“No remnant thyroid tissue” on Tc-99m pertechnetate scan, can we trust it? Comparison with I-123 scan and correlation with ablation success

Objectives This study is to compare the diagnostic performance of Tc-99m pertechnetate scan (Tc-99m scan) and I-123 scan for detecting remnant thyroid tissue in post-thyroidectomy states. Successful ablation rate was also compared in groups which were categorized by scan findings.

Methods We enrolled 150 patients who underwent I-123 scan and Tc-99m scan before first I-131 therapy. All scans were done after thyroid hormone withdrawal (TSH>30 μIU/mL). Patient-based analysis was done. Each result was confirmed by post-therapy I-131 scan. Groupings were based on scan findings; Group A, positive on both scans; B, positive only on I-123 scan; C, positive only on Tc-99m scan; D, negative on both scans. Successful ablation was determined by follow-up I-123 scan 6 months after I-131 therapy. With groups, we compared the clinical factors including ablation success, age, sex, serum thyroglobulin, I-131 dose, interval from operation to scan.

Results Findings of Tc-99m scan were significantly different from those of I-123 scan. Tc-99m scan showed much lower sensitivity and NPV for detecting remnant thyroid tissue than I-123 scan (63.2% vs 97.2% and 10.2% vs 60%, respectively). Specificity and PPV of both scans were 100%. The proportions of each group were like these; group A, 60.7%; B, 32.7%; C, 0%; and D, 6.7%. There was fair correlation between group B and successful ablation (phi=0.280, p=0.012). However, logistic regression failed to show predictive ability of group B for successful ablation. Clinical factors were not different between groups.

Conclusions Tc-99m scan misses patients who need ablation therapy in thyroid cancer patients. Therefore, I-123 should be used rather than Tc-99m scan. Differences in each scans does not correlated to successful ablation