A Comparison Between Diagnostic I-123 and Posttherapy I-131 Scans in the Detection of Remnant and Locoregional Thyroid Disease

Aim of Study: The aim of this study was to compare pretherapy diagnostic I-123 scans with 7-day posttherapy I-131 scans days in detecting remnant thyroid disease as well as locoregional metastases in patients who had undergone thyroidectomy for differentiated thyroid cancer. Methods: The I-123 and I-131 scans from 53 patients were reviewed. The number of lesions identified on each scan was compared as well as the relative intensity of each lesion on the I-123 and I-131 scans using region-of-interest-derived lesion to background ratios. For the I-123 diagnostic scans, 1.0 to 1.6 mCi of activity was administered orally 24 hours before scan acquisition. Patients received therapeutic doses of I-131 ranging from 30.7 to 362.2 mCi orally, on the same day but after acquisition of the I-123 scan. All posttherapy I-131 scans were performed Mays after radioablative therapy with I-131. Results: Of the 53 patients, 14 had concordant scans, with an equal number of lesions detected by both scans. Thirty-nine I-123/I-131 scan pairs showed discordance, with 27 patients demonstrating more lesions on the I-123 scans. Of the 142 total lesions, 35% were detected only by I-123 pretherapy images compared with 15% detected only on posttherapy images. In a subgroup of 42 patients I-123 scans showed 56 lesions (out of a total of 116) that demonstrated either a better lesion to background ratio than I-131 or were seen exclusively by I-123. Conclusion: The use of diagnostic I-123 pretherapy scintigraphy seems to be superior in many patients to posttherapy I-131 imaging obtained at 7 days in detecting locoregional metastases or remnant in postsurgical patients. This is likely due to the fact that I week is too long to wait to perform posttherapy imaging.