We're getting better at screening for cancer, and that could be a problem | Popular Science
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We're getting better at screening for cancer, and that could be a problem

Opinions vary among medical professionals.

cancer cell

All it takes is one cancer cell to turn into a whole tumor.

Finding out you have cancer is a bell you can’t unring. As doctors increasingly have the tools to find cancers before they actually pose a problem, we’re going to have to start asking ourselves a tough question: is knowing always a good thing?

Half of Americans say they’d want to be screened even for a type of cancer they couldn’t do anything about. But medical professionals are somewhat divided on just how much we should test people for diseases we’re not able to treat.

Meanwhile, our screening tests are getting broader. A study in the journal Science this week demonstrated a solid first step toward a blood test that detects eight different kinds of cancer. Together, these eight diseases account for 60 percent of cancer deaths in the U.S., and the screen is able to not only identify cancer with reasonable accuracy, but can also sometimes indicate which organ the disease is in. It does this by testing for mutations in floating tumor DNA, and by looking for cancer-specific proteins. This combination makes the test more accurate than many prior efforts.

It’s not ready to go to market anytime soon, but its very existence raises some big issues for the medical community. A significant subset thinks that earlier detection is key to preventing deaths. Another segment thinks that earlier detection isn’t always key, and that screening can actually do substantial harm. While some doctors push for broad screening, others urge caution even as diagnostic technology surges ahead.

To understand why this divide exists, we have to take a step back to the basics.

The first thing to know is that cancer is pretty common. Roughly 38 percent of people in the U.S. will get diagnosed with cancer at some point in their life, and 67 percent of them will live five years or more. It’s in part because of certain screening methods that there are about 14 million people living with the disease right now—for some cancers, early detection really is a lifesaver.

The colonoscopies that everyone loves to complain about, for example, make a huge difference—we can stick a camera inside a patient and actually see cancers forming. Colonoscopies look for polyps, abnormal growths inside the intestines that can eventually turn into cancers. Because we can see them and often remove them before they become serious, we can actively prevent colorectal cancer.

But not every screen does more good than harm. We can screen for prostate cancer fairly easily by testing levels of prostate-specific antigen, a protein that circulates in the bloodstream and becomes elevated as one develops prostate cancer. When that test first came out, the perception was that we’d be able to find cancers early enough to treat them more effectively. But while that was certainly true for some, doctors also wound up finding a lot of pre-cancers and early cancers that never would have actually done the patient harm.

Many think of cancer as a malignant, unstoppable force that inevitably takes over our bodies, but the reality is that some cancers don’t develop that way.

“We’ve ping-ponged on prostate cancer screening, because we know there are lots of cancers that are going to progress so slowly, you’ll die from something else first,” explains Lisa Schwartz, the co-director of the Center for Medicine and Media at The Dartmouth Institute. She’s worked extensively in the field of risk communication and overdiagnosis, and she’s seen the problems that screening can cause. The simple fact is that not all cancers become life-threatening. Some cervical cancers can actually go away on their own, and certain types of very early breast cancer can show up in tests and yet never develop into problematic tumors. “For many cancers, we’re not good at distinguishing which will be dangerous and which won’t,” says Schwartz, and in some cases the treatment can be more aggressive than the disease itself. “If these cancers don’t actually go on to progress, the treatment is pure harm.”

This just isn’t how most people think about cancer. And it doesn’t help that some statistics can appear to show that screens like the PSA test save lives, even though they don’t. Data about the five-year survival rate can actually be misleading for doctors and patients alike, who take certain numbers and interpret them as meaning we’ve saved lives even when we haven’t.

Let’s use an example. In scenario A, doctors diagnose a 67-year-old man with prostate cancer when he shows up to a physical with an enlarged prostate. The cancer kills him three years later. The five-year survival rate of our single-man study is zero percent. In scenario B, that same man gets a PSA test when he’s just 60, but he still dies at 70. Now our five-year survival rate is 100 percent, even though the man didn’t live any longer—he just spent an additional seven years of his life as a cancer patient. This example is extreme (no one bases their survival rates on a single person), but you can imagine how having a whole bunch of cases similar to this one in the mix could skew statistical results. Finding cancer earlier doesn’t always change your prognosis, but it always changes your length of survival as a cancer patient. That makes it seem like a test helps prolong life, even when it doesn’t.

Here’s another example: In scenario X, we have no screening method and 1000 people are diagnosed with progressive breast cancer. 600 of them die over the course of five years, which works out to a 40 percent survival rate. In scenario Y, those same 1000 people get progressive cancer, but another 2000 people get a screening test that shows they have those abnormal but ultimately harmless cells we talked about earlier. Those people all live, and the same 600 people with progressive tumors die. Now our survival rate is 80 percent. But we haven’t saved any more lives, we just told more people they have cancer.

