Re: Skin Dose from Tc-99m

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

Sorry, but in my first response to your query, I only provided the
information you needed to calculate the skin dose once you knew the
amount of Tc-99m activity on the skin.  That was half the information
you wanted.  the other part was quantifying the activity.

I hate to use a GM pancake to try to quantify  pure gamma emitters,
because the intrinsic detection efficiency is really lousy. For gamma
emiiters measured with a Ludlum 44-9 GM pancake probe, depending on the
isotope (energy and abundance of gammas emitted),  the detection
efficiency is always less than 1%.   Several years ago I hired a summer
student and we did extensive detection efficiency measurements (with 14
different calibrated disc sources - both betas and gammas) at various
source-detector distances on  the portable contamination monitoring
instruments in all our labs, and  subsequently calibrated the
instruments for lab use so that our folks can quickly and easily relate
a detector reading to actual contamination level (Bq/cm^2)   for the
radioisotopes they are using.

If you use a crystal-based detector - for instance the Ludlum 44-3 (1 mm
thick NaI) or the sandwich probe Ludlum 44-21  (plastic and NaI
detector), you will have a much higher detection efficiency (around
9-10% for Co-57), but in my experience are likely to peg the meter.

We have solved this problem by using the gamma camera as the instrument
for quantifying Tc-99m skin contamination.   Turn the camera (with the
low energy collimator on) face up, cover it it a protective underpad,
and have the tech  place the contaminated hand directly on the camera
face surface.  You can either accumulate counts for a given time or a
preset count.  Then do the same with a known amount of  Tc-99m on a
piece of blotter paper.  You can then use the raw data  to calculate the
amount of activity, or you can use the gamma camera software for some
more sophisticated manipulations (i.e. drawing an ROI around the
contaminated spot  to calculate the surface area contaminated, etc.).

I am directly responsible for radiation safety in nuclear medicine in
two university teaching hospitals and consult to NM departments in 5
other community hospitals and two clinics, so have faced this situation
more than a few times.

I hope this is helpful and good luck!!

Karin Gordon
Radiation Safety Office r