Radiologists who read a higher number of mammograms find more cancers, according to a report in the Journal of the National Cancer Institute (Vol. 94, No. 5: 369-375).

The researchers also found that the high-volume radiologists were better able to tell when a mammogram is normal.

Screening mammography in the US is often done at special screening centers. But it is also done at local radiologists' offices located in or near where women see their primary doctors.

This means that while the radiologists at the centers are reading a large number of mammograms, those at the local offices are most likely reading fewer.

In the US, a radiologist needs to read 480 mammograms a year to qualify as competent. In other countries such as the United Kingdom, they must read 5,000 a year. Laura Esserman, MD, and her colleagues wondered if this made a difference in the accuracy of the readings.

One clue was that a large number of breast biopsies performed in the US because of suspicious mammograms turned out to be normal. Some groups report only 11% of their biopsies are positive for cancer although others report rates as high as 40%. But in the UK, the positive rate goes up to 60%.

To find out if these rates are caused by the fact that UK radiologists are more accurate because they read more mammograms, Esserman tested the radiologists.

First, they took a group of radiologists in the US who read mammograms and divided them into three groups based on the number they read. The low-volume group read less than 100 per month, the medium group read 100 to 300 per month, and the high volume group read more than 300 per month. They compared these groups with a group of British radiologists.

Each group was given a set of 60 mammograms in which there were 13 cancers. The high-volume radiologists were better at detecting cancers and more accurately judged normal mammograms.

The US and UK high-volume radiologists were equally accurate. But the low- and medium-volume US radiologists failed in both of these categories.

This means that when a low-volume radiologist reads mammograms there is a greater chance that a cancer will be missed. And, a normal mammogram may be called abnormal and force a woman to undergo an unnecessary biopsy. Along with the pain and anxiety they cause, these biopsies also drive up health care costs, experts have said.

According to Esserman, one reason there are so many unneeded biopsies in the US may be radiologists' fear of being sued for malpractice if they miss a cancer. Delayed breast cancer diagnosis is one of the most common reasons for malpractice suits.

Convenience may explain why there are low-volume radiologists. Women might prefer to have the mammograms close to home or work, rather than travel long distances, as they might have to do in the UK.

New technology may help. Digital mammography, which is increasingly being used, will allow the pictures to be transmitted to a central location where high-volume radiologists can interpret the films. Esserman concluded, “What is necessary are creative and new approaches to the implementation of mammography screening."