# Radiation question **URL:** \ **Category:** Factual Questions\ **Created:** [February 2, 2017, 10:01pm UTC](https://boards.straightdope.com/t/radiation-question/778858 "2017-02-02T22:01:42Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![race\_to\_the\_bottom](https://avatars.discourse-cdn.com/v4/letter/r/c4cdca/32.png) [@race\_to\_the\_bottom](https://boards.straightdope.com/u/race_to_the_bottom)\ **Post date:** [February 2, 2017, 10:01pm UTC](https://boards.straightdope.com/t/radiation-question/778858/1 "2017-02-02T22:01:42Z")
I recently had a low-dose CT scan of my chest in a program for the early detection of lung cancer. As I was leaving, I asked how many gray the procedure involved. The tech went into the control room and reported “114”. I asked about the prefix, milli, mega, kilo, etc. She didn’t answer but said “centimeter”, I said “square centimeter”, she said “cubic centimeter”. This left me confused. What was the 114 likely to be, and how much radiation is that? ---
**Author:** ![Blue\_Blistering\_Barnacle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_blistering_barnacle/32/3386_2.png) [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)\ **Post date:** [February 2, 2017, 10:24pm UTC](https://boards.straightdope.com/t/radiation-question/778858/2 "2017-02-02T22:24:07Z")
I’m not sure what she is reporting. Mostly people care about the volumetric CT Dose Index (CTDIvol), which is usually reported in milliGray (mGy). The goal for Low Dose Chest CT for an average size patient is around or just under 3 mGy, but could be higher in a larger diameter patient. There is also a Dose-Length Product (DLP) which can be used to estimate exposure (best for populations rather than individuals). This is mGy-cm. ---
**Author:** ![Blue\_Blistering\_Barnacle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_blistering_barnacle/32/3386_2.png) [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)\ **Post date:** [February 2, 2017, 10:27pm UTC](https://boards.straightdope.com/t/radiation-question/778858/3 "2017-02-02T22:27:05Z")
In the hypothetical case of a dose reported to be “114”, this is probably the DLP. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [February 2, 2017, 10:32pm UTC](https://boards.straightdope.com/t/radiation-question/778858/4 "2017-02-02T22:32:33Z")
I’m no expert on this, but [Google University](https://en.wikipedia.org/wiki/Gray_%28unit%29#Absorbed_dose_in_tissue) tells me that the average radiation dose from a CT scan of the abdomen and the pelvis is 14 mGy, so I’m guessing that is more or less what you got and the 114 anything is wrong. (If you had received 144 full Grays, I’m thinking that you wouldn’t be asking this question because of being dead.) ---
**Author:** ![Blue\_Blistering\_Barnacle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_blistering_barnacle/32/3386_2.png) [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)\ **Post date:** [February 2, 2017, 11:11pm UTC](https://boards.straightdope.com/t/radiation-question/778858/5 "2017-02-02T23:11:27Z")
> [@Blue\_Blistering\_Barnacle](#): > > In the hypothetical case of a dose reported to be “114”, this is probably the DLP. BTW, not out of expected range for a low dose chest CT. ---
**Author:** ![race\_to\_the\_bottom](https://avatars.discourse-cdn.com/v4/letter/r/c4cdca/32.png) [@race\_to\_the\_bottom](https://boards.straightdope.com/u/race_to_the_bottom)\ **Post date:** [February 3, 2017, 12:48am UTC](https://boards.straightdope.com/t/radiation-question/778858/6 "2017-02-03T00:48:27Z")
Thanks everyone, I was in error in stating that the woman I talked with was the tech. The tech had left the room. I never saw him again. This woman came in to wrap up an let me know that the procedure was complete, and she may have said something like 14. My impression was that she was not too knowledgeable. I was thinking milligrays as well. The radiologist’s report emphasized that the scan was low dose. ---
**Author:** ![race\_to\_the\_bottom](https://avatars.discourse-cdn.com/v4/letter/r/c4cdca/32.png) [@race\_to\_the\_bottom](https://boards.straightdope.com/u/race_to_the_bottom)\ **Post date:** [February 3, 2017, 1:01am UTC](https://boards.straightdope.com/t/radiation-question/778858/7 "2017-02-03T01:01:45Z")
A bit of Googling indicates that a low dose scan for early detection of lung cancer is 0.15 mgy, so 0.14 mgy seems right then. That is the same as 0.15 msv. Average inhaled exposure from Radon and its decay products is about 1.25 msv/year, so it looks like I didn’t get fried. Not more than the equivalent of 6 weeks exposure to background. > **[Background radiation](https://en.wikipedia.org/wiki/Background_radiation#Air)** > > Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is not due to deliberate introduction of radiation sources. > Background radiation originates from a variety of sources, both natural and artificial. These include both cosmic radiation and environmental radioactivity from naturally occurring radioactive materials (such as radon and radium), as well as man-made medical X-rays, fallout from nuclear weapons testing and nuc... BTW, I highly recommend anyone who has done any serious smoking and is past 60 or so seriously consider doing this yearly. It will catch cancers of about 5mm ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [February 3, 2017, 1:40am UTC](https://boards.straightdope.com/t/radiation-question/778858/8 "2017-02-03T01:40:27Z")
