Self-stunning in thyroid ablation: evidence from comparative studies of diagnostic 131I and 123I

Abstract

Twenty-six patients who had undergone recent surgery for differentiated thyroid cancer were investigated using iodine-131 iodide (120 MBq). Uptake in the thyroid bed was measured at 3 days using a dual-head gamma camera. An ablation activity of 131I iodide (4,000 MBq) was administered 3-38 (median 14) days later and uptake in the thyroid bed measured once or twice, 1-3 days post therapy. For measurements post therapy, the gamma camera was operated in the high-count rate mode with appropriate correction factors to compensate for any count loss. A further 16 patients were given iodine-123 iodide (200 MBq) as the diagnostic agent and uptake was measured at 24 h. The ablation activity was administered 5-47 (median 19) days later and uptake again measured at 24 h. In some cases, a further measurement of uptake was made within the period 1-3 days post therapy. Reduced uptake of the therapeutic administration ( P<0.001) was observed in all 26 patients given diagnostic 131I, with a median value of 32.8% (range 6%-93%) of the uptake in the diagnostic study. In the patients given diagnostic 123I, reduced uptake of the ablative radioiodine was observed in 15 of the 16 patients ( P<0.001), and overall the median value was 58.8% (range 17%-130%) of the diagnostic uptake. In one case the uptake post therapy was increased. The stunning observed in the group given 123I was significantly less ( P<0.001) than in the group given 131I. In the patients given diagnostic 131I, stunning appeared to increase in severity the longer the time interval between the diagnostic and therapeutic radionuclides, for intervals up to 25 days. Thereafter, there seemed to be some recovery of uptake capability. Overall there was no evidence of a large rapid loss of radionuclide from the thyroid bed 1-3 days post therapy. The stunning observed using 123I could not be explained by errors in the estimation of relative uptake due to different tissue absorption of the 131I and 123I photons, nor by the radiation dose delivered by the 123I. However, the ablative 131I itself may cause stunning because the cumulated activity, over the first few hours of uptake, is not insignificant when compared with all the cumulated activity from a diagnostic administration of 131I. The resultant radiation dose to the thyroid remnant, as the therapeutic radioiodine is being taken up, may be sufficient to inhibit the uptake process, thus leading to a reduction in maximum uptake when compared with that of a diagnostic activity of radioiodine.

Radiation-Induced Thyroid Stunning: Differential Effects of 123I, 131I, 99mTc, and 211At on Iodide Transport and NIS mRNA Expression in Cultured Thyroid Cells

“…Therefore, it is not surprising that radiation-induced loss of iodide transport develops gradually and rather slowly after the NIS gene transcription is significantly suppressed. Moreover, these in vitro findings support the many clinical reports of a delay in detection of stunning until several days after the administration of radioiodine for diagnostic purposes (1)(2)(3)(4)(5)(6)(7)36,37).…”

“…To avoid this problem, 123 I has been proposed to replace 131 I for diagnostic uptake measurements. Stunning has nevertheless occurred in thyroid remnants and metastases after administration of 123 I within a rather wide range of diagnostic activities (50-200 MBq) (7,12,13). However, whether 123 I irradiation triggers cellular changes leading to loss of NIS expression and reduced iodide uptake has not been experimentally investigated.…”

“…The reason for this effect is not fully understood, although several mechanisms have been proposed. One explanation, rejecting an effect from the diagnostic 131 I, is that a low 131 I content measured in treated tumors reflects a rapid loss of viable tissues and that dying cells damaged by irradiation are incapable of keeping accumulated iodide (7,8). In this case, thyroid stunning would be regarded as an artifact due to an early therapeutic effect of the ablative 131 I and hence not a clinical problem.…”

Impact of Post-Thyroidectomy I-123 Versus I-131 Whole Body Scan on the Outcome of Radioactive Iodine (I-131) Ablation for Differentiated Thyroid Carcinoma

“…They concluded that I-131 ablation has the same outcome whether (2 mCi) 74 MBq of I-131 used as a diagnostic agent or (400 µCi) 14.8 MBq of I-123 used for imaging prior to I-131 ablation. Although these conflicting results could be attributed to the fact that iodide transport and NIS messenger RNA (NIS mRNA) expression were reduced by both I-131 and I-123 (29) , the stunning observed following I-123 could not be explained by errors in the estimation of relative uptake due to different tissue absorption of the I-131 and I-123 photons, nor by the radiation dose delivered by the I-123 (30) . The resultant radiation dose to the thyroid remnant, as the therapeutic radioiodine is being taken up, may be sufficient to inhibit the uptake process, thus leading to a reduction in maximum uptake when compared with that of a diagnostic activity of radioiodine (30) .A unique advantage of this study is that it is one of a few studies that compared outcome of I-131 ablation in age-, gender-, histopathology-matched patient groups following either post-thyroidectomy I-123 or I-131WBS.…”

Calculations in nuclear medicine — application of free online software

“…In cases of therapy of the thyroid and its tumors, it is of course very important to keep in mind that pre-therapeutic contamination with (non-radioactive) iodine leads to diminished uptake of the radioiodine not only by competitive inhibition of the iodine transporter (similarly to a blockade of a receptor), but also through a specifi c arrest of thyreocytes (phenomenon referred to as Wolff-Chaikoff-effect) [26]. An issue of changing the maximal uptake and/or effective half time by irradiation of the target tissue (even during the dosimetry measures) is controversial, but also exists [27][28][29]. These observations may lead to avoiding errors in radionuclide therapy and to its further optimization.…”