RE Xe133 vs Tc-99m

[VERNIG.PETER@FORUM.VA.GOV]

We switched shortly after I got here.  Generally, I'd say that Tc-99m is
easier to deal with.  You do not need to worry about negative presure and
evacuation times.  If a vial of Tc-99 is dropped, first it well may not 
break as 5 and 10 ml vials have relatively thick walls wrt length.  If it
does, you have floor contamination not a gaseous release.
 
There will be some generalized contamination from vent patients, usually in
the 100 of dpm/100cm^2 range.  This can go to 3 or 4000 dpm/100cm^2 wipe
with an uncooperative or [more likely] confused or unresponsive patient.
The patients are supposed to keep their mouths closed arround the mouth
piece, if they don't that will increase the release.  In this respect,
a recommended action level of 22,000 dpm/100cm^2 is given in Reg Guide 8.23
for low risk beta or x-ray emitters.
 
A high work load of vent studies on predominantly "poor" [as defined above]
might conceiably lead to exceding the  action level, but I am inclined to
think it isn't very likely.
 
We did have a background problem that we resolved by discovering thtat
that an air return duct was FILTHY.  Probably 5 years of accumulated
dust, which trapped the Tc-99m aerosol.  We cleaned it up and had our
Engineering Dept install a filter housing & furnace filter.  If we ever
have a background problem we just swap out the filter.  Generally they are
not visibly dirty at all and are retained.  They do load up very fast.
If your camera room was suitible for Xe-133, negative air pressure and 
reasonably short clearance time [high air replacement rate] it should be
very adequate for Tc-99m.  I would recommend checking the return air 
duct and cleaning as necessary befor starting.
 
Hope this is helpful.  BTW I would recommend putting e-mail address in 
body of message, responses can be private.
 
Peter G. Vernig, VA Medical Center, Denver, vernig.peter@forum.va.gov
 
I guess I should note these are all my opinions only and do not repre-
sent those of VA Denver, Dept. of Veterans Affairs, or US Government.
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