Comparison of 123I scintigraphy at 5 and 24 hours in patients with differentiated thyroid cancer

This study was performed to determine differences in 123I image quality at 5 and 24 h for the detection of residual thyroid or recurrent disease in patients with differentiated thyroid cancer and to evaluate which of these approaches provides the optimal yield compared with scans obtained after 131I therapy.

The study included 99 patients (70 women, 29 men). Histopathology included 88 papillary cancers, 7 follicular cancers, and 4 Hürthle cell tumors. Each patient previously had a near-total thyroidectomy. Seventy patients were receiving initial 131I therapy, and 29 had undergone prior radioablative therapy with 131I. Whole-body images and spot views of the neck and chest were obtained with a dual-head scanner at 5 and 24 h after the oral administration of 56 MBq (1.5 mCi) Na123I. In addition, tomographic images of the neck and chest were obtained in 26 patients. The images obtained at 5 and 24 h were evaluated for the number of lesions visualized and the image quality. Seventy-four patients received radioablative therapy after this diagnostic work-up. Whole-body images were obtained on these patients 7 d after 131I therapy and were compared with the diagnostic scans.

Overall, images acquired 5 and 24 h after oral administration of 56 MBq (1.5 mCi) 123I were concordant in 73% of patients. For 25 patients (25%), 24-h scanning was superior by detecting additional neck foci or confirming equivocal neck foci (20 patients) and confirming equivocal pulmonary uptake (5 patients). In the subset of patients undergoing surveillance imaging after prior 131I ablation therapy with positive scans, 24-h images were superior in 66%. In 3 patients, SPECT revealed additional foci compared with planar images at 5 h. These foci were also clearly identified on the planar images obtained at 24 h. Images obtained after therapy did not identify additional sites of tumor involvement compared with those noted on 24-h images but did reveal more foci of residual thyroid tissue in 5 patients.

The diagnostic yield of planar diagnostic 123I scintigraphy at 24 h was superior to that at 5 h for lesion detection and image quality, and images obtained after 131I therapy did not reveal unknown metastatic foci.