises whether organized screening programs “rescue” distinct subgroups of women who fall through the opportunistic screening net? In the present study, we did not find any socio-demographic, lifestyle or health factors, which could explain the large difference in BC screening attendance between the French- and the German-speaking regions of Switzerland. None of the interaction terms were statistically significant.
In contrast to previous observations demonstrating that a socio-economic gradient is evident in BC screening attendance in opportunistic but not in population-based programs , in the present study, neither attendance to opportunistic nor organized BC screening were associated with educational level, nationality, living in urban or rural areas or marital status. Previous studies observed that women with higher educational levels reported greater screening participation [11, 22], although not consistently so [23–25]. It should be noted that in Switzerland, all residents are obliged by law to have a health insurance, thus, BC screening costs are basically covered . This might explain why education, as potential indicator of the women’s socioeconomic status, was not associated with BC screening attendance in the present study. In many studies [11, 27, 28], marital status was associated with higher attendance to BC screening. Partners seem to encourage each other towards health conscious behaviors . A survey carried out in Geneva, Switzerland, found that men had more favorable attitudes towards BC screening than women . Living with a partner, but not being married or registered (registration is in Switzerland only an option for couples of the same sex), is considered in the Swiss Health Survey as being single. This may explain why we did not observe an association between marital status and BC screening attendance.
In our study, additional socio-demographic factors such as nationality and living in a rural or urban area also had no effect on screening attendance rates in both regions. Lower screening participation in urban areas has previously been reported from various European countries [25, 31, 32]. The different demographic composition of urban and rural areas in relation to migrants, education and economic level of the inhabitants may contribute to such differences . Participation to the Swiss Health Survey was limited to German-, French- or Italian-speaking individuals. Thus, the group of immigrants in the present study is restricted to those who speak German, French or Italian. It does therefore not reflect reality in Switzerland in relation to the various immigrant groups living in this country, and may have excluded minorities prone to non-attendance of BC screening [11, 33]. Using more detailed data of the 2002 SHS, attendance to BC screening was higher in Swiss women than in women from Italy, former Yugoslavia, Portugal and Spain living in Switzerland .
In relation to lifestyle factors, represented by smoking habits, alcohol intake, physical activity and attention to diet, only moderate alcohol consumption in the French-speaking region showed an association with BC screening; however, no statistically significant interactions between the two regions were observed. There is some evidence from other studies that former smokers have a higher attendance to BC screening [35, 36], but many previous studies failed to distinguish between former smokers and those who had never smoked. As Vander Weg  pointed out, former smokers may see themselves with a higher cancer risk due to their smoking habits in the past and, therefore, may be more motivated to adhere to screening recommendations than non-smokers. The findings of lower rates of mammography among current smokers, which have been rather consistently shown in other studies [7, 35, 36], were not confirmed in the present study and in another survey .
In our study, alcohol consumption had no effect on the rates of having had BC screening with the exception of higher BC screening attendance of women from the French-speaking region who reported drinking moderate amounts of alcohol. Results from other studies [7, 28] indicate a J-shaped association between alcohol consumption and BC screening use, where both abstainers and heavy drinkers have lower attendance than women drinking modest amounts. The absence of an association between high and low alcohol intake and mammography attendance in our study may be due to information bias and a lower participation rate of heavy drinkers in research studies [7, 37].
In relation to physical activity, our findings are not in line with the results of previous studies, in which non-attendance to BC screening was associated with lower leisure time physical activity . There are too few studies analyzing the association between nutritional habits and BC screening attendance to come to a meaningful conclusion [38, 39].
Impaired overall health and more frequent contact with medical doctors, respectively, on the other hand, may have implications for BC screening. Studies have shown that factors such as diabetes, cognitive decline, and depression might negatively affect the receipt of cancer screening [8, 22, 28]. The presence of several comorbid conditions might increase the opportunity of receiving cancer screening or diagnosis, however, because of more frequent contact with medical doctors. This could be an explanation for the positive association observed between visits to a physician (compared to none) and higher attendance to BC screening in the present and other studies .
Schumacher et al.  observed in their Education and Research Towards Health study that those women who had had other screening tests (cervical and colorectal cancer screening) were much more likely to have received mammography in the past two years. This was true for opportunistic and organized screening, as in the present study. Similar results were seen for organized BC screening in a study carried out in Geneva . Individuals who participate in multiple cancer screenings may be more health-conscious, and/or more knowledgeable about cancer screening tests than those who do not [9, 28, 42]. Those with low attendance to screening tests, on the other hand, may be more critical about the benefits of the screening, including anxiety related to false positive results  and inaccurate knowledge and negative mammography beliefs such as that mammography may be harmful, is only needed when symptoms are present, is not necessary, is painful, etc. .
Study strengths and limitations
One strength of our study is the large representative sample of individuals 15 years and older living in Switzerland, allowing for limitation of our analyses to women aged 50–69 years. This study also has several limitations. It is a cross-sectional survey, thus, causality cannot be inferred. In addition, data were self-reported. Women, for example, tend to overestimate their attendance to cancer screening according to the recommendations . This could result in reporting and/or recall biases. Furthermore, 46 % of eligible subjects did not participate in the SHS survey. However, we hypothesize that with respect to mammography attendance rates our survey cohort represents the general population very well because the attendance to the mammography screening programs in the French-speaking cantons were similar to the official data of Swiss Cancer Screening in 2011 (51 %) . In addition, the use of weighting factors allowed for the extrapolation of the results in relation to age, gender, region and nationality from the sample to the total population living in Switzerland . Furthermore, attendance to opportunistic or organized screening was not clear-cut in the present study, i.e. in the German- and in the French-speaking regions, 10.1 % and 55.1 %, respectively, had a mammography in the context of a screening program, i.e. in the German-speaking region 16.8 % attended mammography on their own initiative, 73.1 % followed the advice of a medical doctor (French-speaking region: 8.5 % and 36.5 %, respectively). Women with a personal or family history of breast diseases might undergo regular radiologic breast imaging and thus, do not attend a screening BC program. In particular, women who had mammography in previous years in private radiology practices might prefer to continue these examinations in this trusted setting.
Finally, other factors that may be significant determinants of BC screening utilization have not been recorded in the current study [20, 21, 42, 45], such as promotion of BC screening by the government, cancer leagues, medical organizations and lay press. Furthermore, personal or family BC history, knowledge and beliefs in the benefits of screening, and attitude of gynecologists and other medical doctors [46, 47] towards BC screening may be of importance.