Are posttherapy radioiodine scans informative and do they influence subsequent therapy of patients with differentiated thyroid cancer?

Posttherapy scans (PTS) with a gamma camera are typically used after therapeutic doses of I-131 to visualize metastases that may not be seen with lower dose diagnostic scans. During a 16-month period, we studied 81 patients (64 with papillary thyroid cancer and 17 with follicular thyroid cancer), who had both a diagnostic whole-body scan (I-131 dose 3 mCi) and a PTS. A total of 117 PTS were evaluated. At the time of PTS, clinical or radiologic evidence of metastatic or residual disease was present in 68 patients (84%). The anatomic sites of known disease included, neck (63), mediastinum (23), lung (35), bone (14), trachea (16), esophagus (5), and brain (2). PTS showed focal areas of abnormal uptake not seen in diagnostic scans in 15 scans (13%). Areas with abnormal new uptake included: neck (5), lung (5), mediastinum (4), bone (2), and adrenal (1). In 7 patients (9%) the PTS results impacted future decisions regarding plans for subsequent diagnostic scanning and I-131 therapy or changed the patient's risk group category. In conclusion: (1) 13% of 117 PTS demonstrated abnormal foci of I-131 uptake not seen on pretherapy scans and (2) PTS changed management strategy in 9% of the studied patients.