3-D mammograms find more cancers, but do they save lives?

A new study shows that so-called 3-D mammograms find more breast cancer than regular digital mammograms, leading some doctors to predict that the exams will someday become the standard of care.

Other breast cancer advocates caution that doctors still don't know whether finding these extra cancers will actually save lives, however, or simply lead more women to treatment.

The study focused on a relatively new technology called tomosynthesis, first approved in 2011, which provides three-dimensional images of the breast by using a technology similar to CT scans, or computed tomography. The imaging machine moves around the breast in an arc, taking multiple X-rays that a computer forms into a 3-D image.

Doctors get better pictures of the breast, because the machine cuts the images into digital slices, so that doctors can look at the breast as if turning pages in a book, says Emily Conant, chief of breast imaging at the University of Pennsylvania's Perelman School of Medicine.

Her study was the largest of its kind, with 454,850 women in 13 medical practices.

Researchers compared results of women screened in 2010 and 2011 using standard, digital mammograms, with women screened from 2011 to 2012 with a combination of digital mammograms and tomosynthesis.

The combination approach found 41% more invasive breast cancers, or ones that have grown beyond the milk ducts. While standard mammograms detected 2.9 tumors per 1,000 women, the combination found 4.1 cancers per 1,000 women, according to the study, in the Journal of the American Medical Association.

Both approaches found about the same number of non-invasive cancers – ones confined to the ducts – called ductal carcinoma in situ, or DCIS, the study says. While some doctors consider these an early cancer, others say they are less worrisome precancers.

Conant says women screened with tomosynthesis were 15% less likely to need repeat screenings, with 16 fewer repeat exams per 1,000 women. These second screenings can be stressful, as well as time-consuming, because women often must return to the office for a second appointment on another day.

But authors acknowledge that 3-D mammograms have drawbacks, as well. While conventional mammograms are covered by insurance, women often have to pay an extra $50 to $100 to add the 3-D tests, Conant says. Women are also exposed to twice as much radiation from 3-D mammograms, compared to standard mammograms. That radiation dose may fall in the future, due to improvements in technology.

The absolute differences between the two types of exams, in terms of the number of cancers found, is very small, says Fran Visco, president of the National Breast Cancer Coalition. And other benefits are uncertain.

The study didn't follow women over time. So doctors don't know whether undergoing tomosynthesis actually saves lives, Visco says.

Many cancer researchers estimate that 1% to 10% of breast cancers are "overdiagnosed," or treated even though they would never have become life-threatening. A screening that finds more breast cancers only helps women if the test also reduces their risk of dying, Visco says.

Without following women for many years, doctors also can't tell whether tomosynthesis missed a lot of cancers, according to an accompanying editorial by Etta Pisano of the Medical University of South Carolina and Martin Yaffe of the University of Toronto. In their editorial, they urge the National Cancer Institute to fund a trial to answer these questions.

Even if that trial is conducted, its results would come long after tomosynthesis has hit the market, says the American Cancer Society's Robert Smith. Many breast centers have been heavily marketing the screenings for years.

"The good news is that early data is encouraging," Smith says.