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RE: Discharge of a I131 patient from the hospital
Actually, assuming I am doing the unit conversions correctly, the figure of
0.9 uSv/hr from a patient whose burden is 40 MBq
is close to the result from our clinical experience. From a database of 350
administrations of I-131 to in-patients for thyroid cancer, we get an
average exposure rate per mCi (37 MBq) of 0.113 mR/hr (1.13 uSv/hr). These
measurements are taken immediately after the administration of the
capsule(s) comprising the dose (mean = 121 mCi or 4.48 GBq) so that we are
dealing with a partially shielded point source in the stomach. However,
there is a lot of variability in the readings because of wide variations in
body size/shape and in the thickness of tissue overlying the gastric pouch.
At discharge you are not dealing with a point source if the patient has had
a thyroidectomy, and all of our patients have. At that point the patient is
an extended source with maybe a few small foci of metastatic uptake.
In any case, we do not discharge I-131 patients based on residual activity,
but on the potential exposure to others, the limit for that being 500 mrem
(5 mSv). We estimate the effective half-life with serial surveys of each
patient, and discharge them if the effective half-life is less than 24 hr
and the maximum exposure rate at one meter is less than 30 mR/hr (sorry, our
meters are all in traditional units). This almost always means that the
patient goes home after one night in the hospital. Sometimes we hold onto a
patient into the afternoon of the day after administration, but we have had
no one stay two nights since we adopted our present protocol about 2 years
ago. On the other hand, USNRC Reg Guide 8.39 would seem to allow for
discharge immediately after administration of a dose as large as 200 mCi
(7.4 GBq) if you believe the assumptions that are made in Appendix B. We
don't. Thyroid cancer patients being treated with I-131 excrete more slowly
than normal people due to reduced kidney function, a result of the severe
hypothyroid state they are all in at the time of treatment. We just
published this in the September issue of Health Physics. We also like to
keep them one night to hopefully contain most of the inevitable
contamination.
David L. North, Sc.M. DABR
Associate Physicist
Department of Medical Physics
Main Bldg Rm 317
Rhode Island Hospital
593 Eddy St.
Providence, RI 02903
ph: (401)444-5961
fax: (401)444-4446
dnorth@lifespan.org
> ----------
> From: LAM HOI CHING
> Reply To: LAM HOI CHING
> Sent: Thursday, September 27, 2001 22:56
> To: radsafe@list.vanderbilt.edu
> Subject: Discharge of a I131 patient from the hospital
>
> Dear Radsafers,
> What appropriate measurement of dose rate from an I131 patient should be
> taken in determining its residual activity before discharging that
> patient?
>
> The practice is < 150MBqMeV (quite a magic figure) = 24uSv/sec in
> emission energy rate for most diagnostic radionuclides.
> For I131, if the residual activity is less than 400 Mbq, then the
> patient can be released and he/she can get any public transport without
> violating the local regulation.
> However not all centers can have whole body counter to measure the
> patient residual activity, second the whole body counter is not designed
> for such purpose too. Our usual way is to measure the dose rate at 1m from
> the patient body surface, so that the measurement result is in terms of
> uSv/hr.
>
> 1. The question follows: what is the appropriate conversion factor
> from
> (dose rate in uSv/hr at 1m from patient) --->(residual activity Mbq)
>
> 2. From IPSM report 65 (Table 3.2 : Calculated dose rate from
> radionuclides used for diagnosis, pg 44) the row for I131 follows:
>
> Radionuclide : I131
> Usual maximum activity : 40Mbq
> Typical dose rates(uSv/hr) at 1m from point source = 2.3
> Typical dose rates(uSv/hr) at 1m from patient = 0.9 ( without literature
> quotation).
>
> I'm rather doubted the figure of 0.9uSv/hr at 1m from the patient because
> it seems, by my intuition, to me too low. Because I cannot locate any
> reference article for that, I don't believe in that figure.
>
> If assuming the patient residue as point source (thyroid gland may be
> treated as a point when measurement is at 1m), dismissed patient dose rate
> for 400Mbq ===> 23 uSv/hr.
>
> If instead, believing that 0.9 value, the dismissed patient should have
> dose rate well below 9uSv/hr!!
>
> The consequence is which limit should be adopted? 23uSv/hr ? 9uSv/hr?
> In the case of ablative thyroid patient treating with 80mCi with his/her
> effective half-life for decay about 4 days long, it implies that the
> practice changing from 23 to 9 will make the hospitalization for 4 more
> days!! The justification of course relies on the accuracy.
> I rather not believe that figure of 0.9, becuase HVL(I131, 364keV,
> water) = 6.3 cm water. Whether the body attenuation > 7cm seems unlikely
> to
> me? Moreover I noticed a thyroid phantom usually has depth less than 6cm.
> So I would like to ask for any advice or information for this issue.
> Thank you.
>
> Sender:
> John Lam, Physicist, Eastern Hospital, HK
> lamhc@hkusua.hku.hk
>
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