Purpose: The use of I-123 in lieu of I-131 for diagnostic whole-body thyroid tumor scanning (DxRal) in patients with differentiated thyroid cancer obviates the risk for stunning and affords significantly improved image quality. Because of the shorter half-life (13 hours) of I-123, images have been acquired primarily 6 or 24 hours after injection, potentially decreasing the sensitivity for detecting weakly avid thyroid tumor or remnant.
Materials and methods: The authors evaluated the use of 111 to 185 MBq (3 to 5 mCi) I-123 for DxRal under withdrawal conditions, imaging at 6, 24, and, in most cases, 48 hours. DxRal with I-123 was compared in 13 evaluations performed in 10 patients, with post-I-131 treatment scans acquired early (2 to 3 days) and late (7 to 10 days) in all cases but one.
Results: Of 37 sites of tumor or remnant identified in post-treatment scans, 26 were found in the DxRal I-123 scan (sensitivity, 70%). Of the 11 sites missed by I-123, 7 were seen only in the late post-treatment scans. Therefore, the sensitivity of I-123 imaging compared with the early post-I-131 treatment scans was 26 of 30, or 86.7%. In 10 cases, 48-hour I-123 imaging was attempted, yielding images of acceptable quality in eight of them. Lesion identification was improved on the 48-hour images; in one case, this allowed the identification of a site of tumor recurrence that was confirmed positive on the I-131 post-treatment scan.
Conclusions: I-123 doses of 111 to 185 MBq for DxRal provide acceptable levels of sensitivity overall and may permit 48-hour imaging for improved detection of weakly avid tumor or remnant without any risk for "stunning."