Discharge of a I131 patient from the hospital

[LAM HOI CHING <lamhc@HKUSUA.HKU.HK>]

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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