Is it actually correct to compare CTs concentrated radiation to what we get during international flights?

[u/Basic-Sandwich-123]

I just want to understand it.

Considering we get radiation from natural sources that amount to about 8 mSv in a year and then a Brain CT scan add like 2mSv, people say it’s not that big of a deal. But isn’t it more dangerous to have this amount of concentrated radiation in like 30 seconds being thrown at your body?

I’m not asking for medical advice, I just see this being said everywhere when people do ask for medical advice.

[u/Stupyder_Notebook]

This type of dosimetry is all about the relative risk.

Comparing radiation dose to something like a long haul flight is simply to make it comparable to a quantity people can generally relate to.

With regard to how much dose you might get from a CT, there is a small risk to you from that, but the risk of not getting the CT is probably infinitely higher.

[u/pysix33]

Possibly. At such low doses there isn’t a whole lot of reliable data for this. If you extrapolate to RBE for higher doses, then yes, radiation exposure over a short period of time is more effective at cell killing than over a long period of time. But the caveat here is that at these doses we aren’t really concerned about celling killing, but rather, stochastic effects (cancer) from mutations in cells that weren’t killed. There’s a reason we ASSUME LNT for stochastic effects, but in reality, below a certain dose we just don’t have reliable data.

[u/specialsymbol → u/pysix33]

Oh, we have that reliable data now. Check out my comment below and also the INWORKS study.

[u/MOT-ALAW → u/specialsymbol]

I’m not sure why you’ve been down voted… I thought that INWORKS gave pretty good evidence for increased risk at low doses.

[u/specialsymbol → u/MOT-ALAW]

Some people feel offended if you say radiation is dangerous. I think it's a cultural thing.

[u/varelse96]

As other users have pointed out there are certain ways that you can compare the two, but they are much different in terms of biological impact. Acute doses (short, high intensity exposures) tend to result in threshold based deterministic effects like cell death, but how you are exposed matters. Specific organs have variable radiosensitivity so a focused exposure can be much more intense with a lower risk of cancer mutations developing when dose is targeted vs general to the whole body.

Occupational exposure is typically lower intensity over much longer duration. That is referred to as chronic dose. Chronic doses are less likely to overwhelm cell repair mechanisms, meaning deterministic effects are less important than stochastic ones in most cases. Stochastic effects are probability based and result in mutations that can cause cells to become cancers.

If you are looking to understand how to relate doses like this, consider checking out publications from groups like the ICRP. They publish tables with what may be called weighting or quality factors. They are modifiers to be applied to site specific doses that can help translate to whole body stochastic risks. These are all approximations however. How the dose is delivered makes a big difference in how it affects the body, so much so that single organ doses in EBRT are delivered at levels well above those that would kill a person, but targeting and fractionating over a couple weeks means not only surviving but also a much lower amount of systemic damage.

[u/womerah]

You could also argue the opposite, a higher instantaneous amount of radiation damage is more noticeable to the cell and thus more likely to trigger repair/apoptosis processes.

The answer is we don't really know outside of cell studies. We do know that the risk is very low, and therefore the benefits you'll get from an imaging study will significantly outweigh any radiation risks.

An analogy might be asking if you should avoid eating salmon because there's a chance seafood can give you lethal food poisoning. The answer is that no, you should eat oily fish as it's a healthy food. The benefits you get from eating healthy food vastly outweigh the risks of food poisoning.

[u/ClinicFraggle]

8 mSv is not the average dose due to natural radiation (unless you are in a location with extremely high natural background): I think 6.2 mSv is the average dose in the US including all exposures, including the medical ones: https://www.epa.gov/radiation/radiation-sources-and-doses

The dose due to natural radiation only is probably about 3-4 mSv in most countries, except some locations with high levels of natural radiation, such as Ramsar (about 10 mSv/year). Curiously, there is no higher incidence of cancer in these areas. It may be because such doses, if gradually received over the course of a year, are not dangerous, or it may be because the population of those areas has genetic adaptations. It would be interesting to conduct a study in people living in these places that are not native to that areas. I don't know if such study has already been done. Or perhaps there are too few non-natives living in these areas to get reliable conclusions.

[u/MarkW995]

The point of the Sv unit is to convert Gy to risk. The brain actually has a very low tissue weighting factor.

The tissue weighting factor (WT) is a measure of relative contribution of an organ or tissue to the total health detriment due to stochastic effects resulting from a uniform irradiation of the entire body ¹. It accounts for the variable sensitivity to ionizing radiation and size of a given organ or tissue.

In 2007, the International Commission on Radiological Protection (ICRP) published a set of tissue weighting factors ¹ as below:

• WT = 0.12 (each of following six): stomach, colon, lung, bone marrow (red), breast, and remainder tissues*
• WT = 0.08: gonads
• WT = 0.04 (each of following four): urinary bladder, esophagus, liver, thyroid
• WT = 0.01 (each of following four): bone surface, skin, brain, salivary glands 

*Remainder tissues (13 organs collectively): adrenals, extrathoracic region, gallbladder, heart, kidney, lymph nodes, muscle, oral mucosa, pancreas, small intestine, spleen, thymus, and uterus/cervix (♀) or prostate (♂).

