Re: X-Ray Dosimetry for Children

[Philip Hypes <phypes@lanl.gov>]

>1.  How does one calculate organ doses from x-rays in children where no data
>(i.e., organ dose per entrance exposure) is available?

I'd say the best way to obtain defensible numbers would be by duplicating the
intended projections on a phantom.  Record the dose, and then calculate the
dose to the organ in question (say, bone marrow), taking into account the
amount of the organ that is irradiated and the shielding provided by other
organs.  You could either use adult phantoms if you have them and calculate
down to the necessary dimensions, or make some water phantoms that are
the right size to begin with.  I know you are just searching for ways to spend
your time, so you'll be glad to realize that for the age range you mention,
you will probably need to use 3 or 4 sets of phantoms to cover the
anticipated range of body sizes.

>2.  Would it make sense to calculate the organ doses to adults (much more
>data available) and multiply by 2 or 3 (ICRP 62 states risk to children is 2
>to 3 times that of an adult)?

For equivalent tissue volumes, that would probably be a fair approximation.
I would be inclined to correct for the difference in irradiated volumes
between
a child and an adult before multiplying by 2 or 3, however.  Unless the ICRP
values are risk per unit volume of irradiated tissue, just multiplying the
adult
values would probably be excessively conservative.  Of course, that might
not be a bad thing;  your call.

>3.  Even the available data do not address organ doses (e.g., to the bone
>marrow) from extremity x-rays.  Is it necessary to calculate effective dose
>(equivalent) from x-rays of the extremities e.g., lower legs, hands, feet,
>etc.) and if so, how would one go about it (This applies to adults as well)?

Short answer;  get your extremity doses and then decide if organ doses
are necessary.  Calculating the dose to the exposed volume might give you
enough information to make a recommendation to the doctor.  That can be
done easily enough with phantoms as I discussed above.  I wouldn't want to
decide if organ dose calculations are necessary until I had the extremity
doses
from the machines in question in hand.  Which leads me to point out that
unless you have a truly extraordinary QA program to ensure equivalent output
among the machines in your department, it would be best to make your
measurements on the actual machines that will be used in the study.

The first step in determining the organ dose would be to multiply the dose
rate in the beam by the transmission factor for the tissue between the tube
head and the organ of interest.  Unless you can find a reference, you will
probably have to determine the transmission factor from the densities of
the tissues and the kVp/HVL of the beam.  For fatty tissue, water is probably
a good approximation.  Bone densities are between 1.7 and 2.0 g/cm^2,
according to HPRHH p. 153.  I would assume this value refers to hard bone,
not marrow.  I would expect muscle to fall somewhere between water and bone,
probably closer to water than bone (See p.l80 of Cember).  Now do a volume
correction to account for the difference in volume between the chamber you
used to get the dose rate and the volume of the organ being irradiated.  That
should give you a fair value for the organ dose.

>4.  Has anyone attempted to translate the effective dose equivalent quoted
>in NCRP 93 (360 mrem/yr) to effective dose?

Sorry, not I.

>Any ideas or references (I have looked up some references, but they don't
>appear to have all of the necessary info) would be appreciated.

Just a general comment;  keep in mind that you are not determining an actual
dose/risk for a particular patient.  These sorts of procedures are intended to
give an indication of the dose levels that might be expected from a projection
to a theoretical patient who matches the phantom very well.  In order to be
able to discuss more precise doses to a particular patient, you would need to
use the same machine, with as accurate a phantom as possible, and precise
physiological data about the patient.  The precision-limiting factor in any
discussion of risk to such a patient would be the accuracy of our knowledge
about dose to risk conversion, and we all know that individual variations can
be considerable.

Hope this helps, and that everyone feels free to point out any errors in my
comments.
___________________________________________________________
Philip Hypes
Los Alamos National Laboratory
Safegaurds Science and Technology Group (NIS 5)
(505) 667-1556  phypes@lanl.gov

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