Objective: To determine whether 123 I can be used as a safe and effective alternative tracer to 131 I for imaging remnant tissue and for searching for metastatic lesions of well-differentiated thyroid cancer.
Methods: We studied a series of 16 patients (10 women and 6 men; 29 to 73 years of age) who had well-differentiated thyroid cancer and had undergone thyroidectomy and subsequent radioiodine treatment. Diagnostic 10 mCi (either preablation or postablation) were compared with the 5- to 7-day 131 I posttherapy scans (doses, 75 to 200 mCi). Scans were jointly interpreted by nuclear medicine and endocrinology staff members.
Results: Of the 16 patients, 15 had concordant findings between the 123 I diagnostic images and the corresponding 131 I posttherapy scans. Only one patient had additional lesions noted on the posttherapy 131 I scan, which was attributed to the higher dose the patient received during radioiodine treatment (a 6% discordance rate). This discordance, however, would not have changed the patient treatment. No advantage was noted for delayed imaging at 48 hours; the 24-hour images were satisfactory. The 123 I scans were of higher resolution than the 131 I scans.
Conclusion: Diagnostic 123 I scans are an acceptable alternative to 131 I scanning for whole-body imaging in well-differentiated thyroid cancer and demonstrate adequate imaging of thyroid remnant tissue or metastatic lesions from thyroid carcinoma comparable to the post-therapy scans. 123 I scans also offer greater convenience to patients because they do not have to return for 48- or 72-hour scans. Furthermore, they facilitate earlier institution of therapy by decreasing the interval between diagnostic scanning and radioiodine treatment.