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Re: Weighting Factor for Hot Particle Skin Doses per Cm^2
I agree with the the points raised by Eddie Goldfinch. Just in case
there has been any misunderstanding - my earlier comments to Carol
Marcus were on the basis that the problem was how to make an
estimate of the contribution made to the effective dose by a local skin
exposure (small area, high dose - not necessarily a hot particle
exposure - maybe a high dose to the hands). ICRP in publication 60
included a tissue weighting factor for the skin - this means that strictly,
for the first time, the effective dose must include a contribution made
via skin exposure. ICRP publication 59 (Biological basis of skin dose
limitation) pointed out the variation in sensitivity of various skin areas -
related to UV exposure. ICRP 59 mentions the possible need to take
this into account in evaluating the effective dose. Neither ICRP 59 or
ICRP 60 gave any guidance on how this should be done. In the case
of a small area skin exposure, averaging the dose over the whole body
skin is a seemingly obvious thing to do - as Eddie suggests, and as I
mentioned in my earlier e-mail. This may however not reflect the
actual carcinogenic risk. If there is a high local dose to the skin of the
chest (usually shielded from UV) the carcinogenic risk will be very
significantly less than if the same dose was delivered over the same
area of the hands or the face (usually exposed to UV). If you average
the dose over the whole body skin area then you will significantly over-
estimate the skin cancer risk from the chest exposure and
underestimate the risk from the face exposure. Radiation
epidemiology studies (described in ICRP 59) are the basis for this view.
The NRPB document to which I referred considers how this variation in
sensitivity of various skin areas can be taken into account if necessary.
There are some practical situations of skin exposure in the workplace
and in the environment in the UK where NRPB indicate this may be
appropriate. In many cases where the skin dose is small the
contribution to effective dose is very small and this level of finesse is
probably inappropriate. My only point is that if you are interested in
these kinds of detailed considerations then NRPB have tried already
and you may wish to see how they developed their approach. If it is an
important wider issue then certainly ICRU/ICRP should look at it and
integrate it into their recommendations as Eddie suggests.
Monty Charles
**************************
On 24 Jan 2003 at 11:46, E.P. Goldfinch wrote:
> I rarely get involved in scientific issues discussed on the list. Skin
> dosimetry, especially in relation to extremities or small areas of skin, is
> an area to which ICRP and ICRU should be encouraged to consider more
> deeply. My understanding is that it is generally considered to be true that
> if the effective dose is within limits, then the skin is adequately
> protected from stochastic effects. The skin of the whole body is regarded
> as an organ and so the contribution of the dose to a small area of skin has
> to be averaged over the whole skin. This does not take account of
> deterministic effects, for which ICRP recommends averaging over 1 cm2, but,
> I understand, the USNRC now recommends 10 cm2.
>
> The subject is discussed further in The Journal of the ICRU Vol 2 No 2
> (ICRU Report 66) and in an Editorial in Radiation Protection Dosimetry Vol
> 102 No 1 (2002)
>
> E.P. Goldfinch
> Nuclear Technology Publishing
> P.O. Box No 7, Ashford, Kent TN23 1YW, England
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> Fax: (+44) (0) 1233 610021
> E-mail: <goldfinch@ntp.org.uk>
> Web site: <http://www.ntp.org.uk>
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>
Dr Monty Charles
Reader in Radiation Physics
Head of Radiation Biophysics Group
School of Physics & Astronomy
The University of Birmingham
Birmingham B15 2TT
Tel +44 (0) 121 414 3483
Fax +44 (0) 121 414 4725
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