The Matka of Radiation from CT Scans

There was quite a brouhaha earlier this week when a senior doctor on television said that the radiation from one CT scan equals that from 300 chest X-rays - he said this in the context of the fact that many CT scans of the chest done for Covid-19 in India are perhaps not required, something that I have also spoken about in The Matka of “Doing Something” Medicine and The Matka of Covid-19 Treatment. His statement however led to a lot of anguish among non-doctors, doctors, and radiologists who perform these CT scans (I am one) and many also issued statements on social media about how wrong the calculations were and that the radiation from a CT scan of the chest is equivalent to only 100 chest X-rays or 50 or less or more.

But that’s missing the point.

The only question that matters is…

Does exposure to radiation from X-rays or CT scans or mammograms cause cancer?

Let’s start with the answer right away. “No”.

This post is free to read, but you will need to subscribe with your email ID to read the rest of the post and to  listen to the accompanying audio/podcast.

You can listen to the audio/podcast hosted on Soundcloud by clicking the Play button below within the browser itself.

The highest radiation (15-25 mSv) is from a PET/CT scan, a modality used to diagnose and stage cancer. A standard plain CT scan of the lungs gives a radiation of 1-3 mSv. The radiation from a chest X-ray is on an average 0.1 mSv or less and the background radiation we are exposed to in our environment is 3 mSv per year. This table from the American College of Radiology (ACR) gives an idea of the radiation from different X-ray based procedures [1].

Virtually every physics society in the world [2,3] has a position statement that says there is no known harm or increased risk of cancer from exposure to the radiation from medical imaging (CT scan, PET/CT, X-rays, mammography, etc) when the level of radiation is below 50-100 mSv.

It has been 126 years since the discovery of X-rays, 125 years since the invention of the X-ray machine and 49 years since the first CT scan images. In this period,  not one single study has conclusively proven that radiation from CT scan causes cancer.

When people like me make these statements, many people turn around and say “the absence of proof does not mean proof of absence”, implying that just because we don’t have evidence, does not mean that the statement, “CT scan radiation causes cancer” is not true.

So where does this belief come from?

In 2001, there was a paper by Brenner and his colleagues [4] that used a modeling study based on data from the atomic bomb explosions in Hiroshima and Nagasaki to extrapolate the risk to radiation from CT scans. What they did was to use the “linear no-threshold (LNT)” theory that says that if you are exposed to 1000 mSv of radiation from an atomic bomb explosion and your risk of cancer is x, then an exposure of 10 mSv of radiation from a CT scan will lead to a risk of cancer of x/100, for an exposure of 1 mSv, x/1000 and for a 0.1 mSv (chest X-ray) exposure, x/10,000. According to this paper and the LNT theory there is no “safe” radiation level, while the overarching evidence shows the opposite to be true and that below 50-100 mSv of radiation, there is no evidence of harm (Figure).

The LNT theory is like the aspirin analogy that James Conca came up with [5]. It assumes that if it is dangerous to consume 1000 aspirin tablets at one time by one person (who will likely die), the risk is the same if 1000 people each take one aspirin on one single day or if one person takes one aspirin a day for 1000 days. This is a crazy assumption.

Brenner’s article caused an explosion. The lay press latched on to the findings. There were  follow-up articles in the New England Journal of Medicine [6] and other similar high-impact journals, which led to a clamor from multiple stake-holders to control radiation dose, which led the CT scan manufacturers to sink billions of dollars into low radiation research and development (which turned out to be quite a boon, because this led to the development of new technologies that have extended the lives of the X-ray tubes that power the CT scans and some really new cool technology like dual energy), and so on and so forth.

But modeling studies as we know now from all the predictive models that have almost always been wrong about the spread and severity of Covid-19 need to be proven with on the ground studies.

Hence started the retrospective and prospective studies that examine long-term, longitudinal data and compare those who have received CT scans to those who have not, to see if there is any increased risk of cancer. The first and still the most quoted is the Lancet paper by Pearce et al [7] that suggested an increased risk of leukemia (1:10,000) in those who received CT scans of the brain in the UK. What they did not account for though is the concept of reverse causation where the reason why the CT scan was done in the first place may actually be the cause of the cancer and not the radiation from the CT scan itself. A small study by Shibata and colleagues from Japan beautifully shows how CT brain non-exposed and CT brain exposed populations are significantly different [8]. Multiple subsequent studies from different countries have suffered from the same bias of not accounting for reverse causation.

