Influence of Diagnostic Radioiodines on the Uptake of Ablative Dose of Iodine-131

Abstract

The uptakes of thyroablative doses of 131I by postoperative thyroid remnants and/or thyroid carcinoma metastases following diagnostic surveys with 131I or 123I were retrospectively compared by visual inspection. Only those patients with a diagnostic scan demonstrating functioning tissue, remnant, and/or metastasis, following thyroidectomy for differentiated thyroid carcinoma, were evaluated. The 131I survey group (n = 26) had received a diagnostic dose of 3 to 10 mCi of 131I. The 123I group (n = 14) had received the usual diagnostic dose of 300 microCi of 123I. The age, sex, and tumor type in the two groups were not statistically different. The uptake of the ensuing thyroablative dose of 131I appeared, by visual inspection, to be impaired in 20 of 26 patients in the 131I group and in none of the 14 patients in the 123I group (p < 0.00003). In the 131I group there was suggestion of a dose-response, that is, the higher the administered activity of 131I for the diagnostic scan, the more reduced was the subsequent apparent uptake of the thyroablative dose (p = 0.0007). Thyroid remnants or cervical lymph node metastases appeared to be affected more frequently than were the distant (pulmonary or skeletal) metastases (p = 0.004). This study suggests that iodine uptake function may be suppressed by the absorbed radiation from the 3 to 10 mCi "diagnostic" scanning dose of 131I. In this regard, 123I may be a better initial diagnostic agent to be used prior to radioablation therapy.

5mCi pretreatment scanning does not cause stunning when the ablative dose is administered within 72 hours

“…Park et al (13) showed reduced activity on post-treatment (100-200mCi iodine-131) scans compared to DxSCANS in 40% of patients treated with 3mCi, 67% treated with 5mCi and 89% treated with 10mCi. Muratet et al (11) showed differences in the ablation of thyroid remnants (ablative dose of 100 mCi) when using a tracer dose of 1 and 3mCi (76 and 50% of success, respectively), and McDougall et al (10) did not detect stunning with a dose of 2mCi (only 1.4% of patients showed reduced therapeutic uptake).…”

“…Although stunning can be suspected to be responsible for the difference between pre-and posttreatment uptake (9,10,(13)(14)(15)(16), treatment efficacy seems to be the fundamental parameter in the definition of the impact of this phenomenon which, however, has not been considered in all studies. Morris et al (8) did not observe differences in the success of ablative therapy between patients submitted or not to pretreatment scanning with 5mCi iodine-131 (64.9 and 66.7%, respectively).…”

“…The lack or decrease of iodine uptake on posttreatment scans suggests the occurrence of the stunning effect (9,10,(13)(14)(15)(16), but other parameters should also be taken into account, mainly the efficacy of ablation which is the main end-point (8,9,11,14,(16)(17)(18).…”

Effect of stunning of diagnostic 131-iodine doses on ablative doses for differentiated thyroid cancer patient’s outcome

“…However, in 1994, Park [5] and his associates showed that amounts of 131I larger than 2mCi have a sufficiently harmful effect on thyroid tissues to interfere with subsequent uptake of therapeutic amounts of 131I on the post ablation whole body scan for detection of any thyroid remnants or tumor foci. This had been noted at 3mCi dose, and becoming progressively greater with larger doses.…”

“…The latter is considered a radiobiological suppressive phenomenon defined as a temporary suppression of iodine trapping function of the thyrocytes and thyroid cancer cells as a result of the radiation given off by the scanning or first dose of RAI-131 [5] . Concern for stunning has led some physicians to prescribe treatments of RAI-131 without prior diagnostic imaging in hopes of increasing the therapeutic effect [6,7] .…”