Neck and whole-body scanning with 5-mCi dos of ¹²³I as diagnostic tracer in patients with well-differentiated thyroid cancer

Objective: To determine whether a 5-mCi dose of ¹²³I can be used as an effective radiotracer for assessing the presence of remnant thyroid tissue and for searching for metastatic lesions in patients with well-differentiated thyroid cancer as well as to attempt to ascertain whether a scan performed only at 4 hours is sufficient for accurate diagnosis and might replace the conventional protocol of scanning at both 4 hours and 24 hours. Methods: We prospectively studied 27 patients who had undergone near-total thyroidectomy and had a documented diagnosis of well-differentiated thyroid carcinoma. Patients underwent scanning after receiving a 5-mCi dose of ¹²³I, at a time when they had discontinued thyroid replacement therapy and had a thyrotropin level in excess of 30 μIU/mL. Whole-body images at 4 hours and 24 hours were obtained and were compared with posttherapy scans obtained 5 to 7 days after administration of ¹³¹I. Scans were interpreted by two board-certified nuclear medicine physicians. Results: Of the 27 patients, 2 (7.4%) showed discordance between the ¹²³I scan performed at 24 hours and the posttherapy ¹³¹I scan. When 4-hour images after administration of ¹²³I were compared with the posttherapy ¹³¹I scans, a discordance rate of 14.8% (4 of 27 patients) was noted. In addition, two of these four patients showed lesions on the 24-hour images that were not seen on the 4-hour images (one with new lung metastatic involvement and the other with a local recurrence in the lower neck area). The prognosis and treatment of these two patients were substantially changed by the results of the 24-hour images. Conclusion: On comparison of scans obtained after administration of a 5-mCi dose of ¹²³I with those obtained after ¹³¹I therapy, we conclude that 5 mCi of ¹²³I produces images that have excellent quality and resolution and also compare favorably with those obtained after ¹³¹I therapy. Furthermore, a decrease in the dose of ¹²³I from 10 mCi to 5 mCi lowered the cost of the study without compromising the diagnostic accuracy or image quality. Finally, use of 24-hour images will occasionally disclose additional areas of radioiodine uptake not detected on the 4-hour scans and is therefore recommended.