[ RadSafe ] Radiotracer dose at a distance.

[Geo>K0FF]

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,  ©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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