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Studies suggest that if you made full-body scans part of your regular routine, you’d find a bunch of cancers over the years, but it wouldn't change your odds of having a life-threatening cancer. This is mostly because our definition of cancer is a little too broad.

Hosted by: Hank Green

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Sources:
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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4322920/pdf/nihms658721.pdf
https://ascopubs.org/doi/full/10.1200/JCO.2005.04.7985
https://www.ncbi.nlm.nih.gov/pubmed/16170016
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(Intro)

In 1985, a post-mortem study of 101 people in Finland found that 35.6% of the subjects had thyroid cancer.  The researchers went on to argue that if they had examined the tissue more thoroughly, they expect most, if not all, of their subjects would have had at least one small tumor in their thyroid and for clarity, these were people who were not diagnosed with thyroid cancer prior to their death.

Similarly, in 2005, an American study did full-body CT scans of almost 1200 people of all ages and found that 86% of them had at least one abnormal finding, and that was just one scan on one day of their lives.  Studies like these suggest that if you made full body scans part of your regular routine, odds are you'd find a bunch of cancers over the years, which sounds terrifying but it doesn't change your odds of having a life-threatening cancer.

See, what you would be finding are harmless cancers that don't ever make you sick or even go away without treatment.  So really what this is is an indicator that our definition of cancer is just kinda crummy.  Now, for a long time, cancer has been defined as any uncontrollable growth of cells in your body, and that definition worked super well back when the only way we could detect tumors was feeling around for weird lumps, but our ability to detect strange cell growths has improved immensely, especially over the past 50 years.

In fact, our ability to scan the human body has progressed to the point where we can identify tiny abnormalities which may or may not be headed for malignancy, the ability to spread between tissues, and this is making many doctors reevaluate what cancers should really be considered cancer.  

The old school definition can encompass any wonky collection of cells in your body, but when a cancer isn't going to be malignant, treating it doesn't really prevent anything, and even malignancy isn't enough to make something worthy of being like a "cancer", because there are a lot of cancers which actually aren't that bad.  Instead, they're indolent. 

Even though they're malignant in the sense that they can spread between tissues, they spread so slowly that ultimately they don't cause symptoms.  Some even regress or disappear all by themselves, like the teeny tiny thyroid tumors found in that Finnish study, for example.  Those almost always stay small or go away.  Only on rare occasions do they become problematic, and researchers report that depending on the organ, between 15 and 75% of diagnosed cancers are indolent, which means doctors have to ask which cancers are actually worth looking for.

For decades, many have thought doctors should err on the side of caution by basically screening everyone for as many potentially dangerous cancers as often as they can, because hey, it catches more cancers and it's true that people who are diagnosed with early stage cancer typically live longer, but screenings have also led to cancer over-diagnosis, and while the tumors in over-diagnosis are harmless, the diagnosis itself isn't, because people are understandably upset when they hear that they have cancer.  They may end up taking medicine or undergoing surgeries they don't actually need, which can take a toll on their bodies and their wallets.

Back when these screening tests were first developed, doctors made the logical prediction that early detection of tumors would decrease the number of people who died from them, but often, that's not what happened.  For instance, between 1975 and 2016, the number of people diagnosed with thyroid, kidney, and skin cancers more than tripled, but the number of deaths for those cancers stayed pretty much the same, because if there are really three or more times as many clinically relevant cancers occurring nowadays, more people should be dying from them, because while our treatments have gotten better, they haven't gotten that much better, so no change in the number of deaths suggests that the screening programs are really good at catching weird cell growths that don't matter medically rather than catching dangerous cancers earlier on, and this is likely thanks to rapid increases in imaging technology.

The bittersweet reality is that a lot of the time, doctors can't tell the difference between cancer that needs treatment and cancer that does not, especially if the cancer is small.  Though there's hope that they will be able to do that someday.  The upside to all of this is that widespread screening programs have helped scientists discover ways to distinguish between dangerous malignant cancers and harmless ones.  For example, with breast cancers, doctors can use molecular identifiers to determine if patients need chemotherapy, but that kind of fine-grained sorting isn't yet available for all cancers, so while scientists continue to search for diagnostic markers, doctors are trying to narrow down who should be screened, when, and how to minimize over-diagnosis.

Also, they're trying to change how we talk about cancer.  Some doctors think the term 'cancer' should be reserved for tumors that are at the highest risk of becoming malignant or already are.  Other overgrowths or abnormal cells should be called something else specific to their nature, and even malignant cancers that grow super slowly and therefore are unlikely to cause real harm, whether or not they're treated, could be rebranded as IDLE disorders, I-D-L-E, Indolent Lesions of Epithelial origin.  

But on the upside, until we redefine cancer in a more meaningful way, at least you like, always have an iron-clad reason to get out of work.  Just tell your boss that if you looked hard enough, you'd probably find some weird cells growing somewhere in your body.  Sorry, I got weird cells.  Gotta go home. 

Thanks for watching this episode.  If you liked learning about cancer over-diagnosis, we also have an episode on why we haven't cured all the cancers yet, which you might like.  Why don't you watch that one next, and if you like that one, too, you should probably just subscribe to the channel because we have so many videos that are good.  I promise.  They're good.

(Endscreen)