Re: S-35 Radioactive Contamination

[Karin Gordon <kgordon@cc.UManitoba.CA>]

Hi Ninni

Our radiopharmacist next door suggested that if there were no skin lesions
(cuts, abrasions, etc) as an entry point for the contamination into the
systemic circulation, it was highly unlikely the S-35 methionine would be
absorbed through or into the skin.  The drop in detected activity would most
likely be due to skin exfoliation. - By the way, did you know that
housedust - which coalesces into all those cute fuzzy things under the bed
is largely comprised of exfoliated skin cells? We shed millions of those
things every day!   

With respect to dosimetry, you might want to reference the following 
article: 

Calculation of the Dose to the Basal layer of Skin from Beta/Gamma 
Contamination.  AR Curran, Journal of the Society of Radiological 
Protection 6 (1) 1986 23-32.

I doubt that most North American RSOs are members of the Society of 
Radiological Protection (UK) and therefore do not have ready access to 
it's excellent quarterly journal - why I joined.  For that reason, 
I'll give you some helpful data from that paper.


When calculating skin dose, you are really interested in the dose to the
deepest or basal layer of the epidermis, where new skin cells are forming,
all the more superficial layers develop from this. The basal layer is
therefore the most radiologically sensitive layer of the skin.

Curran gives the skin thickness for 4 sites on the extremities:

Body site		Mean and standard deviation
			for epidermis thickness in 
				mg/cm-2

Back of hands			8.5   +/- 2.6
Wrists				8.0   +/- 0..8
Fingers				22.0  +/- 12.0
Finger tips			37.0  +/- 11.2

He then provides, for selected radionuclides (C-14, P-32, C0-58, Tc-99m,
I-125, and Cs-137) tables of dose rate for contamination either
distributed evenly on the surface or uniformly distributed within the top
1 mm layer. 

The table for C-14 (beta max is 0.156 Mev, similar to S-35 with 0.167 MeV
-close enough) indicates, at various depths, the dose rate rads/hr for 1 
uCi distributed evenly over the surface of 1 square cm skin.

The dose rates at depths of 8, 20, 30 and 40 mg/cm-2 are respectively,
0.615, 0.0105, 0.00035 and 0.00001 rads per hour.

If you assume the contamination has penetrated the skin surface and is 
distributed uniformly to a depth of 1 mm, the corresponding dose rates 
are 0.0347, 0.0225, 0.0164 and 0.0129 rads per hour. 

You can see the difference in distribution (surface vs penetrated) causes a 
huge difference in basal layer dose.

Obviously, because of the rapid skin sloughing, you will have to take a 
number of measurements over time, graph them and then do the necessary 
mathematics to come up with an integrated dose. It doesn't appear you are 
going to exceed any dose limits.

Thank the good Lord it wasn't P-32, the dose rates are humungous!

If you want a reprint, contact Dr. Curran - his 1986 address was as follows:
PPC Consultant Services c/o NEI Waste Technologies Ltd, PO Box 13, 
Saltmeadows Road, Gateshead, Tyne & Wear, England NE8 1YZ.  Perhaps some 
of our UK members would know if it has changed?

I've given you all the above because the dose rates printed in NCRP no.111
(page 25) are only good for a depth of 7 mg/cm-2, the average skin depth
over the trunk - not suitable for hands, where the skin is much thicker, 
and contamination more likely to occur!!. 
 
Hope this is helpful, Ninni - You owe me a beer at the CRSO meeting. See 
you in LA!!

Karin 

Karin Gordon
Radiation Safety Office			phone (204) 787-2803
Health Sciences Centre			fax   (204) 787-1313
820 Sherbrook Ave			e-mail KGordon@cc.umanitoba.ca
Winnipeg, MB. Canada R3A 1R9



On Thu, 2 Mar 1995, Ninni Jacob wrote:

> There has been a discussion on simulation of contamination, and now I have
> a real one that I need some advice on.
> 
> Yesterday, a student's thumb was contaminated with S-35 methionine. I
> estimated 54 nCi in  a 1/2' X1/2" square area of the thumb? We tried all
> the conventional decon methods. Nothing reduces thge activity.
> 
> Does anyone have a program that would calculate the dose to the skin at
> that area?
> Today the activity is down to 18 nCi. Is this because of the desquamation
> of the skin or is there some absorption also?
> 
> What would be your advice on taking a  urine sample? grab sample? 24 hour
> sample?
> 
> Thanks in advance for any help.
> 
> Ninni Jacob
> Radiation Safety Officer
> University of Rhode Island
>