Re: Thallium 201

[stabin@esper.com (Mike Stabin)]

Darryl Kaurin wrote:
>
>Dear Radsafers,
>
>Regarding the stress test, I had one and they did use 4 mCi of thallium-201. 
>The health physics personal where I worked measured me daily, and the half
>life was 3 days, equal to the physical decay.  I couldn't work in radiation
>areas for 3 weeks.  I was told by the hp staff latter that you might be able
>to request they use Tc-99m insted, which has a much shorter half life, and
>less dose of course too.  I haven't calculated my dose, but it is certainly
>more than the dose I get at work.
>

Well, perhaps less dose.  These are the doses, in mGy/MBq of Tl-201 and
Tc-99m sestamibi, in resting patients or patients undergoing stress testing,
and the effective dose equivalents and effective doses in mSv/MBq:

                               Tl-201        MIBI - rest    MIBI - stress

Adrenals                       6.3E-02         4.3E-03          3.9E-03
Brain                          5.9E-02         1.8E-03          1.9E-03
Breasts                        3.6E-02         1.7E-03          1.6E-03
GB Wall                        8.3E-02         1.8E-02          2.5E-02
LLI Wall                       3.4E-01         3.7E-02          2.9E-02
Small Intestine                4.5E-01         2.7E-02          2.2E-02
Stomach                        1.9E-01         5.2E-03          4.7E-03
ULI Wall                       3.3E-01         5.0E-02          4.0E-02
Heart Wall                     2.8E-01         4.4E-03          4.8E-03
Kidneys                        4.6E-01         1.8E-02          1.5E-02
Liver                          9.9E-02         5.1E-03          3.7E-03
Lungs                          4.7E-02         2.4E-03          2.2E-03
Muscle                         4.6E-02         3.7E-03          3.3E-03
Ovaries                        1.0E-01         1.4E-02          1.2E-02
Pancreas                       7.4E-02         5.0E-03          4.6E-03
Red Marrow                     5.5E-02         4.5E-03          4.0E-03
Bone Surfaces                  8.8E-02         5.8E-03          5.4E-03
Skin                           3.3E-02         1.9E-03          1.8E-03
Spleen                         1.8E-01         5.2E-03          4.0E-03
Testes                         2.0E-01         3.5E-03          3.1E-03
Thymus                         4.6E-02         2.3E-03          2.3E-03
Thyroid                        6.2E-01         2.2E-03          2.2E-03
Urinary Bladder Wall           5.2E-02         3.7E-02          2.7E-02
Uterus                         8.5E-02         1.2E-02          1.0E-02
Effective Dose Equivalent      1.6E-01         1.5E-02          1.3E-02
Effective Dose                 1.4E-01         1.3E-02          1.1E-02
   
Bear in mind, however, that a typical Tl-201 test uses only around 50 MBq,
while a Tc-99m MIBI test may use 500-1000 MBq.  So the radiation doses are
fairly comparable.  In addition, some physicians will use multiple
injections of Tc to study the initial uptake, and later, the redistribution,
while Tl-201 has very good inherent redistribution.  OTOH, the Tc-99m is
better than Tl for doing first pass studies, which may be done in
conjunction and save the additional administration of a separate
pharmaceutical.  But my point is just that the dose picture is not that
markedly different.  I also thought that an accurate picture of the
dosimetry would be interesting.  You can always get access to such numbers
at our web site.  

I also agree with Liz Brackett's comment about the presence of contaminants
and their effect on external exposure.  The numbers above for Tl-201 include
these contaminants, BTW.

Mike Stabin
Radiation Internal Dose
  Information Center
Oak Ridge, TN
http://www.orau.gov/ehsd/ridic.htm