Mammogram benefits are long-lived and increase with time

A substantial relative benefit of regular mammograms on breast cancer mortality persists for as long as 29 years after the start of screening, and the absolute number of breast cancer deaths prevented increases with follow-up time, the long-running Swedish Two-County Trial of mammographic screening shows.

The study randomized 133,065 women, 40 to 74 years of age, to receive either an invitation to breast cancer screening or usual care. The screening phase of the trial lasted about 7 years. Women between 40 and 49 years of age were screened every 24 months on average; women between 50 and 74 years of age were screened at average intervals of 33 months.

Trial data published in 1985 showed a significant 30% decrease in deaths from breast cancer among women in the screening group, an effect that has held steady in subsequent years. Moreover, after 29 years of follow-up, the longest of any breast screening trial, the researchers observed that the absolute number of breast cancer deaths prevented increases with each year of screening.

Researchers found that 1 death was prevented for every 414 or 519 women screened (depending on case status and cause of death source used) over a 7-year period-or 34 or 42 years of life per 1,000 women. “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 1 life,” they write, adding that most of the prevented deaths would have occurred longer than 10 years after the start of screening.

Because the observed number of prevented deaths increases with longer follow-up time, “evaluation of the full impact of screening, in particular estimates of absolute benefit and number needed to screen, requires follow-up times exceeding 20 years,” the researchers note.

The study was published online June 15 in Radiology.