Patterns of 131I excretion in remnants upon 2nd session of the treatment of thyroid cancer are related to the kinetic parameters of the previous one

Full text: Radioioiodine kinetics in thyroid remnants during 131I treatment of thyroid cancer patients is known to differ between sequential courses. The aim of the study was to evaluate the effective half-time of 131I excretion and its accumulation in remnants during the second session as a function of kinetics parameters during the first one. 14 patients with differentiated thyroid cancer have been treated with 131I after thyroidectomy. Thyroid remnants were ablated after the second course of 131I treatment. Activities administered (A) amounted from 1110 to 5200 MBq. The following values have been determined by means of scintigraphy which has been carried out during 4 days after activity administration: ROI was determined in thyroid remnants. Nl- max cpm in ROI in 24 hours after activity administration; N0 - background cpm (upon the shoulder); , - N = N1- N0. η = 3√N η 1 was measured during the first course of the treatment, η2- during the second one. Effective half-time of 131I excretion during the first (T1) and the second (T2) session was determined under the assumption of exponential 131I excretion. Average values of every parameter were equal: Tav1 = 3.99 days; Tav2 = 2.42 days; ηav1= 11.34; ηav2= 4.88; Aav1=2086 MBq; Aav2=2061 MBq. Every parameter has been normalised to the average value: Tn1=T1/Tav1; T22 = T2/Tav2; ηn1=η1/ηav1; η22=η2/ηav2. The assumption was made that T22 and ηη2 are linear function of kinetic parameters of the first 131I treatment. Regression analysis has been carried: Tn2= Tn1 - 0.097-(An1-An2) + 0.037-hN1 R = 0.70 ηn2= hn1- 0.867-An1+ 9.451.An2 + 0.349-Tn1, R = 0.28 Effects of previous course of the treatment on 131I accumulation decrease in remnants is more pronounced than on the effective half-time of 131I excretion. Changes in 131I accumulation and excretion in remnants depend on the activities administered. Copyright (2002) The Australian and New Zealand Society of Nuclear Medicine Inc