RE: Irradiation of mouse thymus

[Bill Kendall <KENDALL@nucusa.com>]

If this irradiator is like the ones I have used and supported, it has a
relatively small cylindrical chamber that is surrounded by tubes in which
the sources are contained when the irradiator is activated.  On this
assumption, your researcher is on the right track.  The only problem is the
thickness of the lead.  You could obtain a lead pig with walls of sufficient
thickness (you can calculate what you need from the dose rate you have vs.
the acceptable dose rate to the rest of the animal). You could then drill a
hole through one wall to make a collimator.  The anesthetized animal would
then be placed (packed) into the cavity and the thymus aligned with the
aperture.  If the lead pig is of sufficient height (of if another lead
cylinder could be place on top), it would not be necessary to use the lid to
the pig.  Anyway the animal needs to breathe.

Careful alignment of the pig in the exposure chamber would align the
aperture with an actual source.  Remember that since this is a highly
collimated beam , special dosimetry would have to be performed.  I would
suggest using TLD.

If lead is not readily available, use Cerrobend which can be fabricated in
your radiation oncology department.  It is reusable, too.

Another thought is to proceed as described above and place Cesium-137
tube(s)that are customarily used in brachytherapy into the aperture.  This
would insure alignment, foster reproducibility and, probably, simplify
dosimetry.  A possible problem would be a provision in your radioactive
material license that limits the use of the sources to human use, but that
provision can be changed by your radiation regulatory authority.  If you
have a broad scope license, you could change it internally.

On dosimetry, the experimenter should be aware of the penumbral effects of
deep collimators.  That is, the dose rate will vary across the field.  The
fall-off at the edge will necessitate a larger field to get full dose to the
edges of the gland.  Other structures will be irradiated, but a less than
adequate dose to all portions of the gland could (and probably would)
compromise the experiment.  If this is not a familiar concept, consult your
therapy physicist.
   
One last comment, the penumbral effects can compromise the dosimetry as
well.  The dosimeter must be small enough to be completely contained in the
low gradient portion of the beam.  However, other dosimeters can be used to
measure the lateral aspects of the beam. Again, TLD rods or chips would do
the job nicely. Film has some utility in this application if the maximum
density does not exceed linear portion of the H&D Curve.  To use film as a
dosimeter, however, is very tricky.  It is highly energy dependent,
nonlinear, backscatter dependent and very sensitive to processing
conditions.

This sounds like fun, wish I were there to participate.

If you want to talk, call me.

Bill Kendall
kendall@nucusa.com
(410) 418-9311

-----Original Message-----
From: Larson, Ron [mailto:RLARSON@RESEARCH.USF.EDU]
Sent: Wednesday, December 01, 1999 4:36 PM
To: Multiple recipients of list
Subject: Irradiation of mouse thymus


We have a researcher here at the university that would like to
specifically irradiate the thymus of a mouse using the Cesium-137
irradiator. He asked our office how he could go about doing this. The
use of the collimator will not be possible (at least I don't think so)
and he wants to wrap the anesthetized mouse with lead foil and have a
small opening exposing the area of the thymus. My initial response was
that the lead foil would not adequately shield the rest of the mouse. He
wants to irradiate with a dose of 7 Gray which would be about 10 minutes
of exposure time.  If anyone has any experience or suggestions that our
office could pass onto this researcher, we would be most appreciative.
Please respond to the e-mail address below. Thanks!

Ron Larson
USF Asst. Radiation Control Officer
3500 E. Fletcher Avenue
Suite 523
Tampa, FL  33613
Office: 813-974-7656
Fax: 813-974-5938
E-Mail: rlarson@research.usf.edu

************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html
************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html