Does thyroid stunning exist? A model with benign thyroid disease

With regard to the treatment of differentiated non-medullary thyroid carcin oma, there is controversy over whether radiation from a diagnostic radioiod ine (I-131) application really does have a suppressive effect on the uptake of subsequent therapeutic I-131 (so-called thyroid stunning). However, inh erent difficulties in exact remnant/metastatic tissue volumetry make it dif ficult to quantify how much diagnostic I-131 is actually absorbed (absorbed energy dose) and hence to decide whether a threshold absorbed dose exists beyond which such stunning would occur. Since in benign thyroid disease the target volume can be readily quantified by ultrasonography, we sought to d etermine definitely whether stunning of thyroid cells occurs upon a second application of radioiodine 4 days following the first one. We therefore stu died 171 consecutive patients with benign thyroid disease (diffuse goitre, Graves' disease, toxic nodular goitre) who received two-step I-131 therapy during a single in-patient stay. For application of both calculated I-131 a ctivities we performed kinetic dosimetry of I-131 uptake, effective half-li fe and absorbed dose. At the second application, patients showed significan t stunning (a 31.7% decrease in I-131 uptake, from 34.7%+/-15.4% at first a pplication to 23.7%+/-12.3% at second application, P<0.0005) without a sign ificant difference in effective half-life (4.9+/-1.3 vs 5.0+/-1.7 days, P>0 .2). ANOVA showed that the extent of stunning was influenced significantly only by the absorbed energy dose at first application (F=13.5, P<0.0005), w hile first-application I-131 activity, target volume, gender and thyroid fu nction had no influence tall F<less than or equal to>0.71, all P>0.4). Ther e was no significant correlation between extent of thyroid stunning and fir st-application I-131 activity (r=0.07, P>0.3), whereas there was a highly s ignificant correlation between thyroid stunning and first absorbed energy d ose (r=0.64, P<0.00005), the latter correlation fitting a logarithmic model best. Multivariate factor analysis also revealed first absorbed energy dos e to be the only decisive stunning factor. In conclusion, our study confirm s that stunning exists in benign thyroid conditions and that it is a purely radiobiological inhibitory phenomenon related to absorbed dose.