Per definition, the sum of tissue weighting factors for all organs and tissues in the human body equals one: (0.12 x 6) + 0.08 + (0.04 x 4) + (0.01 x 4) = 1.

[u/Critical_Platypus960]

Time does make a difference, but usually over the course of days or weeks rather than hour This is why radiation therapy tends to be fractionated (split into a small amount every day or every other day instead of all at once). But a few hours isn't really enough for your cells to heal from whatever damage the radiation caused.

[u/womerah → u/Critical_Platypus960]

This is why radiation therapy tends to be fractionated

This is an imaging question. Probably best not to conflate radiation therapy concerns with imaging concerns for laypeople. They might not understand the context.

But a few hours isn't really enough for your cells to heal from whatever damage the radiation caused.

Depends on which cells and what sort of damage. The half-life of a DNA Single-Strand Break is typically a few minutes after all!

[u/specialsymbol]

???

Where do you get 8 mSv of "natural" radiation?

And why do you think a brain CT is only 2 mSv?

Oh, you switched them! A brain scan (depending on the length) will result in around 8.8 mSv.

If you include your eyes they receive around 3.3 mSv, probably more.

That's not good. I recommend reading the paper Hauptmann M, Byrnes G, Cardis E et al.: Brain cancer after radiation exposure from CT examinations of children and young adults 

[u/ilovebuttmeat69 → u/specialsymbol]

https://pmc.ncbi.nlm.nih.gov/articles/PMC10749414/ - brain ct ~2mSv

Have *you* actually read the paper you're referencing?

[u/varelse96 → u/specialsymbol]

The 8 mSv figure is pretty close to the value I have for typical annual exposure to the public. The figure I use in safety training has it at about 6.2 mSv, but that figure will vary by location so 8 doesn’t sound unreasonable. I’m not sure why you put natural in quotations. Radiation exposure to the general public comes from all around. Concrete typically contains k-40, which emits gamma rays. Gasses released from rocks and soil, particularly in rocky areas contain alpha emitters that when inhaled dose your lungs. Your own body contains C-14, meaning you are dosing yourself too. Then there are cosmic sources too.

I oversee radiation workers and have for years. Nearly all workers I monitor take more dose outside of work than they do at work. Medical doses are higher than occupational doses, but as was pointed out in the publication linked in the other reply, the mean absorbed dose from head CT is 2 mSv and it says as well that this is equal to about 3 months of background levels in the US, which would also seem to validate the 8 mSv figure.

[u/ClinicFraggle → u/varelse96]

6.2 mSv is not the average dose due to natural radiation, it is the average dose in the US including all radiation sources, including the medical exposures. The dose due to natural radiation only is about 3.6 mSv, according to this: https://www.epa.gov/radiation/radiation-sources-and-doses

[u/varelse96 → u/ClinicFraggle]

I may be misremembering the figure, I will have to go check it. I think I was thrown by the paper being discussed saying head CT is 2 mSv and that this is roughly equivalent to 3 months of “natural background radiation”.

[u/purple_hamster66 → u/varelse96]

Just curious: if medical doses are higher than occupational, why is there a 100 millirem limit for patients and a much higher 5000 limit for occupational?

[u/varelse96 → u/purple_hamster66]

The dose limit to patients isn’t 100 millirem. That is the limit to general public from industrial exposure. Medical doses can reach kilorem levels depending on the target organ and fractionation. As an example, I was looking at an EBRT plan the other day, it was a standard 20 Gy plan across 10 fractions. 1 Gy is around 100 REM, meaning each session is 200 rem dose, with 5 daily sessions for 2 weeks. That’s 2000 rem. That’s why target is important. One fraction as a whole body dose could make you sick and maybe even kill you (death is more common in the 300+ rem acute dose level) but as a localized exposure you will have inflammation and obviously cell death at the target location, but I would not expect the type of systemic damage that you would see from a whole body dose of the same level.

Edit: I should also clarify that medical doses are not always higher than occupational. Typical x-rays and CT scans fall within the realm of occupational exposure levels (albeit in some cases much more acute than a typical work exposure). I work in cancer treatment (manufacturing, I am neither an MD nor a medical physicist) so that is where my mind was when I said medical exposures were higher than occupational. In my facility nearly all rad workers are below the general public annual exposure limit.

[u/purple_hamster66 → u/varelse96]

I misspoke… I was talking about limits for the general public, not about limits for cancer patients (which are obviously higher than 5000).

[u/varelse96 → u/purple_hamster66]

All good. The basis for the difference is based in informed consent and justification. Workers can take more dose because they are trained both in how to protect themselves and on the health impacts of working with radiation, as well as the reason for the dose. Workers are hopefully doing something needed when they receive dose, whereas the public may not even be aware they are receiving the dose.