[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Iodine Therapy Dose Calculation

        Reply to:   RE>>Iodine Therapy Dose Calculation

Hi RadSafers,

The effective half life of I-131 fixed in the thyroid is calculated by the following formula:

      Tr x Tb       Where:   Te = Effective Half Life
Te = --------                Tr = Radioactive Half Life
     (Tr + Tb)               Tb = Biological Half Life

Table 25 In the CRC Handbook of Radioactive Nuclides gives a biological half life for iodine of 138 days.  Consequently, the effective half life of I-131 after fixation in the thyroid is 7.6 days.  You cannot have an effective half life longer than either of the smaller of the physical or biological half lives involved!

The biological half life stated above is for a standard man with intact thyroid and normal function.  It should be noted however that the typical thyroid cancer patient has far from a normal thyroid.   The excretion profile of Iodine-131 from a typical thyroid patient is two phased.  The first phase is dominated by biological excretion from the blood stream with an effective half-life of about 15 hours.  In general, approximately 70% of the administered dose is excreted within the first 24 hrs.  After 24 hours, the biological turnover of bound iodine begins to take over but the effective  half life is about 48 hours.  This data is based upon actual patient measurements.  I haven't done a complete analysis of all my patient excretion data but I have data from at least 100 patients treated over the last 12 years.  Once I crunch the numbers more completely, I'll give you all an update with mean and range data.  The data is quite variable depending on patient factors such as residual thyroid tissue, renal function, hormonal status and many other parameters.

Later,

<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><
Michael J. Bohan, RSO   |  e-mail: mike.bohan@yale.edu
Yale-New Haven Hospital |    Tele: (203) 785-2950
Radiological Physics    |     FAX: (203) 737-4252
20 York St. - WWW 204   |    As usual, everything I say may be plausibly
New Haven, CT    06504  |    denied at my employer's convenience ...
><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>