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I-123 Diagnostic Thyroid Tumor Whole-Body Scanning with Imaging at 6, 24, and 48 Hours


Original Articles

Purpose  The use of I-123 in lieu of I-131 for diagnostic whole-body thyroid tumor scanning (DxRaI) 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 DxRaI under withdrawal conditions, imaging at 6, 24, and, in most cases, 48 hours. DxRaI 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 DxRaI 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 DxRaI 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.”

From the Nuclear Medicine Section,* San Francisco VA Medical Center,

and Departments of Radiology† and Medicine,‡ University of California, San Francisco, San Francisco, California

Received for publication July 19, 2001.

Accepted August 22, 2001.

Reprint requests: Stephen Gerard, Nuclear Medicine (115), San Francisco VA Medical Center, 4150 Clement Street, San Francisco, California 94121. E-mail:

© 2002 Lippincott Williams & Wilkins, Inc.