The routine use of mammography to screen healthy women for breast cancer is leading to the widespread detection and treatment of tumours that would never have caused symptoms, a study in the New England Journal of Medicine asserts today1. The results inject yet another dose of controversy into an area that is already hotly debated. The study examined the effects of mammography screens on breast-cancer incidence between 1976 and 2008 in US women over 40. The authors conclude that more than one million women diagnosed with the disease would never have developed symptoms. In 2008 alone, they estimate, more than 70,000 women had such breast tumours diagnosed, accounting for 31% of all breast cancers diagnosed in women 40 and older. “Our study raises serious questions about the value of screening mammography,” the paper concludes. “The harm of overdiagnosis [is] probably larger than has been previously recognized.” The diagnosed women, the authors add, have undergone treatments involving surgery, radiation, hormones and chemotherapy “for abnormalities that otherwise would not have caused illness”.
…The study authors — oncologist Archie Bleyer of the St Charles Regional Cancer Center in Bend, Oregon, and H. Gilbert Welch, a professor of medicine at Dartmouth College’s Geisel School of Medicine in Hanover, New Hampshire — examined the change in incidence of early-stage and late-stage breast cancer after large numbers of women began to have screening mammograms in the mid-1980s. As expected with the advent of a widespread screening programme, they found that diagnoses of early-stage disease more than doubled during the three decades, with an increase of 122 cases for every 100,000 women. The authors argue that if the screening was working as intended and stopping those cancers from progressing to a more harmful disease, then one would expect to see a roughly equivalent decrease in late diagnoses. Instead, they found that, assuming a steady underlying rate of breast cancer incidence, the number of late-diagnosed cancers decreased by only 8 cases per 100,000 women. Their inference: many of the early cases being detected through screening would not have gone on to cause symptomatic disease. They also conclude that mammography’s contribution to the sharp decline in breast-cancer mortality rates during the same period must therefore be small.