Letter to the Editor

Comments to the letters by Per-Henrik Zahl and Jan Maehlen and by Peter C. Gotzsche concerningour article: Increased incidence of invasive breast cancer after the introduction of service screeningwith mammography in Sweden

Hakan Jonsson*, Robert Johansson and Per Lenner

Department of Radiation Sciences, Oncology, Umea University, Sweden

Dr. Gotzsche questions our statement that the randomizedtrials are less well suited for estimation of the level of over-diagnosis. In fact, we wrote that ‘‘in general’’ the trial time,i.e., the time the study group was invited to screening and thecontrol group not invited, was too short in the Swedish trialsand elsewhere to study the incidence when it has stabilizedafter the start of screening. We agree that the Malm€o trial hada trial time long enough to study the stabilized incidence.However, in the Canadian trial, only 5 annual mammographyexaminations were accomplished for the invited group.1

Gotzsche gives a relative risk figure for incidence in thetrials at Canada and Malm€o.2 However, there are some reasonswhy this figure is not comparable with ours. The relativerisk incidence figures were based on the first 7 and 8.8 yearsof follow-up. In our paper, we analyzed the incidence 7–15years after the screening started. The first round was includedin their analysis. The objective of mammography screening isto detect cancer early, which will lead to an increased inci-dence in the first round. It is, therefore, misleading to includethe first round when studying incidence/overdiagnosis. Nonin-vasive cancer was included in the Malm€o trial,3 while it wasnot in our study. Noninvasive cancer is more frequentlydetected in screening.

We acknowledge the comment by Dr. Zahl and Prof. Maeh-len about spontaneous regression. They made a comparisonbased on our figures, which showed that only a minor part(27%) of the difference between a screened and unscreenedpopulation can be explained by slow growing cancers. How-ever, we believe that the figure is somewhat underestimated.There are two reasons for this. The attendance can be lower inwomen aged 70–74 years than in younger women. We do notknow the attendance in all 11 counties in our study, but for thethree counties where we know the figures, the attendance was7–8% lower in 70–74 years than in 50–69 years. Women whohad screen-detected tumor in age 50–69 that hypotheticallywould have been clinically detected after 70 years of age with-out screening and died before 70 years contribute to the figure1648 but not to the figure 452. The probability of deathbetween 50 and 69 is 10–15%.

However, these remarks can only explain a minor part of theremaining cancers, and we agree that this may indicate thatspontaneous regression may occur. However, the interpretationhas to be careful because the estimation of expected incidencewas based on historical data and we cannot exclude a break inthe incidence trend because of changes in background factorscontemporaneous with the introduction of service screening.

References

1. Miller AB, To T, Baines CJ, Wall C. Canadian National BreastScreening Study-2: 13-year results of a randomized trial in womenaged 50–59 years. J Natl Cancer Inst 2000;92:1490–9.

2. Gotzsche PC. On the benefits and harms of screening for breast can-cer. Int J Epidemiol 2004;33:56–64; discussion 69–73.

3. Andersson I, Aspegren K, Janzon L, Landberg T, Lindholm K, Linell

F, Ljungberg O, Ranstam J, Sigfusson B. Mammographic screening

and mortality from breast cancer: the Malmo mammographic screen-

ing trial. BMJ 1988;297:943–8.

*Correspondence to: Oncological Centre, Umea University Hospital,S-901 85 Umea, Sweden. Fax:146-90-127464.E-mail: hakan.jonsson@oc.umu.seReceived 12 September 2005; Accepted 13 September 2005DOI 10.1002/ijc.21633Published online 13 December 2005 in Wiley InterScience (www.

interscience.wiley.com).

Int. J. Cancer: 118, 2649 (2006)

' 2005 Wiley-Liss, Inc.

Publication of the International Union Against Cancer