The Capital Region (RegionH) with Copenhagen and neighboring areas includes nearly one-third of the Danish population. The roll-out of breast screening in the entire RegionH took place in 2008–2009 [9]. At the implementation of the program, all women aged 50–69 years were personally invited if they lived in the region at the time when women with their respective birth-month were invited. Women were re-invited in the next invitation round, provided they still lived in the region, were 50–69 years, were not in treatment/control for breast cancer (which would normally last 18 months after the diagnosis of breast cancer), had no mammography within the last 6 months, and had not asked the screening center to be exempted from invitation. Women turning 50 and women aged 50–69 moving to RegionH were invited to screening according to their birth-month. From 2018 onwards, women with previous breast cancer were furthermore invited up to the age of 79 years. Time and screening location were indicated in the invitation letter, but changeable by web- or telephone contact to the screening center. Non-responding women were sent one reminder. The screening program was organized in invitation rounds of approximately two years, Supplementary Table 1.
For the present study we retrieved data from 2008 to 2020 from the RegionH mammography screening database. For each woman, the data retrieved included the unique Danish personal identity number, date of birth, invitation dates, and participation defined as a mammography after an invitation date and before the next invitation date or two years.
Some women included in this dataset will have been close to the upper end of screening age in 2008, while other women will recently have entered the dataset when they turned 50 years. Depending on their age at entry, the women will therefore have been eligible for screening between 1 and 6 times. Furthermore, after one invitation a woman could inform the screening center that she did not want further invitation. In order to take these two limitations into account, we restricted the analysis to women who had been eligible for at least three consecutive invitation rounds. Our analysis therefore covered women who turned 50 years before 17 November 2016 (end of fourth invitation round) and who were not older than 62 years on 9 January 2008 (start of first invitation round). A woman in this population was defined as participant in screening if she had at least one participation registered during the period 2008–2020, and as non-participant if she had no participation registered.
The women were stratified by age 50–54, 55–59, and 60–62 at first invitation after start of the RegionH program. Sociodemographic data defined by status at the date of first invitation were retrieved from registers in Statistics Denmark. Marital status was divided into married/living in registered partnership or not, including divorced, separated, widowed, or single. Birthplace as being in Denmark or elsewhere. Occupation was divided into “self-employed persons”, “employees”, “retired persons”, and “others” including other economically inactive women.
Education was highest achieved education, and it was divided into “low education” including for instance unskilled workers and assistant nurses; “short term education” including short sales and administration education; “professional bachelor education” including nurses and teachers ; “academics” including persons with at least a master´s university education; and “missing” including mostly immigrants with no education registered in Denmark. Data on hospital contacts including diseases in the Charlson co-morbidity index; for codes see Supplementary Table 1, were retrieved for the period 2003–2020 and classified into 0 or 1 + contact within five years prior to first invitation.
Crude, age-adjusted and mutually adjusted ORs for non-participation vs participation by independent variables were calculated with 95% confidence intervals (CIs). SAS 9.4 was used for statistical analyses, Copyright (c) 2016 by SAS Institute Inc., Cary, NC, USA.
Approval by Faculty of Health and Medical Sciences, University of Copenhagen, Ref. no.: 514 − 0238/18-3000, served as ethical clearance. According to Danish legislation, informed consent is not required for register-based studies without contact to patients, relatives and/or treating physicians.