An effective halflife estimate of that sort is what they gave one of
my co-workers. It was wrong in practice. He was 'hot' for well over
the 23 days indicated by that thumbrule for a 201 Tl stress test (and
far over the week-and-a-half the lab tech told him). It was more like
2-3 months before he was able to pass through a portal monitor without
an alarm.
If there was an isotopic analysis to check for impurities that might
have skewed the figures, I never heard about it.
Dave Neil
neildm@inel.gov
______________________________ Reply Separator _________________________________
Subject: Re: Thalium Stress Test (fwd)
Author: RADSAFE (INELMAIL.RADSAFE) at _EMS
Date: 7/18/97 7:06 AM
Hello All,
Bruce Busby stated in part:
<snip> the reading was coming off the hand of a man
>four rows back in the audience who had had a heart thalium stress test
>earlier in the week. Of course the question came up on what was the
>approximate exposure for an individual who receives a stress test and is
>their any exposure to others. Can you help me out???
Typical thallium injected activity is 4 mCi or less. The effective
half life of thallous chloride is 56 hours (T 1/2 physical = 73 hours). The
specific gamma ray constant for 201-Tl is 4.7 rads/(mci-hr) at 1 cm
distance. We can compute the total dose at one meter for example, ignoring
attenuation within the body, which is very substantial, using the formula:
total dose = 4 mCi x 1.44 x 56 hours x 4.7 rads / mCi-hr x (1 cm / 100 cm)^2
=150 millirads at one meter distance.
This assumes another person remains next to this person the entire
time that this material is present in their body (10 half lives = 560
hours!), with no attenuation of the material by the body.
On a more practical note, our nuclear med technologists used to
inject 7 patients/day 5 days a week with 201-Tl, and remain in the room
while SPECT acquisitions were acquired (20 minutes each x 2 studies, stress
and rest). (We now use mostly 99mTc instead of 201-Tl.) The average
technologist annual film badge dose was 250 millirem per year. This
includes working in a hot lab, injecting patients and patient studies
utilizing other isotopes. Clearly, the diagnostic use of radioisotopes does
not constitute a substantial dose risk to the public. It does fire off a
survey meter nicely, however.
Regards,
Mark S. Rzeszotarski, Ph.D.
Medical Physicist