Objective: The aim of this study was to investigate the appropriate time for performing an iodine-131 post-therapy whole-body scan (TxWBS) through a qualitative and semiquantitative analysis of early and late scans.
Materials and methods: This study evaluated pairs of scans of 134 patients who underwent TxWBS on the third and seventh day. The scans were analyzed to evaluate sites, intensity of uptake, concordance or discordance between the scans, relationship with risk factors, and serum thyroglobulin (Tg) levels. To evaluate early and late radioiodine kinetics in thyroid remnants and metastases, 65/134 pairs of scans (48.5%) were subjected to a semiquantitative analysis.
Results: The early and late scans furnished concordant images in 108/134 patients (80.5%). In 10/134 patients (7.5%), early scans provided more information compared with late scans, showing lymph node and distant metastases in seven and three patients, respectively. In 16/134 patients (12%), late scans provided more data compared with early scans, with thyroid remnants and lymph node and distant metastases demonstrated in four, seven, and five patients, respectively. Negative early/positive late TxWBS results in patients were found to be significantly correlated (P=0.007) with elevated serum levels of Tg and a high-risk for recurrence (P=0.003).
Conclusion: This study suggests that in about 20% of patients early or late TxWBS can miss the visualization of thyroid remnants or lymph node or distant metastases, which can be achieved performing both studies. High-risk patients with elevated serum Tg levels should be considered for a late TxWBS, which can demonstrate a possible metastatic involvement that was not diagnosed or that was downstaged by early TxWBS.