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¹²³I Isotope as a Diagnostic Agent in the Follow-Up of Patients with Differentiated Thyroid Cancer: Comparison with Post ¹³¹I Therapy Whole Body Scanning

Departments of Medicine (A.S.A., A.A.-H., A.A.), Radiology (S.B.), Surgery (M.A.M.), and Biomedical Physics (A.A.H.), King Faisal Specialist Hospital and Research Center, Riyadh 11211, Saudi Arabia

Address all correspondence and requests for reprints to: Dr. Ali Alzahrani, Department of Medicine (MBC-46), King Faisal Specialist Hospital and Research Center, P.O. Box 3354, Riyadh 11211, Kingdom of Saudi Arabia. E-mail: aliz{at}kfshrc.edu.sa

Abstract

Radioactive iodine (¹³¹I) plays a major role in the diagnosis and management of differentiated thyroid cancer (DTC); however, data on the use of the ¹²³I isotope in DTC are limited. We compared 238 diagnostic whole body scans performed 24 h after oral ingestion of 185–555 MBq ¹²³I with their corresponding ¹³¹I posttherapy whole body scans obtained 4–5 d after ¹³¹I therapy. We studied scans in 3 clinical situations: with the first ¹³¹I therapy, with the second ¹³¹I therapy, and in cases of elevated Tg and negative diagnostic scan. One hundred and seventy-seven pairs were obtained with the first ¹³¹I therapy and showed complete concordance between pretreatment and posttreatment scans in 166 pairs (concordance rate, 93.8%). Six other posttreatment scans showed more foci in the thyroid bed than the pretreatment scans, but no evidence of uptake in new areas. Only 5 posttreatment scans showed foci in new locations: 3 in cervical lymph nodes (CLN), 1 in the lung, and 1 new bone metastasis in a patient with known skeletal metastases. With the second ¹³¹I therapy, 34 pairs were obtained and showed complete concordance in 28 pairs (concordance rate, 82.4%). Five discordant pairs showed additional foci in areas that were already positive on pretreatment scans. Only 1 posttreatment scan showed a new bone metastasis in a different site from the bone metastases that were seen on its corresponding pretreatment scan. Of 27 pairs of scans in patients with elevated Tg and negative pretreatment scans, 15 posttreatment scans remained negative, 6 posttreatment scans showed an uptake in the thyroid bed, and 3 other posttreatment scans showed lung uptake in patients whose computed tomography scans of the chest showed only bronchiectasis (in 2 patients) and lung scarring (in the third patient) without evidence of lung metastases. Three posttreatment scans showed definite uptake (in thyroid bed, thyroid bed and lung, and CLN) compared with their corresponding pretreatment scans, which were initially reported negative but were retrospectively thought to have had faint uptake. In 56 pretreatment scans, the ¹²³I diagnostic activity was 185 MBq, and the results showed complete concordance in 54 pairs. Two posttreatment scans showed additional uptake: 1 in the bone and 1 in CLN. These data suggest that pretreatment scanning using ¹²³I is highly comparable to ¹³¹I posttreatment scanning and that ¹²³I is an excellent diagnostic agent in DTC.

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