A mammography screening based cohort? Absolutely!

Since 1994 all women in Sweden, aged 40–74 years, are invited every 18–24 months to the national mammography-screening program. About 80% of eligible women attend screening regularly, and attendance is among the highest recorded in the world.

The Karma project takes advantage of this national tax-funded health care system to recruit participants and continuously collect mammograms. Women were invited when conducting either screening or clinical mammography at any of four large mammography-screening units in Sweden.

The comprehensive design of the Karma study allows easy and continuous collection of follow-up mammograms of all participants attending screening mammography. This simple approach enables us to focus our research on reducing the incidence and mortality of breast cancer by creating individualised screening and prevention strategies.

Establishing the cohort

The prospective cohort study was Karma project was established in 2010 by professor Per Hall at the Karolinska Institute in Stockholm, Sweden. Establishment of the Cohort was made possible through a donation from Märit and Hans Rausing.

Recruitment to the Karma study was conducted at the Karma Test Centres in four hospitals in Sweden: Stockholm South General Hospital in Stockholm, Helsingborg Hospital in Helsingborg, Skåne University Hospital in Lund, and Landskrona Hospital in Landskrona.

All women invited for screening from January 2011 to March 2013, at the four hospitals, were invited to participate in the study. Additionally, women who had a clinical mammography at any of the participating mammography screenings centres during this time were also invited. During the recruitment period, a total of 210,233 women were invited to participate in the Karma study and 70,877 women (34%) joined the study. The majority of the participants were recruited during year 2012.

Locations of the Karma Study centres in Sweden

Collecting personal data

All study participants responded to the Karma online questionnaire upon study entry; of these, 68,426 (97%) participants completed the questionnaire. The comprehensive questionnaire includes more than 250 questions covering breast cancer-related topics, i.e. reproductive health, alcohol and tobacco use, medical history, menopausal status, use of oral contraceptives, hormone replacement therapy and other medications, family history of cancer, quality of life, physical activity and diet. A second follow-up of the web-based questionnaire started in May 2016 and is currently on- going.

To further enrich the data of the cohort, we have used national registers available in Sweden. Matching to the registers is done using Swedish Personal Identity Numbers, a 10-digit number that is unique for every individual and used in all health-based registers. All Karma participants are biannually matched to the registers.

Biological samples in the biobank – the makings of a rich cohort

All Karma participants donated blood upon study entry. In total, blood samples from 69,440 (98%) study participants were collected during the recruitment period. Blood, plasma and DNA is continuously processed and stored in the Karolinska Institutet high-throughput biobank. Approximately 20,000 participants have thus far been genotyped within the Collaborative Oncological Gene-environment Study [COGS] project: [www.nature.com/icogs], in collaboration with the Breast Cancer Association Consortium.