Organ dose and effective dose

[CSW69]

I recently recieved a Chest CT with and without contrast for a total of 20 mgy (msv). When measuring potential risk of cancer, would I assume the 20 mgy? It's my understanding the effective organ dose is much higher for the lungs, maybe as much as double so 40 mgy.

When looking at cancer risk probabilities, there is a large differnce in my dose being the 20 mgy vs 40 for the organ. Which is more accurate?

Thanks

[yellowbean]

Effective dose is organ specific.  You would have to know the organs of interest, the initial dose, and the tissue weighting factor of the organs or tissues irradiated to determine the mSv.  Basically each organ will have a different Organ Effective dose which is determined based on the sensitivity of the organ to the effects of radiation.  You add all of these organ effective doses to get your total effective dose for the scan.  

A Gray is a measure of the amount of radiation absorbed, it does not take into account the effects/sensitivity of that radiation on a specific tissue.  However gray and mSv are proportional since

Effective Dose = Dose Received * Tissue Weighting Factor

If Dose received is doubled the effective dose also doubles since there is a 1 to 1 mathematical relationship between the two, and the tissue weighting factor is a fixed value that you can look up on a table.  

However when comparing one organ to another, Dose of radiation as measured in Gray there can be very different than Effective Doses which is dependent on the tissue that is being irradiated, since tissues vary in sensitivity and Effective Dose is an attempt to quantify this difference between tissues.  

[CSW69]

That is a great exlanation, one of which I could not find in 100's of articles. Likely because each article has a particular slant and thus the trutht that's always in the middle never gets much attention.....

One last question - Given my dose accoring to the radiologist tech was right at 20 mGy, what is a best estimate of effective dose in mSv?

The reason I ask is that many artilces have suggested a one to one relationship for mGy and mSv.

Again, thanks for the explanation as well as the encouragement. Now I know my true organ dose is not 40 but as you said, 20 or less which means much less overall radiation risk than I had come to expect.

[yellowbean]

Well, you are kinda on the right track...but thinking about it a bit backwards....Effective dose is measured in Sieverts, which is a completely different (but related) unit that takes into account both the dose of radiation recieved and the sensitivity of the tissue irradiated based on the stochastic effects of radiation on the tissue in question.  

If you have a 20 mGy exposure, then that is the maximum dose any tissue in the path of the radiation can recieve.  In reality that is the dose exposed to at the skin on the beam side, and the dose decreases with tissue depth as some of the radiation is absorbed at each level as it penetrates tissue and is therefore gone and cannot expose the deeper tissues.  Many people don't understand this, but tissues actually absorb radiation, and only a small % of the radiation exits the body to form the image.  Say 100% radiation enters the skin, maybe a few percent leaves on the backside.

So, this is just a long way of saying that if the dose was 20 mGy, then at most the dose seen at the lungs will be 20 mGy, the only way to increase the dose of exposed radiation is to increase the time or energy of the beam (or both).

Risk of cancer induction is dependant on tissue as far as what type of cancer can/will be induced, but Effective Dose tries to take this into account by having a tissue weighting factor which is factored into the dose to determine relative risk between different tissues for any given dose.