By Sonni Efron

The U.S. public has been hit with persuasive messages that more and earlier cancer screening is better – but the evidence isn’t yet in, said Dr. Jennifer Croswell of the National Institutes of Health’s Heathcare Delivery Research Program.

In a briefing for National Press Foundation fellows, Croswell (bio, Twitter), a medical officer at the NIH program’s Division of Cancer Control and Population Sciences, said that journalists should distinguish between screening, which targets healthy populations, and treatment, which focuses only on people who have symptoms and are seeking help.

“It’s hard to make healthy people better off than they already are,” Croswell said.

Most people who undergo mass screening never get the cancer in question – so by definition, they cannot benefit, she said. Still, they are exposed to potential harms in the screening process. Those could include invasive, painful or costly treatments that may not be proven to extend their lives.

There are several biases in the research itself, Croswell said. One is lead-time bias: People who are screened may be found to have cancer sooner, so they may end up living longer with a cancer diagnosis even though screening may not be followed by treatment that extends their total lifespan.

“More time that they’re living as a patient is not equal to more time alive,” Croswell said.

Another problem in cancer screening research is “length-bias sampling,” also known as “stacking the deck.” Screening tends to catch patients with slow-moving or so-called “indigent” cancers, but it misses people with fast-growing, aggressive ones.

Bottom line: More screening is not automatically better, Croswell said.  “The more we look, the more we find,” she said – especially with ever-more sophisticated detection technologies. The assumption is that early detection saves lives, but in some cases, doctors are diagnosing disease that would never have harmed the person.

Beyond that, not all cancers behave the way researchers expect. “We cannot differentiate between things that are destined to be malignant and things that are designed to be indigent,” she warned.

The risks and benefits of mass cancer screening – as opposed to screening people known to be at high risk – are in part a matter of personal perception, she said. It’s important to consider the proper balance between risk and benefit.

“The point of screening is to reduce suffering and death. It’s not just to find as much cancer as we possibly can,” Croswell said.