[ RadSafe ] Radiotracer dose at a distance.

[Brennan, Mike (DOH)]

Nicely done.  I am impressed by how well the measured (and corrected)
dose rate and the RADPRO projection matched.  There is certainly food
for thought here. 

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On
Behalf Of Geo>K0FF
Sent: Thursday, September 06, 2007 6:51 PM
To: radsafe at radlab.nl
Subject: [ RadSafe ] Radiotracer dose at a distance.

Interesting experiment. 
Goal was to determine the second-hand dose at a distance from a patient
injected with a radiotracer.
This is the dose delivered to a bystander in the general area of a
patient who has had a radioactive isotope injection. 

Tc-99m is the most common medical diagnostic isotope being used today in
the U.S.
The 140 keV Gamma ray (89%) is fairly penetrating, making it the isotope
of choice for many different  diagnostic scans. Perhaps the most popular
scan of this type is the heart imaging, rest and stress, Myocardial
Perfusion Scan. Sometimes Thallium-201 is used for this test, or one of
a couple different Tc-99m compounds. The Cardiolite or Tc-99m sesamibi
is the one that I had access to.

There are two phases to the scan, the rest scan and the stress scan. For
the rest scan, an intravenous injection of the radioisotope Tc-99m in a
suitable carrier (Cardiolite) is injected and a 30 to 60 minute delay is
required while the radioactive material has time to localize in the
heart muscle. 
After the delay, a SPECT scan is done (Single Photon Emission Computed
Tomography) (note 1) (note 2)

Again, this post-injection delay period is when I could gain access to
the patients. In Missouri where these tests were performed (06 Sept
2007), the patients are allowed to commingle with the general patient
population in a common waiting room. There is no dose limit, monitoring
requirement or signage required by the Bureau of Health Services
Regulation (note 3).

Assuming a 20 milliCurie dose of Tc-99m, the RADPRO online Point Source
Gamma Dose-Rate calculator indicates that a dose rate of 1865 uR/H (
1.86 mR/H) at 3 feet. (note 4).

The instrument used today was a Neutron RAE pager sized scintillator,
equipped with a small CsI(Tl) crystal/photodiode. Previous field trips
verified by Gamma Spectroscopy that it was indeed Tc-99m are dealing
with.
Background rate measured outside the building is 4 uR/H, typical for
this geographical area.
Readings taken from 3 different patients were similar, and varied
slightly according to distance and positioning, but all were in the same
ballpark. At 3 feet the Neutron RAE read 5500 to 6000 uR/H and quickly
over-ranged at 7000 uR/H ( 7 mR/H and the max reading on this detector)
if brought closer (note 5). The inverse-squared law at work very
plainly. 

Very few scintillators are energy linear, the CsI(Tl) is no exception.
Referring to the Gamma Energy Response charts supplied on  by the
detector designers (note 6) an energy correction factor for Tc-99m's 140
keV of X 3 is applied. This means that the detector is 3 times more
sensitive at 140 keV than at 662 keV, the "Cs-137 normalized" energy, to
which calibration is pegged. Therefore the readings of 5500 uR/H is
divided by 3 yielding a real measurement of 1833, very close to the
calculated dose-rate and remarkably well witching statistical and
metrological error. 

Half-life of Tc-99m is 6 hours, so there is very little loss during the
measuring period. 

Total dose to a person sitting a little less than 3 feet from 4 patients
would be 8 mR/H and if one hour passed, 8 mR, whole body.

10 mR is accepted by many as the average dose from a chest X-Ray.

I'm a scientist, not trying to make any point beyond mathematics, but I
must mention that there were multiple  Tc-99m patients in the same
waiting room, commingling with pregnant women waiting for their
appointments with other departments.








(note 1) A Manual of Laboratory & Diagnostic Tests 6th ed. Frances
Fischbach, Lippincott,  (c)2000.
(note 2) SPECT: http://en.wikipedia.org/wiki/SPECT
(note 3) Contact info and name available off-list by email request to
author.
(note 4) http://www.radprocalculator.com/Gamma.aspx
(note 5) A previous test with different instrumentation and isotopes
(F-18) also showed 7 mR/H as the external dose-rate (note 6)
http://www.polimaster.us/download/pm1703om.pdf

George Dowell, "Geo" 
NLNL/ New London Nucleonics Laboratory

573-221-3418

GEOelectronics at netscape.com



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