Although breast cancer mortality has fallen in recent years, improvements in treatment rather than mammography screening appear to have affected the decline, according to a study published online July 28 in the British Medical Journal.
The number of deaths from breast cancer has fallen in many developed countries, but it is difficult to determine how much of that reduction over the past 20 years of mammography screening is due to earlier detection or to improved management, according to Phillippe Autier, research director of the International Prevention Research Institute in Lyon, France, and colleagues.
To untangle various causative factors, an international team of researchers from France, the U.K. and Norway mined the World Health Organization (WHO) mortality database to better determine when trends in breast cancer mortality began to change.
The researchers used cervical cancer mortality reductions as a basis for their study. Reductions in cervical cancer mortality were realized earlier in countries (Iceland, Finland) that implemented nationwide screening programs than in those that delayed nationwide screening (Norway). Because access to treatment was similar in the three countries, the researchers attributed the clear differences in mortality trends to time differences in the implementation of screening.
The team used a similar approach to compare trends in breast cancer mortality within three pairs of European countries: Northern Ireland versus the Republic of Ireland, the Netherlands versus Belgium and Flanders, and Sweden versus Norway.
Countries of each pair had similar healthcare services and levels of risk factors for breast cancer mortality, but were different in that mammography screening was implemented about 10 to 15 years later in the second country of each pair. The researchers expected that a reduction in breast cancer mortality would appear sooner in countries with earlier implementation of screening.
The researchers studied data from the WHO mortality database on cause of death covering the period 1980 to 2006 and data sources on risk factors for breast cancer death, mammography screening and cancer treatment.
Results showed that from 1989 to 2006, deaths from breast cancer fell by 29 percent in Northern Ireland and 26 percent in the Republic of Ireland; by 25 percent in the Netherlands, 20 percent in Belgium and 25 percent in Flanders; and by 16 percent in Sweden and 24 percent in Norway.
These trends in breast cancer mortality rates varied little between countries where women had been screened by mammography for a considerable time compared with those where women were largely unscreened during that same period, stated the authors.
“This is in sharp contrast with the temporal difference of 10 to 15 years in implementation of mammography screening and suggests that screening has not played a direct part in the reductions of breast cancer mortality.”
Furthermore, the greatest reductions were in women ages 40-49, regardless of the availability of screening in this age group.
Autier et al identified several strengths to the study, noting that it improves on population level data because the countries were closely matched with the exception of the timing of a nationwide mammography screening program. However, while the authors did assess the potential impact of confounders (and found none), they acknowledged that an unknown confounder might exist.
Recognizing that the length of the observation period could be questioned, the researchers pointed out that an observation period of 18 years should suffice. Twenty years of follow-up data for the Dutch city of Nijmegen, which started screening in 1975, indicated that mortality did not decline earlier than the city of Arnhem or the Netherlands as a whole.
Moreover, Autier et al referred to earlier studies which found that advanced breast cancer incidence, which would be expected to decline prior to a drop in mortality, held steady in Northern Ireland and the Netherlands after screening.
"The contrast between the time differences in implementation of mammography screening and the similarity in reductions in mortality between the country pairs suggest that screening did not play a direct part in the reductions in breast cancer mortality,” the authors concluded.
Autier and colleagues suggested, "It seems more likely that the downward inflexions observed from 1991 to 1996 [in Norway, Belgium, the Republic of Ireland, Northern Ireland and