Breast cancer incidence trends in Norway-explained by hormone therapy or mammographic screening?

A decline in breast cancer incidence has been observed in several countries after 2002. Reduced use of menopausal hormonal therapy (HT), as a consequence of the publication of results from the Women's Health Initiative, has been argued to be the main reason. In Norway, the governmentally funded Norwegian Breast Cancer Screening Program (NBCSP) was implemented during the same time period as the increased use of HT. This study investigated trends in breast cancer incidence by use of HT and introduction of the screening program. We obtained rates of breast cancer from the Cancer Registry of Norway and sales data of HT preparations from the Norwegian Institute of Public Health. Mammography rates were estimated from published reports. Breast cancer incidence rates increased steadily from 1956 to the end of the 20th century, particularly in women aged 5569 during 19962002 residing in the counties where the NBCSP was first introduced. The rates declined after 20022003. HT use increased in 19872001, peaking around year 2000. In particular, sales of combined estrogen and progestogen preparations declined after 2002. Among women aged 5559, rates of hormone receptor positive breast cancers peaked in 20002003. No such trend was seen in other age groups. In conclusion, the interpretation of breast cancer incidence trends in Norway from 1987 to 2009 is complicated because the NBCSP was introduced during a period with increasing HT use. Both factors likely contributed to the observed trends, and the role of each may vary across age groups.