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Archived Comments for: Estimate of overdiagnosis of breast cancer due to mammography after adjustment for lead time. A service screening study in Italy

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  1. Clarification of the Malmö study

    Sophia Zackrisson, Department of Clinical Sciences in Malmö, Lund University, Malmö University Hospital, Sweden

    15 February 2007

    Overdiagnosis is a potentially harmful effect of screening. Every effort to assess the magnitude of the problem is essential. Therefore, the paper by Paci et al is welcome.(1) As the validity of the Malmö study of overdiagnosis (2) was questioned we would like to make a few comments.

    Ideally, the rate of overdiagnosis at screening should be estimated by direct observation in a randomised trial where the control group was not invited at the end of the trial and during a long follow-up. This was the case in the Malmö Mammographic Screening Trial (MMST). Most other estimates were based on various sorts of modelling (3) or from non-randomised settings. (1;4-7) In the MMST the control group of the age cohort 55-69 years at entry was never invited. The follow-up comprised 15 years after the end of the trial and 25 years after the start of the trial. At the end of follow-up the incidence was 10% higher in the invited group compared with a control group.(2) There are two mechanisms that can explain the overdiagnosis: the lead time of screening detected cases and death in intercurrent disease. Thus, a long follow-up is necessary to account for the longest lead times. Also, with a long follow-up a substantial proportion of the population will be deceased. Therefore, we do not understand what the authors mean by stating “… as 60% of the women died in the Malmö study, there is need to assess the impact of competing causes of death on the overdiagnosis estimate”.

    Not much easier to understand is the subsequent statement: “… women could have mammograms within or outside of the programme… So it is difficult to assess the excess over such a long period of time”. Of course, women in the invited as well as in the control group had mammograms (and cancers detected) after the end of the trial, just as women in the control group also had during the trial. We were studying the impact of mammographic screening in a society with “normal” access to clinical mammography. We assume that the same applies to Italy in the present study by Paci et al and it is unclear in what way this may have affected the estimates.

    Naturally, estimates may vary depending on screening setting and on several factors such as criteria for recall, underlying breast cancer incidence, mortality and age-distribution in the study population etc. What is true for southern Sweden is not necessarily true for northern Italy.

    Sophia Zackrisson, MD, PhD

    Ingvar Andersson, Associate professor

    Lars Janzon, Professor

    Jonas Manjer, Associate professor

    Jens Peter Garne, MD, PhD

    References:

    (1) Paci E, Miccinesi G, Puliti D, Baldazzi P, De L, V, Falcini F et al. Estimate of overdiagnosis of breast cancer due to mammography after adjustment for lead time. A service screening study in Italy. Breast Cancer Res 2006; 8(6):R68.

    (2) Zackrisson S, Andersson I, Janzon L, Manjer J, Garne JP. Rate of over-diagnosis of breast cancer 15 years after end of Malmo mammographic screening trial: follow-up study. BMJ 2006; 332(7543):689-692.

    (3) Duffy SW, Agbaje O, Tabar L, Vitak B, Bjurstam N, Bjorneld L et al. Overdiagnosis and overtreatment of breast cancer: estimates of overdiagnosis from two trials of mammographic screening for breast cancer. Breast Cancer Res 2005; 7(6):258-265.

    (4) Jonsson H, Johansson R, Lenner P. Increased incidence of invasive breast cancer after the introduction of service screening with mammography in Sweden. Int J Cancer 2005; 117(5):842-847.

    (5) Paci E, Warwick J, Falini P, Duffy SW. Overdiagnosis in screening: is the increase in breast cancer incidence rates a cause for concern? J Med Screen 2004; 11(1):23-27.

    (6) Peeters PH, Verbeek AL, Straatman H, Holland R, Hendriks JH, Mravunac M et al. Evaluation of overdiagnosis of breast cancer in screening with mammography: results of the Nijmegen programme. Int J Epidemiol 1989; 18(2):295-299.

    (7) Zahl PH, Strand BH, Maehlen J. Incidence of breast cancer in Norway and Sweden during introduction of nationwide screening: prospective cohort study. BMJ 2004; 328(7445):921-924.

    Competing interests

    None

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