shielding requirements for intravascular HDR units in the cath lab

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

>From: Douglas Simpkin <dsimpkin@EXECPC.COM>
>
>My institution is investigating the use of Ir-192 source intravascular HDR
>in existing cath labs. My present concern is for the adequacy of the
shielding.
>
>A back-of-the-envelope calculation for 0.46 Ci of Ir-192 (gamma factor =
>0.4 R/hr m2/Ci per Johns & Cunningham) at 4 m distance yields an exposure
>rate of 11.5 mR/hr. For a 20 minute treatment that's 3.8 mR or about 3.8
>mrem, which exceeds the NRC's uncontrolled dose limit of 2 mrem in any one
>hour (presuming not more than 1 treatment per hour... is this a reasonable
>assumption?). The hourly dose in an uncontrolled corridor outside the room
>would therefore seem to be in excess of regulations, since NRC
>traditionally disallows occupancy factors < 1.
>
>The standard 1/16" Pb shielding in the cath lab walls would seem to offer
>little protection (transmission = 0.8 or so from NCRP-49 Fig. 11). Even
>doubling the Pb thickness to 1/8" still has a transmission of 0.6 or so.
>
>Our 5" = 12.7 cm thick concrete floors would seem to fare better, with a
>transmission = 0.16 from NCRP 49 Fig. 12.
>
>The manufacturer says that they've not had any concerns at other
>institutions. What experience with this have others had in this regard?
>
>Thanks,
>
>Doug
>Douglas J. Simpkin, Ph.D., D.A.B.R.
>St. Luke's Medical Center
>2900 West Oklahoma Avenue
>Milwaukee, WI 53215
>phone: (414)649-6457
>fax: (414)649-5118
>email: dsimpkin@execpc.com
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