The real-life versions of these extreme hypotheticals motivate experts like those who serve on the U.S. Preventive Services Task Force. Part of their job is to evaluate the evidence for screening tests. Members of the task force, like family physician and researcher Alex Krist, have to balance the benefits with the harms.

“Sometimes it feels like finding something early should always be helpful,” Krist says, “and if we can demonstrate that on the net, the benefits outweigh the harms, we want to encourage screening. But just finding things earlier is not always better.”

This is how some tests, like CT scans to screen smokers for lung cancer, get approval with an A or a B rating (lung cancer screening got a B for smokers between 55 and 80 years old). Others, like PSA testing, a C, noting that "many men will experience potential harms of screening" but that there may be some small potential benefit, and so the decision should be a personal one (though it gets a D for anyone over 70.

It might seem like it can only be a good thing to know more about your health, but Krist and Schwartz both say there can be psychological and physical impacts that people don’t necessarily anticipate. “One [thing to consider] is that you’ve changed somebody’s world,” Schwartz says. “They’re not just a person anymore, they’re a patient. There’s a tremendous worry that comes with a cancer diagnosis.” And patients, she says, have to start making choices about whether or not to undergo treatments, often chemotherapy, that have serious side effects. If the treatment doesn’t change your outcome, Schwartz says, “all we’re doing is subjecting you to more life as a cancer patient.”

These are all downsides that people often don’t consider, so physicians like Krist have to work with their patients to help them understand.

“For something like prostate cancer, I do have a conversation with my patients where I say, ‘if we do this and find out you have a high PSA value, that’s going to permanently change the conversations you and I have and the way you think about yourself,’” Krist explains. He says he tries to get every patient to think carefully about how they’ll feel living that kind of future—how they feel about maybe becoming a person who has cancer—and whether that possibility is something they’re okay with. “I try really hard to engage them the way they want to be engaged, and to get them to think about who they are as a person and what their values are.”

Handing out screening tests without first carefully considering the implications can be a mistake, these experts say. But that’s not how every doctor sees it.

Anne Marie Lennon, who worked on the new blood-based cancer test, is the Director of the Multidisciplinary Pancreatic Cyst Program at the Johns Hopkins Kimmel Cancer Center, where she treats patients with gastric, pancreatic, and esophageal cancers. She’s passionate about developing a screen, even if it only helps a small number of people. “I think screening is a good thing,” she says, “there’s no question that if you can pick up a cancer earlier, that you’re doing your patients a service.”

That doesn’t mean she thinks the test they’ve developed is ready for primetime, of course. The specificity rate, meaning the percentage of the time that it’s correct about a positive result, varies a lot by cancer type, and many of them don’t have high enough success rates to justify introducing the test just yet. It also hasn’t yet been validated on a group of people who may or may not have cancer—it’s only been tested on people who were already diagnosed. Lennon says that they’re going to need more studies to prove this works, as well as to improve the mechanics, but she thinks we could have a screen like this in a decade or so. And she’s excited about that.

“As somebody who deals with patients, there’s nothing more heartbreaking than telling someone they have cancer,” she explains, “because their first question is always ‘but you’re going to be able to cure me, aren’t you?’ And unfortunately with pancreatic cancer, we often can’t. You have to be honest with your patients and tell them no, it’s not curable.” To Lennon, catching a cancer earlier seems to give her patients a better chance, as well as giving her a reprieve from the unending tragedy. “I would much rather be able to go in and say ‘the great news is it’s early, we’re going to be able to remove this, and you’re going to live.’”

Lennon is driven by her pancreatic cancer patients, who she tries to help even though her toolkit is limited. Ironically, pancreatic cancer is also one of the clearest examples for other researchers of a situation where we shouldn’t test early. The five-year survival rate for pancreatic cancer in the U.S. is just 7 percent. Almost everyone dies from their disease, and right now the U.S. Preventive Services Task Force gives testing for it a D: do not screen. There aren’t many useful therapies for it—the majority of even stage I patients die—and the current diagnostic tools are invasive and often inaccurate.

But it’s possible that someday, there will be a test that will actually help. Perhaps it will catch the cancer so much earlier that treatments will be effective, or maybe the tests will be far more accurate, or our therapies will be more advanced. When or if that day comes, the decision to screen won’t divide the medical community. But for the moment, doctors remain uncertain. For now, we tread lightly.

Note: This article has been updated to reflect the newest draft recommendation for prostate cancer screening, which the USPSTF is in the process of reviewing. The old recommendation was a D for men of all ages.

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