Mjerenje retencije J-131 s ciljem optimalizacije radioablacijske doze

The aim of our study was to establish a dosimetric approach for determination of ablation or therapeutic I-131 activity for every single patient after thyroidectomy for thyroid cancer. The retention of 74 MBq of I-131 diagnostic activity was measured by whole body counter and by urine assay in 38 patients. Also, the absorbed dose was estimated for thyroid remnant. Retention in the body and thyroid remnant tissue 72 hours after administration of diagnostic activity was compared with retention measured 72 hours after administration of 4.4 GBq of ablation activity. The average effective half-life of retention, measured by whole-body counter was 15.4 (s.d. 4) and 39.2 (s.d. 44) hours for whole body and thyroid remnant respectively. The average absorbed dose in thyroid remnant was 2.7 (s.d. 2.2) Gy. The retention values measured by urine assay were higher because of the method impreciseness. The correlation coefficient between two methods was 0.673. Patients with and without thyroid tissue remnant were not significantly discriminated by effective half-life of iodine retention. The retention of ablation activity in thyroid remnant 72 hours after administration was markedly reduced than expected, and it was only 36 (s.d. 32) % of those predicted by diagnostic study. Marked uptake reduction was observed in patients with higher absorbed diagnostic dose, in patients with longer interval between operation and diagnostic study, and in patients who had received ablation activity no longer than seven days after diagnostic study. Such results make doubts about precise determination of ablation activity upon dosimetry of diagnostic activity. Regarding our results, first diagnostic study after operation should be performed by as low as possible diagnostic activity because of possible 'stunned' thyroid tissue, and application of ablation dose should be prolonged as long as possible to allow thyroid tissue function recovery. (author). 3 figs., 12 tabs., 80 refs