Comparison of whole body 131Iodine scan results in four, seven and nine days after radioiodine therapy of differentiated thyroid cancer

Introduction: Finding optimum time of post ablation whole body iodine scan in patients with differentiated thyroid cancer(DTC) treated with I131.Material and Methods: 20 patients with DTC, who were treated with I131 underwent post ablation whole body iodine scan (WBIS) in days 4, 7 and 9 after treatment. A dual head gamma camera (ecam, Siemens) equipped with high energy parallel hole collimator was used for imaging. The images were acquired with 7cm/min and stored in a 1024 ×256 matrix. Results: 3 Patients had negative WBIS in all three sets of imaging and 17 patients had postsurgical thyroid remnants on all 3 scans. On days 4 and 7 we detected 11 patients with cervical lymph node metastases while on day 9 only 9 patients showed cervical lymph node metastases.(P=0.135)On all 3 sets of images, we encountered 4 patients with mediastinal lymph node metastases and 1 patient with bone metastasis. In addition, all 3 sets of images detected lung metastases in three patients. The total number of affected foci did not have any statistical differences in whole body scan of day 4, 7 and 9. (P = 0.083)Conclusion: According to the radiation safety hazards for staff and technicians of nuclear medicine department and lack of difference in scan findings between 4 and 7 days after RAI, scanning the DTC patients in the day 7 after RAI administration , is more practicable, with less probability of missing the sites of involvement. Performing whole body iodine scans after 1 week is not recommended.