Should Healthy People Use CT for "Physical Examinations"?#
#Omnivore
Highlights#
Use it if there is a medical necessity, otherwise don't bother. Extra attention to risk-benefit considerations is needed when using it for children and adolescents, who are more sensitive to the risks. ⤴️ ^1a644e08
This article is from the WeChat official account: A Biologist's Science Park (ID: Story-about-Science), written by Y's Science Park, original title: "Healthy People Should Not Use CT for 'Physical Examinations,' but the Claim that One CT Scan Increases Cancer Risk by 43% is Misleading," cover image from: Visual China
This article discusses whether healthy people should use CT scans for physical examinations and the radiation risks associated with CT scans. The article points out that using CT scans for full-body cancer screening in healthy individuals is misleading because the effectiveness of this technique is very low in healthy populations. In addition, CT scans and other imaging tests such as PET-CT scans produce radiation, and excessive use of these advanced medical imaging tests can increase the risk of cancer. However, the claim in the hot search that one CT scan increases the risk of cancer by 43% is misleading. In fact, there is insufficient research on the cancer risks of low-dose radiation, especially in children and adolescents who are more sensitive to radiation risks. Therefore, risk-benefit considerations are necessary when using CT scans, especially for sensitive populations.
• ⚠️ Using CT scans for full-body cancer screening in healthy individuals is highly inefficient and a waste of medical resources.
• ☢ CT scans and PET-CT scans produce radiation, and excessive use increases the risk of cancer.
• _right: The claim that one CT scan increases the risk of cancer by 43% is misleading; the actual risk is influenced by the dose and individual sensitivity.
A hot search a few days ago claimed that one CT scan increases the risk of cancer by 43%:
The source of the news is an academician who proposed that healthy individuals should not undergo cancer screening, probably targeting the widespread use of PET-CT scans and other techniques for physical examinations in recent years, claiming to screen for full-body cancer. It is completely correct to call for not using CT scans and other advanced medical imaging tests for routine physical examinations in the general population.
First, it is misleading to claim that these techniques are used for full-body cancer screening during physical examinations. Previous studies have clearly shown that the efficiency of using techniques such as PET-CT scans for cancer screening in healthy populations is extremely low and meaningless, and it is a waste of medical resources. Second, CT scans, PET-CT scans, and others produce radiation, and according to the "high-level comprehensive physical examination" approach, the radiation dose received by the examinee is very high, undoubtedly causing harm, including an increased risk of cancer.
However, the claim in the hot search that one CT scan increases the risk of cancer by 43% is also misleading. CT scans are indispensable in many cases, and such sensationalism causes panic and unnecessary distress to the public.
Speaking of which, the first article I wrote for my public account was about why PET-CT scans should not be used for full-body cancer screening, and it also mentioned radiation issues. If you are interested, you can take a look: PET-CT Radiation
The claim in the hot search that one CT scan increases the risk of cancer by 43% comes from a paper published in Nature Medicine in November last year:
Despite being published in a top academic journal, using it in news headlines is definitely misleading.
There are two types of radiation we encounter in our daily lives: non-ionizing radiation and ionizing radiation. Non-ionizing radiation, such as cellphone radiation and microwave radiation, has low energy and no carcinogenic risk (online claims that cellphone signals cause cancer are all rumors).
However, ultraviolet rays and X-rays, for example, have high energy, to what extent? They can damage the DNA molecules inside cells, which leads to the risk of genetic damage and genetic mutations. The basis of cellular carcinogenesis is genomic variation, so ionizing radiation carries a risk of cancer. This is why ultraviolet rays can cause skin cancer.
CT scans and PET-CT scans both involve ionizing radiation. In addition, the harm caused by ionizing radiation can be divided into two categories: acute injuries caused by high doses in a short period, manifested as acute radiation syndrome, also known as radiation sickness. For example, in TV series like "Chernobyl," those who were present during or later entered the scene for rescue and cleanup quickly experienced nausea, vomiting, and even death, which belong to this category.
Radiation-induced cancer belongs to the second category of harm caused by ionizing radiation, which refers to the impact on health when the exposure dose is not very high.
In terms of radiation hazards, the higher the dose, the greater the harm, which everyone knows, but another point is that the higher the dose of radiation, the more scientifically clear its harm is.
For example, there is a lot of evidence that radiation increases the risk of cancer when the dose exceeds 100 mGy. However, there is a lack of evidence for cancer risks below 100 mGy. Some risk analyses simulate based on studies above 100 mGy, which introduces several assumptions, such as the carcinogenic risk caused by radiation will not decrease to 0, and there is a certain range of linearity, etc. With so many assumptions, the results are naturally highly controversial. The paper in Nature Medicine was conducted because of the insufficient research on the cancer risks of low-dose radiation.
