Clinical implications of the differences between diagnostic 123I and post-therapy 131I scans

123I has been promoted for diagnostic imaging as a means of avoiding 'stunning'. It has also been suggested that the more favourable physical characteristics and consequent enhanced imaging resolution provided by 123I offers a more accurate diagnostic assessment of the extent of disease prior to therapy. This study evaluated pairs of diagnostic 123I and post-therapy 131I scans for differences in patterns of radioiodine uptake.

Thirty-eight patients (31 women and seven men) with a history of differentiated thyroid cancer underwent 41 diagnostic 123I studies. 131I therapy was administered to 29 patients as soon as possible after positive diagnostic findings were confirmed (with one patient being treated twice during the study period, making 30 treatments). Post-treatment scans were obtained an average of 5.8 days after therapy.

Qualitative comparison of diagnostic 123I and post-therapy 131I scans revealed a decrease in the extent of post-treatment uptake in four of the 30 treatments (13%). Seven patients (23%) demonstrated increased uptake on their post-therapy 131I scan.

Because the physical characteristics of 123I make it inconceivable that it could cause stunning, the decrease in post-treatment uptake seen in 13% of patients from this series increases the likelihood that this pattern is due to factors other than stunning, such as differential rates of radioiodine turnover. In addition, the increase in extent of post-therapy uptake seen in 23% of patients suggests that diagnostic imaging with 123I is vulnerable to the same pre-therapy staging inaccuracies as is low-dose diagnostic imaging with 131I. Further work is needed to determine whether larger diagnostic doses of 123I might mitigate this problem.