Radiology: Mammo screening saves lives, 29-year study shows
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To estimate the 29-year relative and absolute effects of screening mammography on mortality, researchers leveraged data collected from the Swedish Two-County Study, which randomized women into two groups. The active study population (ASP) of 77,080 women was invited to undergo one-view screening mammography and the passive study population (PSP) of 55,985 women received usual care.
The trial launched in 1977 in Dalarna and 1978 in Ostergotland, Sweden, with the screening phase lasting seven years. Women age 40 to 49 were invited to screening every 24 months on average, whereas those age 50 to 74 were invited every 33 months on average.
Initial mortality results published in 1985 showed a 30 percent reduction in mortality among women invited to screening.
In the current analysis, researchers used two sets of results: those from the original local trial endpoint committee and those from an overview consensus committee established by the Swedish Cancer Society.
Local endpoint committee data showed that 35 percent of tumors in the ASP were symptomatic over the maximum 29-year follow-up, according to Laszlo Tabar, MD, from the departments of mammography, surgery and pathology at Falun Central Hospital in Falun, Sweden, and colleagues. However, these tumors contributed to 53 percent of cancer deaths.
In the ASP group, 1,426 cancers were detected and 351 women died from breast cancer. In the PSP group, 1,042 cancers were detected and 367 women died from breast cancer.
Overview committee data showed differences of less than 10 percent for the numbers of cases and deaths.
Referring to local endpoint committee data, and “[taking] into account the different sizes of the ASP and PSP groups, there was a highly significant reduction in breast cancer mortality in the population invited to screening,” wrote Tabar and colleagues, who calculated that the results translate into 42 years of life saved per 1,000 women invited to screening.
The mortality effect observed with consensus data was less, but still highly significant, and showed an estimated years of life saved of 34 per 1,000 women invited to screening.
The researchers stressed that only 45 percent of prevented breast cancer deaths were observed in the first 10 years and that most prevented deaths would have occurred, in the absence of screening, after the first 10 years of follow-up.
This finding has two major implications, continued the researchers. Some breast cancers would remain asymptomatic for several years and would take some years after symptoms appear to lead to death, and “long-term follow-up is necessary for considerably more than 10 years to estimate the absolute effect on clinical outcome.”
Tabar et al continued, “One breast cancer death was prevented for each 414 women (local committee data) or 519 women (consensus data) screened for seven years. Had the screening continued for 10 years, with the same benefit per screening episode, the absolute benefit would have been higher, with approximately 300 women needed to screen to save one life.”
In addition, the researchers acknowledged the differences between the study design and clinical practice in the U.S., where two-view mammography and shorter intervals represent the standard of care. They suggested that two-view mammography and shorter intervals could have yielded a larger impact on breast cancer mortality.
“In conclusion,” Tabar and colleagues wrote, “the results of the Swedish Two-County Trial of mammographic screening are qualitatively the same at 29-year follow-up as when they were first published: A substantial and significant reduction in breast cancer mortality was associated with an invitation to screening.”