Thyroid stunning after (131)I diagnostic whole-body scanning.

TO THE EDITOR: From their study of patients referred for radioiodine ablation, Cholewinski et al. (1) concluded that diagnostic whole-body scanning can be performed effectively with a 185-MBq (5-mCi) dose of 131 I 72 h before radioiodine ablation with no evidence of, and therefore no concern for, thyroid stunning. Their study group consisted of 122 patients who were given a diagnostic dose of 185 MBq 131 I and who were scanned 72 h later. On the day of scanning, after its completion, the patients received an ablation dose of 131 I (5,550 MBq in most cases); whole-body imaging (with spot views) was undertaken 72 h later. The diagnostic and postablation scans were inspected visually, taking note of the number of foci of uptake and the intensity of uptake. Analysis of their observations led to the conclusion that no stunning had occurred and that this was a consequence of the short time interval between the diagnostic and ablation doses. The phenomenon of stunning has been investigated using both qualitative (2,3) and quantitative methods, the latter using profile scanning (4) or an external neck probe (5). In our center we use a twin-head gamma camera in the measurement of uptake after the diagnostic dose (120 MBq) and, more recently, therapeutic dose (4,000 MBq) of radioiodine, taking into account correction for the effects of high counting rates. Uptake on the diagnostic scan is measured at 72 h and uptake of the ablation dose is measured on 1 or more occasions in the time interval 24 –72 h. Shorter time intervals after the ablation dose were used to investigate the possibility of rapid turnover of the " destructive " ablation dose in the thyroid remnants. To date, 26 patients have been investigated. Thyroid uptake after the ablation dose was reduced in 25 of the 26 patients, being 39.4% Ϯ 22.9% (mean Ϯ SD) of the diagnostic uptake (range, 10%–100%). The mean uptake (ϮSD) of the diagnostic dose was 7.6% Ϯ 6.4%, and the mean time interval (ϮSD) between the diagnostic dose and the ablation dose was 16 Ϯ 10 d. Within the study group, 2 patients received the ablation dose on the day of the diagnostic scan and another patient received the ablation dose 4 d after the diagnostic dose. In all 3 patients, the uptake of the ablation dose was reduced, being 86%, 59%, and 40% of the diagnostic uptake, respectively. No differences …