Comparison of I-131 whole body scans performed one and two weeks post-therapy in patients with differentiated thyroid cance

Objectives To compare post-therapy scans using iodine 131 (RxWBS) performed at two time points (early 3-5 days & delayed within 10-12 days) in patients with differentiated thyroid carcinoma (DTC).

Methods This ongoing prospective study recruited 23 patients with DTC referred for RxWBS after ablation or therapy doses of I131. RxWBS was performed using a standardized protocol. For every patient, two scans were performed early at 3-5 days and delayed at 10-12 days. The scans were read blindly regarding the time point. The number of neck & extra-neck lesions was counted. A standard region-of-interest (ROI) was drawn around each lesion and the total counts were recorded. ROI was copied and used for recording the liver (LiBkg) and neck background counts (NBkg). Lesion to NBkg (L/N) and lesion to LiBkg (L/Li) ratios were calculated. Radiation exposure (RE) from the patients was measured at 1.0 meter prior to each scan.

Results The number of neck or extra-neck lesions did not change between the early and delayed scans. Lesion counts were significantly higher in early scans compared to delayed scans (163186 ± 565621 vs. 11065 ± 22888 counts; P < 0.001). Similarly NBkg was significantly higher in early (1411 ± 1640 counts) vs. delayed scans (295 ± 256 counts); P < 0.001. L/N ratio was significantly higher in early scans (71 ± 142 vs. 27 ± 47; P = 0.006). LiBkg did not change significantly between the scans (3417 ± 2995 vs. 2220 ± 3612 counts; P = 0.9). L/Li ratios were (65 ± 191 vs. 14 ± 37; P = 0.2) Mean RE was 10.3 ± 9.4 μSv/h. compared to 3.1 ± 3.5 at the time of delayed scan (P = 0.002).

Conclusions Early (3-5 days) post-therapy ¹³¹I-whole body scans show significantly better lesion visualization compared to delayed (10-12 days RxWBS); however, the number of lesions did not change the radiation exposure was significantly higher.

Research Support None