Women whose breast cancer was detected by screening mammography have a significantly better prognosis than those whose cancer was detected by another means even when the cancer is node-positive, according to an article in the August 17 issue of the Journal of the National Cancer Institute.

The findings were identified by researchers at The University of Texas M. D. Anderson Cancer Center who looked at outcomes from randomized screening studies of more than 150,000 women. A likely reason for that finding is that mammography can detect tumors that are both slower growing and less biologically lethal than those found symptomatically.

The study is important because the survival benefit seen in this analysis is much greater than one would expect for screen-detected breast cancer, said study lead author Donald Berry, PhD, chair of the Department of Biostatistics and Applied Mathematics. Berry is well-known for his work in designing breast cancer clinical trials sponsored by the National Cancer Institute and for his research in evaluating the effects of screening mammography.

"We know that screening picks up many tumors before they can be detected in other ways and women may benefit from early treatment, but the advantage we found is much larger than what would be expected from the so-called stage shift that is associated with screening mammography," Berry said.

"The important message here for clinicians and patients is that breast cancer detected through mammography has a substantially better survival prognosis," he added. "Of two women who have the same age, size of tumors, and similar stage of cancer and spread to lymph nodes, the one whose cancer was detected with mammography has a reason to be happier than the woman whose cancer was detected symptomatically."

In the study, researchers examined data from three large randomized breast cancer screening trials - the Health Insurance Plan (HIP) of New York, which assigned about 62,000 women to screening or to a control group, and two Canadian National Breast Cancer Screening Studies (NBSS), which included a total of 44,790 women in the screening groups and 44,961 women in the control groups.

They then looked only at women in these studies who were eventually diagnosed with breast cancer, and adjusted for stage and other tumor characteristics as a way to eliminate what is known as "lead-time bias." Lead-time bias occurs because lead time is added to the survival time of women detected by mammography but not to women whose tumors are detected clinically. "Lead time is an artifact of screening and not necessarily a benefit of screening," Barry said.

Rather than finding lead-time bias, the researchers found that all things being equal, the method of detection was a statistically significant independent predictor of breast cancer survival. After adjusting for stage of disease, patients whose breast tumors were discovered after a previous negative mammography screen had a 53-percent greater risk of death from the cancer than women with screen-detected cancer. Patients in the control group (where no mammography was used) had a 36-percent increased risk of death compared with screened patients.

While researchers consider further the research and clinical implications of the finding, Berry suggested that information on method of detection be collected for every clinical trial in order to improve the accuracy of findings, or at least as a way that might account for unexpected results. "I am always surprised that clinical trials of new agents result in better outcomes than expected, and it may just be that screen-detected breast tumors are becoming more common over time," he concluded.