Re: Criteria for release of cats that received I-131 therapy

["Phil Rutherford" <email@philrutherford.com>]

I read, with amusement, your "radioactive cat" regulations.  I have no experience of hot kitty litter, however I do have real life experience with diagnostic radioisotope injections.



I have had a couple of "treadmill stress tests", involving injections of 4 mCi Tl-201 (half life 73 hours) and 30 mCi Tc-99m (half life 6 hours).  These are standard medical tests for heart patients that are conducted thousands of times a day in hospitals and medical clinics throughout the U.S.  Typically the patient will leave the medical facility about an hour to an hour and a half after the injections.  Obviously, there is little decay in that time, and so the patient leaves with most of the 34 mCi of isotopes in his/her body (considerably more than the 0.2 mCi limit mentioned in Lorna's email).  Granted Tl-201 and Tc-99m have somewhat shorter half lives than I-131.



I also took some exposure measurements following my first treatment with a Bicron microR meter.  At 8 hours after my treatment, my chest and stomach were reading 17 mR/hr at contact; 3.5 mR/hr at 1 foot; 1 mR/hr at 3 feet; and 2 microR/hr (twice background) at 30 feet.  Extrapolating back, approximately 6 hours to my release, I figure my 1 foot reading was about 7 mR/hr.  It took 36 hours for my contact readings to reduce to 2 mR/hr, and 19 days for my contact reading to fall to 2 microR/hr (twice background).  My radiation field obviously exceeded a 2 mR/hr limit (or a 0.5 mR/hr limit) for a considerable time after my release.  I have not researched what the release criteria are for humans but if the regulations are so restrictive for cats, it would seem likely that they would also be similar or worse for humans.  However, as a practical matter, human patients are apparently not restricted from release following these everyday procedures.  



Interestingly, the long term term behaviour of the longer lived Tl-201 had an effective half life of about 63 hours, only reduced slightly from 73 hours by the biological half life.  Therefore in order to reduce the body burden down to 0.2 mCi from 4 mCi would require approximately 6 days.  And the cost of staying in the hospital for six days before release is ... well you get the idea.



Furthermore I was not counselled about ALARA ... a meaningless concept in an uncontrolled public setting anyway ... stay 10 feet away from all humans, no hugs, separate meal times, sleep in the guest room, no sex  ...  ha ha!!



Well, nobody ever said radiation regs make sense.



Phil







----- Original Message ----- 

  From: BLHamrick@AOL.COM 

  To: ljb1@CORNELL.EDU ; radsafe@list.vanderbilt.edu 

  Cc: nld2@CORNELL.EDU 

  Sent: Saturday, April 10, 2004 7:58 AM

  Subject: Re: Criteria for release of cats that received I-131 therapy





  In a message dated 4/9/2004 8:08:31 AM Pacific Standard Time, ljb1@CORNELL.EDU writes:

    We are wondering what criteria other vet nuclear medicine facilities are 

    being held to in terms of release of cats that have received I-131 

    therapy.  We are held to a minimum stay time (5 days), a maximum dose rate 

    at a foot from the thyroid (0.5 mrem/h) and a maximum activity remaining in 

    the cat (200 uCi).  We would also like to know what regulatory agency is 

    involved with that criteria.

  This will vary based on the regulatory agency with jurisdiction over byproduct material state to state.  There are currently 33 Agreement States in the U.S., which regulate the possession and use by byproduct material (including I-131) under an agreement with the U.S. NRC.  In the remaining states, the NRC retains jurisdiction over the possessiona nd use of the materials.



  Below is the guidance provided by NRC in NUREG 1556, Vol. 7, Appendix H:



  "Release of Animals



  Before a veterinarian releases an animal that has been injected with a radiopharmaceutical or has had radioactive seeds implanted, the veterinarian must ensure that the dose that members of the public (including the animal's caretaker) will receive from the animal is within limits of 10 CFR 20.1301. 10 CFR 20.1301 requires that the total effective dose equivalent to an individual member of the public from the licensed operation does not exceed 1 mSv (0.1 rem) in a year and the dose in any unrestricted area from external sources does not exceed 0.02 mSv (0.002 rem) in any one hour. Furthermore, licensees should provide instructions to the animal's caretaker to keep doses ALARA."



  States may provide similar guidance, or, as it appears in your case, may provide actual dose-rate limitations (usually via a license condition) on the animals such that they will generally meet the public dose limits upon release.



  Barbara