In a recent article for the website FiveThirtyEight, science reporter Christie Aschwanden does a terrific job of making the case for why all of us — patients and physicians alike — need to rein in our overzealous use of cancer screening.
Not only is unnecessary screening wasting billions of dollars each year and placing an ever-increasing burden on the U.S. economy, it’s also leading to real harm.
Aschwanden begins her article with a discussion of the skyrocketing incidence rate of thyroid cancer in Korea, which has jumped 15-fold during the past two decades. As revealed in a study published last month (which I reported on here in Second Opinion), that so-called epidemic of disease is entirely the result of aggressive screening. Almost all the newly identified thyroid cancers being found in Korea are tiny papillary tumors, which research has shown are very slow-growing and highly unlikely to go on to cause symptoms, much less death.
In fact, the results of a small Finnish study suggest that one-third of all adults may have such symptomless thyroid lesions.
How do we know that those small papillary tumors are not much of a threat? Because the death rate from thyroid cancer has remained flat in Korea over the past 20 years. If early screening were saving lives, the death rate would drop — or at least increase slowly.
The situation with thyroid cancer in the United States is similar, although slightly less dramatic. The U.S. incidence rate for the disease has almost tripled during the past three decades, but without any decline in the mortality rate.
“Researchers have a name for this — overdiagnosis,” writes Aschwanden, “and it leads to another problem, overtreatment.”
For almost everyone in Korea — and the U.S. — who is diagnosed with a papillary thyroid tumors ends up having surgery to remove all or part of their thyroid, which then requires them to take thyroid replacement therapy for the rest of their lives.
An outdated way of thinking
As the thyroid cancer situation reveals, many people — patients and physicians alike — continue to cling to the “relentless progression model” of cancer — the long-outdated idea, writes Aschwanden, that “every cancer cell is destined to spread and become lethal.” A misguided belief in that model is why people became so outraged in 2009 when the U.S. Preventive Services Task Force (USPSTF) updated its breast-cancer screening guidelines, essentially recommending that most women don’t need routine mammograms until age 50 and that a mammogram every other year is sufficient after that.
It was also behind much of the public anger toward the USPSTF’s 2012 recommendation against routine prostate-specific antigen (PSA) screening for prostate cancer. (Two years later, the American Urological Association, which had bitterly opposed the USPSTF’s recommendation, did a complete turnaround and said it, too, no longer recommended routine PSA screening.)
“It’s hard to believe in overdiagnosis and overtreatment if you imagine that cancer behaves in a progressive manner: that is, that it arises from one haywire cell that replicates abnormally before inevitably spreading to other parts of the body, eventually killing the person,” explains Aschwanden.
But we know better today — or we should. “Cancer is not a single disease; it’s many,” writes Aschwanden. “On one end of the spectrum lie cancers that become aggressive and invasive from the start, and on the other end are cancers that remain non-invasive and therefore harmless. Whether a single rogue cell replicates slowly and stays put or goes wild and spreads is determined by a variety of factors that scientists are still working to understand.”
A ‘barnyard’ of tumor behavior
As Aschwanden points out, researchers have identified several general behavior patterns for cancers. In an attempt to help the public better understand these patterns, researchers have compared them, says Aschwanden, “to animals that must be kept in the barnyard to prevent a deadly rampage”:
Papillary tumors are like turtles — they move very slowly and never pose an escape risk. They don’t need screening, because they will never cause trouble. Then there are rabbits, which are eager to hop away to other parts of the body, but can be confined if they’re found and fenced. These are the cancers that can be helped by early detection and treatment. Birds, on the other hand, are so flighty and quick that they can’t be confined. Screening makes no difference for bird cancers, because they’re so aggressive that they can’t be detected before they’ve begun their deadly course.
No cancer screening has ever eliminated the majority of cancer deaths. Instead, the best screening can do is [rein in] the rabbits. Birds remain unstoppable, and they’re the ones responsible for most cancer deaths. This is why … three decades of mammography have failed to put a dent in the rate of women presenting with metastatic breast cancer upon their initial diagnosis. Women with breast cancers that behave like birds will almost never be helped by a mammogram, nor will men with the most aggressive prostate cancers be saved by PSA tests.
“Until tests can more accurately distinguish the rabbits from the turtles (not to mention the dodos — it’s now looking like some cancers detectable by screening may actually disappear or go extinct on their own), cancer screening may harm more people than it helps,” Aschwanden adds. “But cancer is a scary disease, and when there’s potential to save a life, some people will gladly take on the risks. Getting screened for cancer is like playing the lottery — your chances of winning aren’t great, but if you do the payoff’s a jackpot.”
Making the case — statistically
Aschwanden’s article includes much more on “the case against cancer detection,” including two great charts (one of the wonkish FiveThirtyEight’s specialties) that illustrate statistically why routine mammogram and PSA screening lead to more harm than good.
Whether such levelheaded statistical arguments will change the opinions of people still clinging to the outmoded relentless progression model of cancer remains unclear, however.
“Some of the most vehement defenders of mammography are women who say that a mammogram saved their lives,” writes Aschwanden. “On an individual basis, no one can tell her she’s wrong. But the statistics suggest that chances are, she’s mistaken. Studies show there’s only a 13 percent probability that a woman whose breast cancer was diagnosed with a screening mammogram will actually avoid dying of breast cancer. At the individual level, the woman with the rabbit breast cancer who gets a mammogram receives treatment and does not die of breast cancer looks identical to the one with the turtle breast cancer who lives on after a needless treatment for a cancer that never posed a threat to her life.”
You can read Aschwanden’s article on the FiveThirtyEight website.