Re: Thalium Stress Test (fwd)

[msr7@po.cwru.edu (Mark S. Rzeszotarski, Ph.D.)]

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