Utility of additional delayed post-therapeutic ¹³¹I whole-body scanning in patients with thyroid cancer

The purpose of this study is to compare early (3 days after administration) and delayed (7-9 days after administration) post-therapeutic ¹³¹I scanning for detecting metastatic lesions from differentiated thyroid carcinoma.

This study included 24 patients (10 men and 14 women) with differentiated thyroid carcinoma who underwent early and delayed whole-body scanning after ¹³¹I treatment. The early and delayed scan images were visually analyzed, and the region-to-whole-body ratio (RBR) and its variation from the early to delayed scanning (RBR variation) were calculated.

In the visual analysis, lung metastases in 2 patients and 1 bone metastasis could not be detected on early scanning but were classified as moderate uptake on delayed scanning. The abdomen showed definite uptake on early scanning but was washed out on delayed scanning. The RBR variation of the lung and bone metastases was significantly higher than those of the abdomen and thyroid remnants. This suggests that the distribution of ¹³¹I was more quickly washed out of the abdomen and thyroid remnants than from the metastatic lesions.

Delayed scanning showed a higher detection rate of lung and bone metastases than early scanning because of the washout of the physiological distributions. Additional delayed scanning is recommended, especially for high-risk patients with intensive physiological distribution on early scanning.