[ RadSafe ] medical issues

Wed Jan 28 21:03:02 CST 2009

Radioisotopes used in medicine usually contain impurities at some level. In 
the case of Tl-201 the overlap in energy of the excitation functions for the 
reactions Tl-203(p,2n)Pb-202, Tl-203(p,3n)Pb-201, and Tl-203(p,4n)Pb-200 
lead to the Tl-200 and Tl-202 in the end product.

Manufacturers optimize the entrance energy of the protons to control the 
Tl-200 activity, and the apparent target thickness to control the beam exit 
energy to control the Tl-202. However due to commodity prices of Tl-201 they 
try to control it to leave the maximum acceptable impurities.

I think Mike Stabin and I had a discussion quite a few years ago about 
permissible exposure to the patient due to impurities. I know that I have 
had the discussion with several medical physicists in the last year. It 
seems to be tribal lore that the "limit" is 5 % of the total dose. Although, 
I haven't found a reference.

That said the only limits I have found in U.S. regulations or guidance are 
very specific limits for some radiopharmaceuticals. For example there are 
limits in the U.S. Pharmacopea for Mo-99 in Tc-99m. There are other 
radiopharmaceuticals that have no limits specified.

Dale

----- Original Message ----- 
From: "Franz Schönhofer" <franz.schoenhofer at chello.at>
To: "'Stabin, Michael'" <michael.g.stabin at Vanderbilt.Edu>; 
<radsafe at radlab.nl>
Sent: Tuesday, January 27, 2009 12:32 PM
Subject: AW: [ RadSafe ] medical issues

Mike and others like Phil Egidi,

Thank you for finally introducing science into this thread! All this hearsay
and "I think...." did not help anybody to get a clear picture of the
question under discussion.

The biological half-lives of radionuclides like Cs-137, Tl-201, Tc-99m and
all others can be easily found within a minute at Google, citing also
reliable scientific sources in case one does not rely on Wikipedia. This is
important because the biological half-life is usually much shorter than the
physical one, but is frequently mixed up. Furthermore one should not confuse
encapsulated, sealed sources with administration of soluble radionuclides.
Regarding the latter one I would like to remind you on a very large scale
"experiment" conducted more than 20 years ago, when the Chernobyl reactor
burnt and sent not only all easily volatilized radionuclides, but also
Cs-137 (as well as Cs-134) over extensive parts of Europe. Anybody
interested in the clearance of Cs-137 from humans should study the tons of
literature available.

What I am rather surprised is, that so much "radioactivity" should be
attributed to impurities in the radionuclides used. To my knowledge there
are usually limits set for the concentration of long-lived radionuclides in
the production - of course on radiation protection grounds. Is there
somebody with a better knowledge than me who can enlighten me and other
RADSAFErs about this issue?

Best regards,

Franz

Franz Schoenhofer, PhD
MinRat i.R.
Habicherg. 31/7
A-1160 Wien/Vienna
AUSTRIA

-----Ursprüngliche Nachricht-----
Von: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] Im Auftrag
von Stabin, Michael
Gesendet: Montag, 26. Jänner 2009 22:04
An: radsafe at radlab.nl
Betreff: [ RadSafe ] medical issues

Cs-137 is not administered as a radiopharmaceutical in nuclear medicine and
cannot be used for imaging on a PET system, it does not emit positrons.
Ge-68, Cs-137, and I think some gadolinium isotope sources, as well as CT
sources, have been used in PET scanners to set up attenuation correction
maps. As Bjorn noted, Cs-137 has been used in brachy implants.

If heart imaging is done with Tc-99m (cardiolite or other forms), it goes
through 8 half-lives in two days, and there is very little residual activity
or measurable exposure after 1-2 days. Tl-201 has a longer half-life (around
3 days), but there is biological clearance. BUT there are often longer lived
contaminants in the product (Tl-200, Tl-202, Pb-203, about 1, 12, and 2.5
day half-lives) which can be measured for some days after the test. I had a
student in a radiation detector course who seriously messed up our
scintillation detector lab because of residual thallium activity a few days
after a heart study. She thought it would all be gone, but there was plenty
there for the detectors to see. She didn't mind being sent home early so the
rest of the class could get their measurements!

Mike

Michael G. Stabin, PhD, CHP
Associate Professor of Radiology and Radiological Sciences
Department of Radiology and Radiological Sciences
Vanderbilt University
1161 21st Avenue South
Nashville, TN 37232-2675
Phone (615) 343-4628
Fax (615) 322-3764
e-mail michael.g.stabin at vanderbilt.edu
internet www.doseinfo-radar.com

"I am realistic -- I expect miracles." - Wayne Dyer

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