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While false-positive results, pain during the procedure, and radiation exposure may be among the most frequent harms associated with mammogram screening, “the most serious downside” is now recognized to be something called “overdiagnosis”—so serious as to raise the question: “does it make [the whole thing] worthless?” The value of doing routine mammograms at all is being questioned due to overdiagnosis, which is “the diagnosis and treatment of breast cancer that would never have become a threat to a woman’s health, or even apparent, during her lifetime.”

See, people “think…that once you have a cancer cell in your body, it will progress, predictably and inevitably, to a terrible death.” And “that[’s] simply not true of most cancers.” “Some cancers [may] outgrow their blood supply,” become starved, and wither away. “[O]thers may be recognized by [our] immune system and…successfully contained, and some are simply not that aggressive in the first place.” Meaning, yeah, it may continue to grow unchecked, but so slowly that it would be like 200 years before it was big enough to cause any problems. And so, in effect, you die with your tumor, instead of from your tumor.

Indeed, if you do autopsy studies of young and middle-aged women who just happened to die in a car accident or something, 20% of them had cancer in their breast. So, like one in five women are walking around with breast cancer. Now, that sounds a lot scarier than it is, since at that age range, the risk of dying from breast cancer is less than 1%. In fact, your risk of ever dying from breast cancer in your lifetime is less than 4%, which goes to show that many of these cancers that they found incidentally—in fact, most of them—would likely have just fizzled out on their own.

The problem is that we continue to have a 19th-century definition of cancer, dating back to the 1860s. See, cancer is defined by what it looks like under a microscope, not by what its subsequent behavior is. So, yeah, using that definition, one in five of these women technically had cancer, like this 30-year-old here. But, that doesn’t necessarily mean it would go on and do anything.

The question then becomes: if it’s so common, do you even want to know about it? If it’s going to progress and cause a problem, then definitely; catching it early could save your life. But, if it’s never going to grow, if it’s going to remain microscopic, then finding it could actually be bad for you. They’d be like look, you have cancer; we have to treat it—surgery, chemo, radiation—whatever it takes. Then, you’d suffer all the physical effects of treatment, the psychological hell of fearing for your life, all completely unnecessarily, if, in fact, it was never going to cause a problem. That’s overdiagnosis.

These kinds of car accident-type autopsy studies show that between 7 and 39% of women ages 40 through 70 are walking around with tiny breast cancers. 30 to 70% of men older than 60 have prostate cancers, and up to 100% of older adults have microscopic cancers in their thyroid glands. Yet, only point one percent—one in a thousand, ends up suffering or dying from thyroid cancer. Normally, it just sits there, and doesn’t do anything. Likewise, even though the majority of older men may have tiny cancers in their prostates, or a significant number of women in their breasts, the lifetime risk of death or cancer spread is only about 4%. So, if you had a magic wand that could pick up cancer with 100% accuracy, and waved it in front of people, your overdiagnosis rate—the probability that the prostate cancer you’d pick up would have turned out to be harmless—is like 90%, and nearly every single thyroid cancer, and a significant proportion of breast cancer cases. That’s why screening for these cancers can be tricky, or even potentially dangerous, since, in many cases—sometimes most cases—you would have been better off if they had never found it.

Now, this is not true for all cancers. “There is little evidence of overdiagnosis [for] cervical or colorectal cancer,” for example. Those cancers do seem to just keep growing; and so, the earlier you catch them, the better. So, institute Pap smears, and cervical cancer death rates plummet. And, just a single sigmoidoscopy between the ages of 55 and 65 may decrease one’s risk of dying from colorectal cancer by up to 40%—whereas some studies show that even getting mammograms every year don’t appear to reduce breast cancer mortality at all. But, if we assume a 15% drop, and a 30% overdiagnosis rate—which is what most studies have found—then that would mean for every 2,000 women invited for mammograms for 10 years, one will have her life prolonged, and 10 healthy women would be overdiagnosed. In other words, they “would not have had [a] breast cancer diagnosed” if they had skipped screening, but were instead “treated [for breast cancer] unnecessarily.

And, about a thousand would have gotten false alarms, which can be stressful while you wait for the results. But “[t]he harms caused by [becoming a cancer patient unnecessarily can be] lifelong,” and even mean a shorter life. It’s “important to be aware that some of the [needlessly treated] women will die from that treatment.” For example, radiation treatments can’t help but penetrate down into the heart as well, increasing risk of the #1 killer of women: heart disease.

This raises questions about doing routine mammograms, period, as it “converts thousands of healthy women into cancer patients unnecessarily”—some of whom may not make it out alive. Ironically, though, those who do become mammography’s biggest cheerleaders, thinking mammograms saved their life. The mammogram found a cancer you didn’t even know you had, and yeah, the treatment was rough—surgery, radiation, drugs, but it worked; life was saved. Thank God she got that mammogram; you should, too.

Whereas actually, the more likely scenario—in fact, maybe the 10 times more likely scenario, is that the treatment didn’t do anything, since the cancer wouldn’t have hurt you anyway. So, you went through all that pain and suffering for nothing. That’s the crazy thing about mammograms; the people who are harmed the most are the ones who claim the greatest benefit.

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