DÄ internationalArchive23/2008Benefit and Risk of Mammography Screening – Considerations from an Epidemiological Viewpoint: Stage-Dependent Lethality

Correspondence

Benefit and Risk of Mammography Screening – Considerations from an Epidemiological Viewpoint: Stage-Dependent Lethality

Dtsch Arztebl Int 2008; 105(23): 421. DOI: 10.3238/arztebl.2008.0421b

Schwartz, F W

LNSLNS Becker and Junkermann's article on the benefits and risks of mammography screening is heralded on the title page with the words, "Mammography: Screening Lowers Breast Cancer Mortality by 35%." This statement, which is also one of the authors' three main conclusions, is misleading in two respects:

(1) The authors are speaking, not of mortality in the usual epidemiological sense of the specific probability of death in a defined population group, but rather of lethality, i.e., the conditional probability of death over a certain period of observation after a particular diagnosis has been made and (in the normal situation) followed up with treatment (1).

(2) The authors attempt to circumvent the statistical problem of lead-time bias, i.e., earlier diagnosis in the screened than in the unscreened population, by measuring the effect of screening in terms of mortality/lethality. They write: "Mortality is the only variable quantifiable without bias for studies on the effectiveness of early detection activities." Yet the endpoint that they consider, i.e., lethality after the diagnosis of breast cancer of any type, is variable depending on the stage of the tumor (according to the WHO tumor classification) at the time the diagnosis is made. The distribution of cases in the screened population is selected in favor of earlier and prognostically more favorable stages, and a slower course of disease, in comparison to the unscreened population. Thus, a consideration of mortality/lethality after the disease is diagnosed does not eliminate lead-time bias. To do this, the authors would have had to present and discuss stage-dependent lethality rates over a defined period of time. DOI: 10.3238/arztebl.2008.0421b

Prof. Dr. med. Friedrich Wilhelm Schwartz
Direktor des Instituts für Epidemiologie,
Sozialmedizin und Gesundheitssystemforschung
MHH
30623 Hannover, Germany
Epidemiologie@mh-hannover.de