This is a great article why there is misunderstanding by physicians in presenting statistics of Cancer Screening.
By Todd Neale, Senior Staff Writer, MedPage Today
Published: March 06, 2012
Reviewed by Dori F. Zaleznik, MD; Associate Clinical Professor of Medicine, Harvard Medical School, Boston
Many primary care physicians in the U.S. accept misleading statistics
as proof that cancer screening saves lives, a survey showed.
About three-quarters (76%) of respondents incorrectly said that
increased five-year survival and early detection of cancer proves that a
screening test saves lives, according to Odette Wegwarth, PhD, of the
Max Planck Institute for Human Development in Berlin, and colleagues.
That rate was similar to the proportion who correctly stated that a
reduction in mortality in a randomized trial proves the efficacy of a
screening test (81%), the researchers reported in the March 6 issue of
the Annals of Internal Medicine.
Misunderstanding of statistics ... matters, because it may influence
how physicians discuss screening with their patients or how they teach
trainees," the authors wrote.
"To better understand the true contribution of specific tests,
physicians need to be made aware that in the context of screening,
survival and early detection rates are biased metrics and that only
decreased mortality in a randomized trial is proof that screening has a
Although improved survival rates and earlier detection of cancer are
often used to demonstrate the efficacy of screening for cancer, those
measures are subject to lead-time and overdiagnosis biases, according to
Wegwarth and colleagues.
For example, they wrote, in a cohort of individuals who will die at
age 70, the five-year survival rate for those diagnosed with cancer
because of symptoms at age 67 will be 0%, whereas the five-year survival
rate for those diagnosed through screening at age 60 will be 100%.
"Yet, despite this dramatic improvement in survival ... nothing has
changed about how many people die or when," the authors explained.
Similarly, screening that detects cancer that will not ultimately
progress also can inflate survival rates without having any effect on
Mortality rates in a randomized trial, however, are not affected by
these types of biases, and a committee of the National Cancer Institute
concluded that that measure is the only one that can reliably prove that
a screening test saves lives.
To find out whether primary care physicians -- who often recommend
screening tests to their patients -- understand which statistics are
most meaningful, Wegwarth and colleagues conducted an online survey of a
national sample of 412 U.S. physicians.
The physicians were asked general knowledge questions about cancer
screening statistics and were presented with two hypothetical scenarios
based on real-world prostate cancer data.
The first scenario described a screening test that improved five-year
survival from 68% to 99% and increased the early detection of cancer
(considered irrelevant evidence). The second described a screening test
that reduced cancer mortality rate from 2 to 1.6 per 1,000 people
(considered relevant evidence).
The respondents were more supportive of the screening test backed by
the irrelevant evidence, as illustrated by the percentage who said the
evidence proves that the test saves lives (80% for the test backed by
irrelevant evidence versus 60% for test backed by relevant evidence, P<0.001).
When presented with the irrelevant evidence of improved five-year
survival, 69% of physicians said they would definitely recommend the
screening test. Only 23% said they would definitely recommend the test
that was based on the relevant evidence.
"We believe that many of the physicians mistakenly interpreted
survival in screening as if it were survival in the context of a
treatment trial," the authors wrote, noting that in the context of
screening, the starting point for survival calculations is different for
screened and unscreened populations.
In an accompanying editorial, Virginia Moyer, MD, MPH, of Baylor
College of Medicine in Houston, said that the study suggests that
physicians do not understand statistical concepts well.
She highlighted two possible solutions for the problem: "Medical
journal editors should carefully monitor publications about screening to
ensure that results are presented in such a way as to avoid
misinterpretation, and medical educators should improve the quality of
teaching about screening tests."
Even together, however, those solutions likely will not be enough,
she wrote, noting that journalists and the general public for which they
write also should be targets of education about screening statistics.
Wegwarth and colleagues acknowledged some limitations of their study,
including the fact that recommendations were based on hypothetical
scenarios and not actual practice, the lack of information on the effect
of subjective factors like the fear of malpractice on the
interpretation of the evidence, and the lack of information on testing
harms in the scenarios.
To read the online article: http://www.medpagetoday.com/HematologyOncology/OtherCancers/31513