Understanding Cancer Screening Statistics

Cancer screening statistics do not have to be confusing

/ Author:  / Reviewed by: Robert Carlson, M.D

Your doctor tells you that one of the benefits of having a cancer screening is that it increases early detection. You may also have heard that a screening test can improve your lifespan should  you be diagnosed with that cancer - commonly called the 5-year survival rate.

Are these the true benefits of cancer screenings? The answer to that question is simple:  no.

What statistics matter

The only numbers that matter when it comes to cancer screenings are the number of lives saved and improvement in the quality of life that result from the tests - as demonstrated by randomized clinical trials.

All the others - early detection and improved survival - are irrelevant.

What's worrisome, though, is that many doctors don't know this.

But now you do.

Testing physician knowledge

To test just how much physicians understand about the statistics of cancer screening, a group of researchers from Harding Center for Risk Literacy at the Max Planck Institute for Human Development in Berlin conducted a randomized survey of primary care physicians in the United States.

Participants were selected from a database of U.S. primary care doctors maintained by Harris Interactive.

Those who answered the survey included 297 general practitioners who have both inpatient and outpatient practices, and 115 physicians who practice only outpatient medicine.

Two scenarios

The physicians received information about the effects of two hypothetical cancer screenings. One described the test benefits in terms of improved 5-year survival rates and earlier detection. The other talked about decreased mortality and increased incidence.

Doctor recommendations

Based on these scenarios, 69 percent of the physicians recommended the test that provided improved 5-year survival from 68 to 99 percent.

Problem is: this is irrelevant evidence.

On the other hand, only 23 percent of the doctors recommended the screening based on relevant evidence - that the test saved lives and reduced deaths from 2 to 1.6 in1,000 persons.

Knowing the difference

When asked about what is relevant and irrelevant evidence of the value of a screening test, most doctors didn't understand the difference.

The survey showed that 76 percent of the physicians thought that irrelevant evidence proved that the screening saves lives, while 81 percent correctly chose the relevant information.

Nearly half (47 percent) of those surveyed also incorrectly said that finding more cases of cancer in screened as opposed to unscreened populations "proves that screening saves lives."

What this all means

Statistics are tricky business. To understand more about this study, dailyRx reached out to one of the researchers involved, Dr. Wolfgang Gaissmaier.

He explained that the terms "survival" and "mortality" have very different meanings as they relate to screening.

"Here, the calculation of survival is based only on those people diagnosed with cancer, while mortality is based on the whole study population," Dr. Gaissmaier told dailyRx.

He continues, "To illustrate, imagine a group of patients in whom cancer was diagnosed because of symptoms at age 67 years, all of whom die at age 70 years. Each patient survives only 3 years, so the 5-year survival for the group is 0%."

Okay, so a group of people get cancer and die three years later. Got it.

"Now imagine that the same group undergoes screening. Screening tests by definition lead to earlier diagnosis," he says.


"Suppose that with screening, cancer is diagnosed in all patients at age 60 years, but they nevertheless die at age 70 years. In this scenario, each patient survives 10 years, so the 5-year survival for the group is 100%," Dr. Gaissmaier says.

That makes sense. The people are screened at a younger age - before symptoms appear at age 67. Then they die at 70.

Dr. Gaissmaier concludes, "Yet, despite this dramatic improvement in survival (from 0% to 100%), nothing has changed about how many people die or when."


What you should know about cancer screening

Cancer screening does detect cancer early on, but that's not its sole goal. "Screening aims at reducing mortality and/or improving quality of life," Dr. Gaissmaier says.

"Screening is targeted at people without symptoms to test for hidden disease. To be useful, early detection needs to enable earlier treatment that is either more effective or safer than later treatment."

In other words, if you're screened before symptoms appear, you're more likely to beat the cancer.

The benefits and harms of cancer screenings

While saving lives is the principal benefit of cancer screenings, there are also harms, which Dr. Gaissmaier explains.

"The most important harm is overdiagnosis, which can eventually lead to overtreatment. Overdiagnosis is defined as the detection of abnormalities that would never progress to cause problems in a patient’s lifetime, such as non-progressive prostate cancers," he says

"Treating a non-progressive cancer is obviously not beneficial, and it can even be harmful because of side effects.

In addition to overdiagnosis and overtreatment, participation in screening bears the risk of yielding false positive test results that can lead to psychological distress and unnecessary biopsies." Dr. Gaissmaier says.

Recent example

Recently, it's been suggested that women who are at greater risk of breast cancer because of a family or personal history receive MRI and ultrasound tests in addition to mammography.

dailyRx asked Daniel R. Kopans, MD, professor of radiology at Harvard Medical School, about this recommendation.

"MRI and ultrasound can find cancers that are not evident by mammography, but we do not have proof that finding these save lives." said Dr. Kopans, senior radiologist at the Breast Imaging Division of Massachusetts General Hospital.

A vocal advocate of the clinically proven benefits of mammography, Dr. Kopans continues, "The crazy part of all of this is that state legislatures are suggesting that women with dense breasts be screened with ultrasound and MRI (unproven to save lives and with much higher false positive rates) while efforts are underway to curtail mammography, the only test proven to save lives!"

The bottom line

Dr. Gaissmair urges patients "to get to the bottom of health statistics that are presented to them by physicians or in pamphlets. That is, they should not blindly trust that the statistical information that is thrown at them actually means what it implies, but dare to ask questions. The only statistics you can trust are those you fully understand yourself!"

And so how do you do that? Ask for the break-down of the patient lives saved.

Dr. Gaissmaier explains what you want to know: "Out of all people who participate in screening (or, say, out of every 1,000), how many do actually have a benefit in terms of reduced mortality or improved quality of life? And how many are actually harmed, for instance by unnecessary treatments?"

All you need to know

Know the real, true and relevant benefits of all screenings you undergo.

Know the questions to ask.

And know that it's okay to say "no" to a screening test, the benefits of which you don't fully understand. 

Reviewed by: 
Review Date: 
April 12, 2012