Re: Prostate Implant Exposures

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

We've discussed these issues at some length before on Medphys, so people
might want to search the archives, but here are some brief notes. A typical
exposure rate at one meter directly in front of these patients is 0.05mR/h
(yes, 50 uR/h). This is true for both I and Pd, because while it's true that
Pd has a lower effective E (~21keV vs. 28 for I125), the apparent activity
of the Pd seeds is greater by a factor of 3. The two facts "cancel" each
other. Note that the emissions are highly beamlike. If a patient were in a
supine position, and one were linearly scanning his body, with a radiation
survey meter (e.g., a Victoreen 450), at a distance of, say, 30cm, the
hottest region would be over the prostate, not surprisingly. By the time the
scanning instrument reached his chest, or his knees, the exposure rate would
likely have returned to background. If one is positioned to the patient's
sides, immediately adjacent to the pelvis, there is intrinsic shielding
sufficient to reduce the exposure rate at ~15cm to that seen at 100cm
directly in front. As a practical matter, in order to make reliable
estimates of the radiation exposure at one meter, even in the AP view, I've
normally found it necessary to make the measurement at =/< 50 cm, otherwise
the field is too weak. I've never done a measurement in the PA view (one
doesn't want to ask but so much of these guys, for the sake of science), but
anatomy would suggest that exposure rates there should be comparable to
those found in LAT views, don't you think?

chris alston






At 10:07  12/05/97 -0500, you wrote:
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>It's been a long week, and the discussion of lead-lined underwear is
>great fun, but it has raised a couple of serious questions.
>
>What experience does anyone have with the release of prostate implant
>patients?  What sort of levels are you measuring at 1 ft and 1 m?
>What sort of instructions are you providing the patients, both for
>home life and future hospitalizations?
>
>The effective lifetime of I-125 is 2073 hrs.  For a 1/3 occupancy
>factor (sleeping time), a person at 1 ft will receive 500 mrem if the
>initial exposure rate at 1 ft is 0.72 mR/hr.  We typically see higher
>than this with kidney embolization patients (our only experience with
>I-125 thus far) and so restrict them from sleeping with their spouses,
>at least on the side where the implant is.  Lead-lined underwear (as
>it shall henceforth always be known) could allow prostate patients to
>return to their own beds more quickly.
>
>My real fear is that one day we will implant somebody such that the
>exposure rate a 1 m is higher than the permissible release limit (1
>mR/hr).  I don't think insurance will let us keep the patient
>in-house for a half-life or two.  Would the NRC allow release of such
>a patient with the proviso that he wear a protective garment?  Am I
>worrying needlessly because such a situation could never occur?
>