This study tracked more than 900,000 children and adolescents in nine European countries, collected data on their CT scans, and the occurrence of blood tumors (such as leukemia and lymphoma) in the future, analyzing the correlation between CT scans and the increased risk of blood tumors. The results showed that receiving CT scans did increase the risk of blood tumors.
However, the claim in the hot search that one CT scan increases the risk by 43% is a misunderstanding. In the paper, it is stated that one CT scan increases the risk of cancer by 43%, but with a premise: in the case of receiving multiple CT scans. What happened? The researchers compared people who received more than one CT scan with those who only received one, and observed changes in the risk of blood tumors. This can be seen in Table 7 of the supplementary materials of the paper:
By comparing, it was found that people who received 2-3 CT scans had a higher risk of blood tumors than those who only received 1, and those who received 4-5 scans had a higher risk than those who received 2-3 scans. In other words, there is a certain dose relationship, and by converting this dose relationship into the risk corresponding to each CT scan, the result of one CT scan increasing the risk of blood tumors by 43% is obtained.
It is important to note that this comparison is based on people who have received 1 CT scan as the baseline. It is about how much risk is increased per CT scan in multiple CT scans, not that one CT scan increases the risk by 43%.
In fact, this paper analyzes the dose relationship of radiation in multiple ways, not just for multiple CT scans. For example, in Table 2:
The researchers used receiving radiation below 5 mGy as the baseline and grouped the remaining people according to the dose they received, comparing the risk of blood tumors. It was found that in the group of 10-15 mGy, the risk of blood tumors was significantly higher than that below 5 mGy, and there was a dose relationship thereafter—the more radiation received, the higher the risk of blood tumors. It is worth noting that there is no significant difference between the 5-10 mGy group and the group below 5 mGy, and the risk becomes more apparent with higher doses.
The dose of one CT scan varies depending on the type of examination, but on average, it is around 8 mGy in medical imaging. The researchers of this paper also made calculations. Receiving such a dose of radiation increases the risk of blood tumors by approximately 16%. However, this is for children and adolescents, who are more sensitive to the risks of radiation. We cannot casually extend it to adults in their thirties and forties, using the risk analysis of children, which is not appropriate, right?
Then, based on a dose of 8 mGy per CT scan, increasing the risk of blood tumors by 16%, further calculations were made regarding the impact on society. So, if ten thousand children and adolescents undergo CT scans, there will be an increase of 1.4 cases of blood tumors in the next 12 years due to these scans.
How should we view such risks? Obviously, the risk of CT scans causing cancer is a real existence, just like the fact that CT scans involve radiation, which cannot be changed. However, with fewer scans, the radiation dose received is lower, and the risk is also lower. For example, if ten thousand young people undergo CT scans and there is an increase of 1.4 cases of blood tumors, this absolute impact is acceptable. Especially when we need to consider many situations where CT scans are necessary for medical reasons, not waiting for 12 years to see if there is a slight increase in the risk of blood tumors, but immediate life-threatening issues.
In fact, the insights from the paper in Nature Medicine are consistent with the previous recommendations in the medical community regarding the use of CT scans and other radiation-based medical imaging: ==Use it if there is a medical necessity, otherwise don't bother. Extra attention to risk-benefit considerations is needed when using it for children and adolescents, who are more sensitive to the risks.==
When considering risk-benefit, we can also compare it with the radiation we naturally encounter in our daily lives. Radiation units in medical imaging are often expressed in mGy or Gy (1 Gy equals 1000 mGy), which represents the absorbed radiation dose. Another commonly used radiation unit is mSv (millisievert, 1 sievert equals 1000 millisieverts).
mSv refers to equivalent radiation, and different radioactive substances have different strengths. mSv is a conversion to the same radiation dose for everyone. Generally, the natural radiation we encounter in daily life is expressed in mSv. The average annual background radiation received by each person worldwide is about 2 mSv to 3 mSv. One CT scan with a dose of 8 mGy is equivalent to two to three years of natural background radiation. And some full-body CT scans during physical examinations have a dose of over 20 mGy, which is equivalent to seven to eight years of radiation exposure. This cannot be ignored.
For occupations with radiation exposure, such as nuclear power plant workers, there is an upper limit on the annual allowable radiation dose. It is 50 mSv in the United States, 20 in Singapore, and in Japan, the total dose for five years should not exceed 100, and the dose for a single year should not exceed 50. According to the approach of some high-end physical examinations, the radiation dose received by the target audience is even higher than that of nuclear power plant workers. This kind of exploitation is quite dark humor.
References:
https://www.nature.com/articles/s41591-023-02620-0
https://www.health.harvard.edu/cancer/radiation-risk-from-medical-imaging
This article is from the WeChat official account: A Biologist's Science Park (ID: Story-about-Science), written by Y's Science Park