RE: incontinent patient dosed with ~100 mCi of I-131

["Heinmiller, Bruce" <heinmillerb@aecl.ca>]

I think the discrepancy between the observation below and Karen's 1.7 Gy/h
per MBq over 1 cm^2, is resolvable by noting that, for I-131, this dose rate
is consistent with it being almost exclusively beta dose at 7 mg/cm^2.  The
corresponding photon dose rate is about two orders of magnitude lower.  The
beta dose at about 1 mm depth is also about two orders of magnitude lower
than the beta dose at 70 micrometres.  This means that if there is
appreciable urine absorption into the diaper (which is what they're designed
for) the beta dose rate at the surface of the skin (and thus the total dose
rate) is nominally two orders of magnitude lower than the figure quoted from
NCRP-111.  The ratio of electron to photon dose for the same fluence of each
can be crudely approximated by the ratio of the typical collision mass
stopping power for the electron spectrum emitted (unless the spectrum is
soft), to the product of photon energy and mass energy absorption
coefficient (assuming soft-enough photons that SCPE is achieved).

Also, to predict the more severe deterministic effects from a given absorbed
dose, the depth of concern is much deeper than 70 micrometres, so that even
any I-131 in contact with skin would deliver much less dose at the depths of
concern (for deterministic effects) than at 70 micrometres.

Bruce Heinmiller CHP
heinmillerb@aecl.ca

> As far as skin doses to the patient, well, I haven't done the
> calculations,
> but as we have not had a problem with buttocks skin in the 64 years we
> have
> been treating thyroid cancer with radioiodine (first with I-130, and then
> I-131 starting 1946), and there must have been many incontinent patients,
> I
> believe this is a non-problem.  Unless your hospital is doing skin grafts
> for I-131 radiation burns(!), better find something else to justify your
> time. If the patient is hospitalized, incontinence is usually handled with
> a
> Foley catheter.  With a biological halflife of 8-12 hours, there isn't
> that
> much I-131 coming into diapers if the patient is hospitalized for 2-3 days
> before going home and using diapers.
> 
> Ciao, Carol
> 
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