DID THE NATIONAL PREVALENCE OF COLORECTAL, BREAST, and cervix cancer screening change during the COVID-19 pandemic? I am sad to report that it did, according to a new study from the American Cancer Society. Today, we look at cancer screening basics before turning to how the pandemic has affected breast, cervix, and colorectal cancer screening.
Americans had had health care disruptions beginning with the first quarter of 2020 when the novel coronavirus landed on our shores. In my clinical practice, it seemed that cancer screening services declined quite a bit, even among a very well-educated population.
Now comes confirmation of this bad news: a population-based surveillance system examining this issue. Are my perceptions correct? Are fewer folks coming in for cancer screening (such as for breast, colon, and cervix cancer)?
First, a look at the value of getting screened for cancer.
Cancer screening benefits (and perils)
Screening tests try to catch cancer at an early stage, before the emergence of symptoms. Tests may include a physical exam and history, laboratory tests, imaging, or genetic tests.
Sounds good, right? Unfortunately, there are some downsides. For example, screening interventions can have side effects. Colon cancer screening with a scope (such as a sigmoidoscopy or colonoscopy) uncommonly causes colon lining tears.
And then, there are false-negative results. Sometimes screening test results appear normal, even when cancer is present. A false negative result can lead to delays in medical care.
False-positive results can also occur. Here, a screening test appears abnormal, even when no cancer exists. The result is often anxiety, not to mention additional tests.
Cancer screening and length bias
Imperfect screening tests sometimes do not result in health improvement or longevity. As frightened as we are about cancer, some cancers would never cause symptoms or become life-threatening. For such relatively quiet tumors, unnecessary treatment may lead to side effects.
I want to take a small detour about how cancer screening study results can show false promise. You have heard about false-negative and false-positive results, but do you know about length bias?
Lead-time bias refers to the fact that screening is more likely to pick up less aggressive, slower-growing cancers. Let’s look at how seemingly positive screening research results can fool us.
Here is a hypothetical scenario from the United States National Cancer Institute: A screening test finds “non-progressive” cancer — cancer no destined to progress or threaten life — leading to the overdiagnosis of 2,000 individuals.
Adding these 2,000 overdiagnosed individuals to a pool of 1,000 patients with “progressive cancers,” found because of symptoms, artificially inflates the five-year survival from 40 to 80 percent. But this apparent improvement in survival is not real: The same number of people died. We have overdiagnosis bias.
Here’s the math: With no screening, we have 1000 people with progressive cancer, Five years later, 600 die, yielding a five-year survival of 40 percent. With screening, 1000 patients have “progressive cancers,” and we add 2000 with non-progressive cancer (the latter picked up by screening). The five-year survival is 80 percent. Looks like a home run, but we end in the same place, with 600 dying.
Cancer screening and lead-time bias
Another problem with cancer screening studies is “lead-time bias.” Let’s say a man has a persistent cough and a lot of weight loss, is diagnosed with lung cancer at age 67 and dies of his cancer at age 70. The five-year survival for a group of patients with his stage disease is zero.
Now let’s say this same gentleman is diagnosed by screening at age 60 and ultimately dies of lung cancer at age 70. His life has been extended, and the 5-year survival of men like him seems to be 100 percent.
We “overdiagnosis” 19 percent of screening-detected breast cancers and 20 to 50 percent of screening-detected prostate cancers.
Given the biases described, we need good randomized trials to understand whether screening works. Fortunately, some cancer screening tools work, but the absolute benefit in survival can be small (especially if the risk of a particular cancer is small).
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