THE SWEDISH TWO-COUNTY TRIAL TWENTY YEARS LATER: Updated Mortality Results and New Insights from Long-Term Follow-up
Section snippets
DESIGN AND METHODS
The design of the Two-County study has been described repeatedly in previous publications.6, 8, 9, 10 The trial was a cluster-randomized controlled trial taking place in two counties in Sweden, Kopparberg (W-county), now called Dalarna, and Östergötland (E-county). The trial randomized 77,080 women aged 40 to 74 years to invitation to screening (active study population [ASP]), and 55,985 to no invitation (passive study population [PSP]). Screening began in the ASP in 1977. On establishment of a
RESULTS: MORTALITY AND SCREENING PERFORMANCE
Figure 1 shows the breast cancer mortality in the ASP and PSP. At 20 years follow-up, there is a significant 32% reduction in mortality associated with invitation to screening (relative risk [RR]=0.68, 95% confidence interval [CI] 0.59 to 0.80, P<0.001). The corresponding numbers of cancers, deaths, and person-years are given in Table 1. Figure 2 shows the cumulative mortality further stratified by age group and county. The largest effect on mortality can be seen at ages 50 to 69. Results for
RESULTS: SCREENING, HISTOLOGY, AND PROGNOSIS
Figures 3 to 10 show survival by histologic type, size, and node status. Figure 11 shows survival by size in those of any histologic type that did not receive axillary dissection. From this, one can categorize tumors for each histologic type as having good, intermediate, and poor prognosis. These are summarized in Table 4. Figure 12 shows the survival over time by prognostic group for all histologic types combined. Clearly, the goal of screening is to shift the balance of the distribution from
RESULTS: MAMMOGRAPHIC FINDINGS AND PROGNOSIS OF SMALL TUMORS
The criteria shown in Table 4 illustrate the importance of tumor size for survival. Figure 17 shows survival by tumor size in the invasive tumors in our population. The effect of size on survival is very strong, as one expects with increasingly poor survival with increasing size. From Figures 4 to 6 it can be seen that this dependence of survival on size is particularly strong for ductal carcinoma. Despite the high survival rates among small tumors, there are still some tumors of size 1 to 9 mm
RESULTS: DIAGNOSIS AND DISCRIMINATION OF SMALL BUT HIGH-RISK TUMORS
Mammography is clearly a very useful tool, not only for early detection of cancers but also for successful discrimination between the highly fatal and nonfatal cancers. The four mammographic prognostic features are (Figs. 20 to 23):
- 1
Spiculated tumor mass with no associated calcifications.
- 2
Circular- or oval-shaped tumor mass with no associated calcifications.
- 3
Spiculated or circular- or oval-shaped tumor mass, associated with either pleomorphic-heterogeneous-granular–type or
Updated Mortality Results and Screening Measures
Our results show a 32% reduction in breast cancer mortality that is maintained 20 years after randomization and around 13 years after the end of the screening phase of the trial. That is, the ASP continues to have lower mortality from breast cancer than the PSP despite the fact that for the last 13 years there has been no difference in intervention applied to the two groups. Considered another way, screening in 1977 to 1985 was continuing to save lives in 1998. The absolute costs have remained
SUMMARY
The benefit of invitation to mammographic screening observed in this trial is maintained as a highly significant 32% reduction in breast cancer mortality. Mammographic screening for breast cancer continues to save lives after up to 20 years. Screening derives this benefit by improving the distribution of tumors diagnosed with respect to prognostic categories based on node status, size, and histology of tumors. There is potential for modern screening programs with shorter interscreening
ACKNOWLEDGMENTS
We thank the American Cancer Society and the Swedish Cancer Society for financial assistance with this research.
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Address reprint requests to László Tabár, MD, Mammagraphy Department, Central Hospital, S-79182, Falun, Sweden