> [@race\_to\_the\_bottom](#): > > A bit of Googling indicates that a low dose scan for early detection of lung cancer is 0.15 mgy, so 0.14 mgy seems right then. That is the same as 0.15 msv. Average inhaled exposure from Radon and its decay products is about 1.25 msv/year, so it looks like I didn’t get fried. Not more than the equivalent of 6 weeks exposure to background. > > [Background radiation - Wikipedia](https://en.wikipedia.org/wiki/Background_radiation#Air) > > BTW, I highly recommend anyone who has done any serious smoking and is past 60 or so seriously consider doing this yearly. It will catch cancers of about 5mm Perhaps she said “point-fourteen” and you heard it as “114.” ---
**Author:** ![Blue\_Blistering\_Barnacle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_blistering_barnacle/32/3386_2.png) [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)\ **Post date:** [February 3, 2017, 2:58am UTC](https://boards.straightdope.com/t/radiation-question/778858/9 "2017-02-03T02:58:03Z")
I think your calc is off by a decimal. Dose estimation: Conversion factor for DLP (mGy-cm) to milliSieverts (mSv) for chest CT is 0.014. See Table 2 on pg 4 of ref below: [http://www.ajronline.org/doi/pdf/10.2214/AJR.09.3462](http://www.ajronline.org/doi/pdf/10.2214/AJR.09.3462) 114 \* 0.014 = 1.6 mSv This is “right” for a low dose chest CT. Regular chest CT would be more. Computed Tomography (CT)-Chest. 7 mSv Computed Tomography (CT)-Lung Cancer Screening. 1.5 mSv Radiography-Chest 0.1 mSv Taken from: > **[Radiation Dose](https://www.radiologyinfo.org/en/info/safety-xray)** > > Patient safety information about radiation dose from X-ray examinations and CT scans (CAT scans) ---
**Author:** ![race\_to\_the\_bottom](https://avatars.discourse-cdn.com/v4/letter/r/c4cdca/32.png) [@race\_to\_the\_bottom](https://boards.straightdope.com/u/race_to_the_bottom)\ **Post date:** [February 3, 2017, 3:27am UTC](https://boards.straightdope.com/t/radiation-question/778858/10 "2017-02-03T03:27:16Z")
Thank you Barnacle; that was VERY useful. Yeah, those decimals will get you every time. It looks like I absorbed about a half a year’s worth of average background in the US. That is not too bad, and it is x-ray radiation, not some radioactive particulate matter which could lodge in my lung and radiate forever. ---
**Author:** ![Blue\_Blistering\_Barnacle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_blistering_barnacle/32/3386_2.png) [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)\ **Post date:** [February 3, 2017, 12:32pm UTC](https://boards.straightdope.com/t/radiation-question/778858/11 "2017-02-03T12:32:56Z")
Just a caveat- I’ve been told that those methods of Dose estimation are more appropriate for populations rather than individuals. So that estimate, especially if combined with estimates from many other cases says more about how much exposure that radiology department is handing out (which, based on the single example seems appropriate) than it does about your individual exposure. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [February 4, 2017, 10:12am UTC](https://boards.straightdope.com/t/radiation-question/778858/12 "2017-02-04T10:12:02Z")
[http://www.usa.philips.com/healthcare/product/HCNOCTN193/ingenuity-ct-scanners](http://www.usa.philips.com/healthcare/product/HCNOCTN193/ingenuity-ct-scanners) indicates that lesser DLP would not be used for lungs, but yeah around 100 cGy cm starts to show the lungs… They are choosing DLP to get the lungs as the subject of the scan. ---
**Author:** ![race\_to\_the\_bottom](https://avatars.discourse-cdn.com/v4/letter/r/c4cdca/32.png) [@race\_to\_the\_bottom](https://boards.straightdope.com/u/race_to_the_bottom)\ **Post date:** [February 4, 2017, 9:45pm UTC](https://boards.straightdope.com/t/radiation-question/778858/13 "2017-02-04T21:45:47Z")
> [@Isilder](#): > > [http://www.usa.philips.com/healthcare/product/HCNOCTN193/ingenuity-ct-scanners](http://www.usa.philips.com/healthcare/product/HCNOCTN193/ingenuity-ct-scanners) > > indicates that lesser DLP would not be used for lungs, but yeah around 100 cGy cm starts to show the lungs… They are choosing DLP to get the lungs as the subject of the scan. They use the minimum necessary to show anomalies, but not in great detail. There are side by side comparisons of low dose and regular CT scans online, and the difference is apparent. ---
**Author:** ![Blue\_Blistering\_Barnacle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_blistering_barnacle/32/3386_2.png) [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)\ **Post date:** [February 4, 2017, 11:30pm UTC](https://boards.straightdope.com/t/radiation-question/778858/14 "2017-02-04T23:30:09Z")
Normal lungs are mostly air, containing linear tubes (blood vessels) and hollow tubes (bronchi), plus a bit more airy-fairy tissue and membranes. The is a lot of contrast between nodules and normal lung tissue. Even more so where there is emphysema. If the chief purpose of the scan is to screen for lung nodules, you can accept more noise, and therefore use less radiation. These noise levels would not be acceptable for a routine chest CT, where everything is on the table.