Researchers at The University of Texas M. D. Anderson Cancer Center found that women whose breast cancer was detected by screening mammography had a significantly better prognosis than those whose cancer was found another way - even if the cancer had already spread to their lymph nodes.
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, say the researchers.
The findings are published in the Journal of the National Cancer Institute.
The study is important because the survival benefit seen in this analysis is much greater than one would expect for screen-detected breast cancer, says the study's lead author Donald Berry.
" 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 says.
Based on the results, Berry suggests that method of detection should be considered when a treatment plan for newly diagnosed breast cancer is being devised, and that this information also should be collected by researchers conducting clinical trials of experimental therapeutic strategies.
" The important message here for clinicians and patients is that breast cancer detected through mammography has a substantially better survival prognosis," he says. " 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," Berry says.
" The paradox is that this result does not mean screening is beneficial," he says. " Without screening, some of the women would not have been diagnosed with breast cancer at all, and in that group, some of them could have avoided surgery and treatment without detriment. The rub is that we don't know which ones they are."
This issue has long plagued screening mammography, especially when the detected tumors are very small and have not spread. But this study appears to add a new element to the debate, Berry says.
" Our conclusions apply generally, and are as important in node-negative breast cancer as they are in node-positive disease," he says.
In this 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 is the time between when the tumor was detected by mammography and when the tumor would have been detected in the absence of screening. 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.
If lead-time bias was responsible for improved survival, and if it was eliminated from the screening mammography group, then those patients should have the same survival, statistically, as women whose cancer was detected outside of mammography, the researchers say.
But that is not what they found.
Instead they discovered 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 to screened patients.
" What is new here is that we found an effect that is beyond stage shift," Berry says. "All breast oncologists know that tumors detected by mammography have a better survival than tumors detected otherwise because they are smaller and more likely to be node-negative. Our study shows that these patients are even better off than their clinical characteristics suggest."
The difference likely is due to what is called a "length bias" which occurs because screening detects disproportionately more slowly growing tumors, the researchers say.
This bias statistically incorporates biological characteristics of the tumors which have not yet been discovered, or cannot yet be easily diagnosed, Berry says." If we were able to get down to the molecular level to assess just the right factors at play in cancer development and progression, that might tell us how lethal a tumor is, but we can't do that right now."
Source: University of Texas M. D. Anderson Cancer Center, 2005