Friday, 26 March 2010

New Gotzsche paper dismissing screening - this time in Denmark

In a paper published in the Research section of the BMJ today, Peter Gotzsche once again lines up the guns against organized screening programs, targeting in this instance the Danish Screening Program.

His conclusions are -
"We were unable to find an effect of the Danish screening programme on breast cancer mortality. The reductions in breast cancer mortality we observed in screening regions were similar or less than those in non-screened areas and in age groups too young to benefit from screening, and are more likely explained by changes in risk factors and improved treatment than by screening mammography
We believe it is time to question whether screening has delivered the promised effect on breast cancer mortality."

As this may hit mainstream news media, I have gathered some responses so far -
Danish and Swedish experts have replied saying that -
They claim that mammography screening in Denmark had no impact on breast cancer mortality. This claim is unsubstantiated, firstly because the authors used very crude data, and secondly because the analysis was not geared to answer the question.

Firstly, breast cancer screening can only possibly have an effect on women not already diagnosed with breast cancer prior to screening. Therefore the so-called “refined mortality” should be used in evaluation of screening. Jørgensen et al did not used refined mortality. Furthermore, they merge data from three screening areas starting screening at different points in time, and used age groups instead of cohorts. Together this gave quite “polluted” data.

Secondly, they calculated “annual change in the relative risk of breast cancer death” by time period and areas excluding 1992-1996. The relevant outcome measure is, however, the change in breast cancer mortality in the screening area controlled for the change in breast cancer mortality in the non-screening area.

Even using these “polluted” data, the relative breast cancer mortality decreased for women aged 55-74 covered by screening, while the relative breast cancer mortality did not decrease for women aged 35-54 largely uncovered by screening, and the relative breast cancer mortality was slightly but statistically non-significantly decreased for women aged 75-84 where the majority, but not all, of the person years were uncovered by screening. Although this pattern in the data is actually visible in Figure 1 in the paper by Jørgensen et al, it was missed in their analysis among other things because they excluded data from the period 1992-1996.

As we have reported previously, the measured impact of mammography screening on breast cancer mortality is highly dependent on the data set used for the analysis. Use of “polluted” data leads to biased estimates (2). Using cohort based refined mortality, we found a 25% decrease in breast cancer mortality in the municipality of Copenhagen during the first 10 years following the introduction of mammography screening in April 1991 (3). We deliberately did not include data from Funen and Frederiksberg in that analysis, as cause of death data were not available at that time for the first 10 years of these two screening programmes.
Other commentators also note -

The analysis of population trends in breast cancer mortality in the presence of screening is complicated by the inability to measure exposure to screening, and the long period of follow-up required. Studies such as this one by Jørgensen et al obscure whatever benefit may be present with crude, insensitive methodology. While we expect to see a range of benefits from mammography, some small and some large, based on the design and quality of the screening program, its duration, and the participation rate of the target population, to argue that there is no benefit from modern mammography on the basis of such flawed methods means this paper contributes nothing of substance to the on-going debate