Parents in the Linac Room - Continued

[Chris Alston <alstonc@odrge.odr.georgetown.edu>]

>Tonight I had a chance to do some measurements.  I positioned a humanoid
phantom on the table and set up a lateral field using a 10x10 collimator.
(In retrospect, I should have put in a cerrobend block as well.)
>
>Then I positioned one of our brachytherapy bedside rolling shields
perpendicular to the couch and next to the gantry head.  This shield
contains 1" of lead and shields a zone ranging from 20" to 56" above the
floor.  So, it would protect from the knees to the neck of most parents.
>
>In 6 MVp mode at 200 MU/min, I measured 0.130 rad/min above the shield and
0.013 rad/min behind the shield using a Capintec 0.6 cc chamber and a Co-60
build-up cap.  As you can see, the shield provides 1 TVL of protection.
>
>If the patient is to receive 5000 rad to midline, that represents
approximately 6000 MU.  At 200 MU/min, that requires 30 minutes of beam time.
>
>That means the parent's head (above the shield) would receive (0.130 x 30)
= 4 rem.  The trunk of the body (behind the shield) would receive 10% of
that value = 0.4 rem.
>
>To put that into perspective, a CT study of the head can deliver 4 rad and
the NRC allows family members to receive 0.5 rem from nuclear medicine
outpatients.
>
>Isn't that interesting?
>
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