But the authors found that while deaths have dropped for both cancers over the last 20 years, "the contribution from screening is uncertain.'' They also found that many patients are undergoing treatment from cancers that actually pose minimal risk.
A comparison of prostate cancer incidence rates in the U.S. to the United Kingdom, where PSA screening has not been widely adopted, "did not result in significant differences in mortality,'' the authors write. For breast cancer the relative reduction in deaths from screening has also been limited.
The authors said that breast cancer and prostate cancer screening has not led to a more significant drop in deaths in the U.S. for two primary reasons: Screening increases the detection of slow growing and indolent tumors, and it often misses the most aggressive cancers because many may not be detected early enough for cure.
"In other words, tumor biology dictates and trumps stage, so the basic assumption of these screening programs that finding and treating early stage disease will prevent late stage or metastatic disease may not always be correct,'' they state.
Periodic screening may find some tumors early, but patients may not be screened often enough for lethal tumors to be detected in time to prevent death, the authors conclude: "Without the ability to distinguish cancers that pose minimal risk from those posing substantial risk and with highly sensitive screening tests, there is an increased risk that the population will be over-treated.''
"People will think that we're saying screening is bad, and nothing could be further from the truth,'' said Ian Thompson, MD, who has authored about 400 scientific articles addressing prevention, early detection, and treatment for prostate, kidney, and bladder cancers. "What we are saying is that if you want to stop suffering and death from these diseases,
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