A recent study in January 2021 in JAMA Surgery, from South Korea, [9] studied people with appendicectomies who had received or not received a CT scan for diagnosing appendicitis. They corrected for reverse causation and found 66 excess cancers, mainly leukemias, in 306727 patients (0.02%), especially in the 0-15 years age group. But as the authors themselves say, there are so many other unaccounted variables and differences between the two cohorts that they cannot really guarantee that these results are not just a statistical aberration. An accompanying commentary [10] in the journalthat published Brenner’s first article (and the journal likely rues the fact that they did not publish an accompanying editorial comment at that time) talks about the risk-benefit ratio of lives saved with CT scans for appendicitis versus the small incidence of leukemia…but this type of argument only further legitimizes the fact that radiation from CT scans does indeed cause cancer, but should be considered an acceptable risk for saving lives…that’s a disingenuous and dangerous argument to make.

As we know by now when it comes to vaccines and anti-vaxxers…it is often not about knowledge or science or its lack, but a gut belief that logical, scientific arguments are unable to shake [12]. So, if you truly believe in your heart that radiation from CT scans causes cancer…then no scientific argument to the contrary is likely to change that belief. But if you haven’t given this much thought or are on the fence, then perhaps this piece may help.

So what is your matka here?

If you are scared to get CT scans or mammograms done, because you believe radiation causes cancer (there is an idiotic WhatsApp meme that rears its head every year or so about how mammograms irradiate the thyroid gland and cause thyroid cancers and we have had women asking for thyroid shields during mammograms because of this), then don’t be. In fact, not doing a CT scan or PET/CT or mammogram when indicated, can actually turn out to be an adverse harmful matka, and lead to tragic consequences, as I have personally witnessed on multiple occasions.

Footnotes

1. American Association of Physicists in Medicine – position statement Oct 2018 - https://www.aapm.org/org/policies/details.asp?type=PP&id=439

2. Position Statement of the Health Physics Society, Jan 2019

3. https://www.acr.org/-/media/ACR/Files/Radiology-Safety/Radiation-Safety/Dose-Reference-Card.pdf

4. Brenner DJ, Elliston CD, Hall EJ, Berdon WE. Estimated Risks of Radiation-Induced Fatal Cancer from Pediatric CT. American Journal of Roentgenology. 2001 Feb;176(2):289–96.

5. https://www.forbes.com/sites/jamesconca/2016/06/24/radiation-poses-little-risk-to-the-world/?sh=292baa2b4e16

6. Brenner DJ, Hall EJ. Computed tomography--an increasing source of radiation exposure. N Engl J Med. 2007 Nov 29;357(22):2277-84. doi: 10.1056/NEJMra072149. PMID: 18046031.

7. Pearce MS, Salotti JA, Little MP, McHugh K, Lee C, Kim KP, Howe NL, Ronckers CM, Rajaraman P, Sir Craft AW, Parker L, Berrington de González A. Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. Lancet. 2012 Aug 4;380(9840):499-505. doi: 10.1016/S0140-6736(12)60815-0. Epub 2012 Jun 7. PMID: 22681860; PMCID: PMC3418594.

8. Shibata S, Shibamoto Y, Maehara M, Hobo A, Hotta N, Ozawa Y. Reasons for Undergoing CT During Childhood: Can CT-Exposed and CT-Naive Populations Be Compared? Dose Response. 2020 Feb 17;18(1):1559325820907011. doi: 10.1177/1559325820907011. PMID: 32110172; PMCID: PMC7026821.

9. Lee KH, Lee S, Park JH, et al. Risk of Hematologic Malignant Neoplasms From Abdominopelvic Computed Tomographic Radiation in Patients Who Underwent Appendectomy. JAMA Surg. 2021;156(4):343–351. doi:10.1001/jamasurg.2020.6357

10. Jones AK, Marsh RM. Beyond the AJR: "Risk of Hematologic Malignant Neoplasms From Abdominopelvic Computed Tomographic Radiation in Patients Who Underwent Appendectomy". AJR Am J Roentgenol. 2021 Apr 21. doi: 10.2214/AJR.21.25968. Epub ahead of print. PMID: 33881903.