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Re: Discharge of a I131 patient from the hospital
Dear Radsafers,
Follwing points have to be kept in view while measuring the residual
activity in patients administered high doses of I-131.
1.The reference point of mesurement should be at one meter distance from
the patient where maxima occurs along the midline.
2.The total hospitalization period depends on the many factors
e.g.presence or absence of matastasis lesions, nature of lesions in terms
of concentration of I-131 and the rate of elimination of the I-1312 from
the lesion,post therapy hydration.
3.There may be cases where the elimination is so fast that the patient can
be discharged on the third day even if the administered activity is more
than 100 mCi.On the other hand the lesions having larger retention time
may take about 7 days to acquire the level at which patient can be
discharged.
A.K.Shukla
On Fri, 28 Sep 2001, LAM HOI CHING wrote:
> 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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