131I distribution in thyroid remnants as a function of time interval between previous low dose scintigraphy and radioiodine treatment

During radioiodine therapy of thyroid cancer patients. I-131 distribution in thyroid remnants after administration of low and high I-131 activities is different. The aim of the work was to evaluate. I-131 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 I-131 activity administration and radioiodine treatment (Delta) affects such parameters. 32 thyroid cancer patients have been treated with I-131 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 I-131 activity administration. Average values for I-131 half-life in remnants (T) and remnants/blood activity ratio (a) after administration of therapeutic I-131 activity were such as follows: T-av(d) = 5.73 days; sigma(av)(d) = 55.57; T-av(t) = 2.75 days; sigma(av)(t) = 31.94 for diagnostic and treatment data correspondingly; Delta(av) = 23.4 days (from 2 to 120 days). It was shown that T-t and s(t) could be evaluated on the basis of previous low dose scintigraphy data. T-t does not depend on time interval between low-dose examination and high I-131 activity administration while sigma(t) decreases with increasing Delta, 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.