For years, women have been urged to get screened for breast cancer because the earlier it's found, the better. Now researchers are reporting more evidence suggesting that's not always the case.
A study in Norway estimates that between 15 and 25 percent of breast cancers found by mammograms wouldn't have caused problems during a woman's lifetime but are treated anyway. Once detected, early tumors are surgically removed and sometimes treated with radiation or chemotherapy because there's no sure way to figure out which are dangerous and which are harmless.
"When you look for cancer early and you look really hard, you find forms that are ultimately never going to bother the patient," said Dr. H. Gilbert Welch of the Dartmouth Institute for Health Policy and Clinical Practice, who was not part of the research. "It's a side effect of early diagnosis."
The study is the latest to explore overdiagnosis from routine mammograms -- finding tumors that grow so slowly or not at all and that would not have caused symptoms or death. Previous estimates of the problem have varied.
The researchers took advantage of a staggered decade-long introduction of a screening program in Norway, starting in 1996. That allowed them to compare the number of breast cancers in counties where screening was offered with those in areas that didn't yet have the program. Their analysis also included a decade before mammograms were offered.
They estimated that for every 2,500 women offered screening, one death from breast cancer will be prevented, but six to 10 women will be overdiagnosed and treated.
Study leader Dr. Mette Kalager and other experts said women need to be better informed about the possibility that mammograms can pick up cancers that will never be life-threatening when they consider getting screened. The dilemma is that doctors don't have a good way of telling which won't be dangerous.
"Once you've decided to undergo mammography screening, you also have to deal with the consequences that you might be overdiagnosed," said Kalager, a breast surgeon at Norway's Telemark Hospital. Kalager and her colleagues looked only at invasive breast cancer. The study did not include DCIS, or ductal carcinoma in situ, an earlier stage cancer confined to a milk duct.