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Journal of Clinical Oncology, Vol 22, No 22 (November 15), 2004: pp. 4652-4653 © 2004 American Society of Clinical Oncology. DOI: 10.1200/JCO.2004.04.156
Mammographic Screening Is Dramatically Changing Age-Incidence Data for Breast Cancer
Division of Molecular Genetic Epidemiology, German Cancer Research Center (DKFZ), Heidelberg, Germany; and the Department of Biosciences at Novum, Karolinska Institute, Huddinge, Sweden To the Editor: Population screening for a cancer may cause major changes in the total and age-specific incidence of that cancer, which has been witnessed for cervical cancer.1 If the risk factors of a cancer remain constant, an effective screening should reduce its incidence. However, at least initially, the detection of asymptomatic tumors may cause an increase in incidence in the screened population, a "screening effect," which may either be due to an earlier diagnosis of tumors, a lead time shift, or to a diagnosis of tumors that would never have been detected—a true overdiagnosis.2 In the course of introduction of screening technologies for breast cancer, these issues have been of focal interest and the subject of an unsettled debate.2,3 During the course of 23 years, Sweden implemented a nationwide mammographic screening, in which 81% of the eligible women have participated.4 Invitation of women to the screening has covered age groups of 40 to 74 years in counties with the widest range, and 50 to 69 years in those with the narrowest range; the screening intervals have ranged from 18 to 24 months.4 Because of this pioneering in national implementation of mammographic screening, the success of the Swedish experience has been followed worldwide.2 A number of studies have evaluated the effects of screening, the results of which are outside the scope of this study.2,3,5,6 Instead, we want to analyze the effects of the nationwide screening activity on the age-incidence relationships of breast cancer. We collected data on breast cancer from the nationwide Swedish Cancer Registry, in which all cases are medically verified, and close to 100% of breast tumors are histologically confirmed; annual numbers of breast cancers ranged from 2,637 during 1961, to 6,623 in 2002.7 Figure 1 shows the age-specific incidences of breast cancer between years 1961 and 2000 in five periods, reflecting the country-wide introduction of mammographic screening. Also, the last available year, 2002, is included. In the prescreening period (1961 to 1975), only one county had introduced a pilot screening activity, followed by four other counties in 1976 to 1985. A national screening activity was started in year 1986, and it extended to a complete national coverage of 26 counties by the year 2000.4,5 A drastic increase in the incidence is evident in the age group 50 to 69 years, which was the primary target group for screening. The increase in this age group has caused a shift in the maximal age for breast cancer from older than 75 years in 1976 to 1985, to 60 to 64 years in 1996 to 2000. In the two latest periods, a small decrease in the incidence can be noted at ages older than 70 years, which would be in line with the compensatory fall in incidence due to screening. However, the magnitude of this decrease cannot be assessed because the incidence of breast cancer increased even before screening, and the trend probably continued but it was masked by screening.7 Somewhat enigmatic is also the increase in incidence after full national coverage of the screening program. One explanation is the ever increasing use of imaging techniques as "opportunistic" clinical tools.7
The present data show the large increase in the incidence of breast cancer in the screened population of women. A proportion of the increase is undoubtedly due to changing risk factors related to fertility and child bearing (early menarche, late first pregnancy, low parity), but the main part is likely to be due to mammographic screening, used by a majority of the target population of women. Even though there is no noticeable decrease in incidence at older age, which would be predicted by an effective screening, there is no increase either. As such, these types of ecological data do not allow conclusions about the benefits of screening, particularly, because we do not know how the incidence would have changed without screening. However, a drastic remodeling of the age-incidence relationship in breast cancer has taken place, and the change is likely to continue. Undoubtedly, opportunistic screening will have similar effects, which will be noted in all developed countries when the proportion of screened women will increase. Breast cancer will definitely become a disease of middle-aged women. Authors' Disclosures of Potential Conflicts of Interest The authors indicated no potential conflicts of interest.
Acknowledgment Supported by Deutsche Krebshilfe and the Swedish Cancer Society. REFERENCES 1. World Cancer Report. Lyon, France, IARC Press, 2003 2. IARC Handbooks of Cancer Prevention: Breast Cancer Screening. Lyon, France, IARC Press, 2002
3. Zahl PH, Strand BH, Maehlen J: Incidence of breast cancer in Norway and Sweden during introduction of nationwide screening: Prospective cohort study. BMJ 328:921–924, 2004
4. Olsson S, Andersson I, Karlberg I, et al: Implementation of service screening with mammography in Sweden: From pilot study to nationwide programme. J Med Screen 7:14–18, 2000 5. Jonsson H, Tornberg S, Nystrom L, et al: Service screening with mammography in Sweden: Evaluation of effects of screening on breast cancer mortality in age group 40–49 years. Acta Oncol 39:617–623, 2000[CrossRef][Medline] 6. Duffy SW, Tabar L, Chen HH, et al: The impact of organized mammography service screening on breast carcinoma mortality in seven Swedish counties. Cancer 95:458–469, 2002[CrossRef][Medline] 7. Cancer incidences in Sweden 2002. Stockholm, Sweden, National Board of Health and Welfare (Socialstyrelesen), 2003
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Copyright © 2004 by the American Society of Clinical Oncology, Online ISSN: 1527-7755. Print ISSN: 0732-183X
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