Re: Another question re: dose

[BLHamrick@AOL.COM]

In a message dated 09/06/2002 5:20:46 PM Pacific Daylight Time, idias@interchange.ubc.ca writes:





> A whole body Esub(eff) of 10 mSv/year has a much higher risk than the same
>
> Esub(eff) to a smaller tissue.





I think the original question was why this is so.


And, I think the simplistic explanation is that you're simply irradiating more cells.  I.e., the risk of inducing cancer is proportional both to the dose to an individual cell and the number of cells receiving that dose.


Since cancer induction is a statistical phenomena, the more tissue irradiated, the more opportunities for cancer induction.  Thus, if I irradiate one cell in my body (I realize that's a spherical cow type of problem), and the theoretical risk for that cell cancerously mutating at that dose is 5E-24, then my total risk of cancer incidence increases by 5E-24.  But, if I irradiate the 5E+13 cells in my body at the same dose, I now increase the total risk of cancer induction to 5E-24 x 5E+13 = 2.5E-10.


All this is also tempered by the fact that some cells are more radiosensitive than others.  So, for example, irradiating a finger skin cell does not result in the same effect as irradiating a bone marrow cell.  There will be, effectively, a weighted average that determines the end risk value.


Ultimately, however, these numbers are not known.  The risk factors used by the standard-setting community are extrapolated from high dose/high dose-rate exposures that have caused cancers at some reasonably statistically significant rate.  One can extapolate downward in a simplistic manner to (theoretically) save time and argument over the matter, but, as the Health Physics Society acknowledges, below 5 rem acute dose, and 10 rem lifetime, any assignment of risk is not scientifically sound.  Even well above those doses the matter is still very much up for grabs, but may well fall into the area where it is appropriate to begin a discussion.


Barbara