The idea that detecting cancers early makes them easier to treat has been around since at least the 1930s, when doctors began advising women to conduct breast self-exams.
And in many cases, that strategy works. The American Cancer Society credits widespread use of mammograms for a 2% annual decrease in breast cancer deaths since 1990. Pap smears have slashed deaths from cervical cancer by more than 70% since they were introduced in the 1940s.
But finding cancers that respond to early treatment is only one of the potential outcomes from a screening test. Many tests produce false positives, prompting additional tests that can be invasive, expensive, time-consuming and anxiety-inducing.
A study published this spring in Annals of Family Medicine found that 60% of men and 49% of women had gotten at least one false positive during three years of routine screenings for ovarian, prostate, lung and colorectal cancer. As a result, 22% of those women and 29% of those men had an invasive diagnostic procedure, the study found.
Other screening tests produce false negatives, giving patients and their doctors the incorrect impression that they have nothing to worry about.
Some detect aggressive cancers whose outcomes aren't improved by early detection.
And some identify small cancers that grow so slowly they'd never compromise a patient's health. Many would even go away on their own.
Statisticians and epidemiologists know this for a fact. The problem is, there's no way to tell which of the tumors are dangerous and need to be treated and which are harmless and would be best left alone. So all of them get treated, often aggressively. The medical establishment calls this overdiagnosis.
"Overdiagnosis is the hardest thing to explain to people," said Dr. Stephen Taplin, chief of the Applied Cancer Screening Research Branch at the National Cancer Institute in Washington, D.C. "No individual woman can know if they're overdiagnosed. They know they have cancer, and they're scared to death. It's completely justified.
"But if you look at people overall," he added, "there are some people who suffered that scare unnecessarily. If they had never known about that cancer, it wouldn't have affected their life."
Such nuances weren't considered when screening tests were implemented. Many were introduced before their effectiveness had been established though clinical trails. At the time, doctors didn't see the need. But experience has prompted them to reconsider.
Take the cancer antigen 125, or CA 125, test that has been used to screen for ovarian cancer. Women with the disease often have higher blood levels of this protein, so it seemed to make sense to check for it in asymptomatic women.
The test was never widely adopted and is no longer recommended for women at average risk, because other conditions, such as diverticulitis and endometriosis, can also boost CA 125 levels. And some patients with the disease have normal levels of the protein.
Many doctors are backing off the prostate-specific antigen test to screen for prostate cancer now that two influential studies published this year found that early detection offered little to no benefit in long-term survival. But the PSA tests did prompt aggressive treatment that sometimes left men impotent or incontinent.
Some guidelines, including those for cervical cancer, have been amended to reflect a better understanding of cancer biology. The American College of Obstetricians and Gynecologists this week suggested that most women get Pap tests less frequently because it's known now that cervical cancer progresses slowly, and abnormal cells often resolve on their own, especially in younger women.
Despite outcry from patients who equate reduced screening with reduced care, it makes no sense to ignore scientific data that happen to be unpalatable, said Dr. George Sawaya, a researcher at UC San Francisco whose studies helped prompt the change in Pap test guidelines.
"It would be much more of a travesty if we didn't change our guidelines in response to new information," he said.
Times staff writer Shari Roan contributed to this report.