re: X-ray Shielding Requirements

[Free Spirit <FRANK@rover.bsd.UCHICAGO.EDU>]

>Well, Texas deemed this to be a significant problem.  So TRCR
>21.301(a)(2) [Part 21 is equiv to 10CFR20] allows a TEDE of 500
>mrem/year for members of the public "from infrequent exposure
>to radiation from diagnostic and therapeutic radiation machines".
>
>In addition, a footnote to 21.301(a)(1) exempts radiation machine
>facilities who met the previous dose limits (500 mrem/year) from
>having to retrofit if they were installed prior to 1/94.
>
>Wes Dunn
>Texas Bureau of Radiation Control

	We had a situation in Illinois not to long ago, which occasioned
	a letter from then AAPM President Robert L. Dixon, Ph.D., to
	the Illinois Dept. of Nuclear Safety. Seems Illinois was doing
	some unusual application of this limit:

	1/ Illinois was ignoring the difference between "continuous (or
	   frequent) exposure," and "infrequent annual exposure," apply
	   ing the 1 mSv limit across the board.

	2/ They were not going to grandfather existing units.

	Compliance under these conditions would have required either
	
	1/ Addition of 12" of concrete or the equivalent to shelding walls or

	2/ Lots of study to justify playing with occupancy factors, (just
	   how long does patient Jones' son sit there while Jones is being
	   treated?)