131I DISTRIBUTION IN THYROID REMNANTS AS A FUNCTION OF TIME INTERVAL BETWEEN PREVIOUS LOW DOSE SCINTIGRAPHY AND RADIOIODINE TREATMENT

Kozak O.V.

During radioiodine therapy of thyroid cancer patients 131I distribution in thyroid remnants after administration of low and high 131I activities is different. The aim of the work was to evaluate 131I effective half-life in thyroid remnants and remnants/blood activity ratio after administration of therapeutic activity on the basis of diagnostic scintigraphy data and determine whether time between diagnostic 131I activity administration and radioiodine treatment (D) affects such parameters. 32 thyroid cancer patients have been treated with 131I after thyroidectomy and diagnostic scintigraphy with 74 MBq. Therapeutic activities administered ranged from 1100 to 5120 MBq. Kinetics of radioiodine excretion and accumulation in remnants has been followed during first 4 days after 131I activity administration. Average values for 131I half-life in remnants (T) and remnants/blood activity ratio (s) after administration of therapeutic 131I activity were such as follows: Tdav= 5.73 days; sdav= 55.57; Ttav= 2.75 days; stav= 31.94 for diagnostic and treatment data correspondingly; Dav =23.4 days (from 2 to 120 days). It was shown that Tt and st could be evaluated on the basis of previous low dose scintigraphy data. Tt does not depend on time interval between low-dose examination and high 131I activity administration while st decreases with increasing D, such relationship being more pronounced for first 10 days after diagnostic examination. The suggestion could be made that thyroid cancer patients should be treated with radioiodine as soon as possible after diagnostic scintigraphy for delivering maximal absorbed dose in remnants.

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