Optimal time for 124I PET/CT imaging in metastatic differentiated thyroid cancer

240 Objectives: As 124I has a different half-life and decay scheme compared to conventional radioiodine isotopes, the optimal time for PET/CT imaging to detect metastasis in differentiated thyroid cancer (DTC) is unknown. Therefore, we aim to determine the optimal time for 124I PET/CT imaging by comparing different days of imaging. Methods DTC patients with known metastatic disease were prepared with low-iodine diet and adequate TSH stimulation. 124I PET and low-dose localization CT were performed at 2, 24, 48, 72, 96 hours after oral administration of 62.9 MBq (1.7 mCi) of 124I. Each scan was independently reviewed by two nuclear medicine physicians, and all foci of uptake were graded 1-5 (1 = definite physiological uptake or artifact; 2 = most likely physiological uptake or artifact; 3 = indeterminate; 4 = most likely recurrence or metastasis; 5 = definite recurrence or metastasis). Discordant findings were resolved by consensus. Foci graded 3-5 were determined as positive for metastasis when seen on another 124I PET/CT imaging time point, visualized on other diagnostic images, or confirmed by cytopathology. These foci were tabulated for number, site, and intensity of uptake. The intensity of uptake was scored by a semi-quantitative 3-point grade (1: faint uptake; 2: intense uptake; 3: very intense uptake) and by SUVmax values. Results A total of 70 124I PET/CT scans (14 sets of 5 time points) were reviewed for 13 DTC patients; one patient had two sets of scans, once using recombinant human thyroid stimulating hormone (rhTSH) and once by thyroid hormone withdrawal (THW). In the 13 patients with highly suspected metastatic disease, a total of 37 lesions suspicious for metastasis were identified on 124I PET/CT, of which eight were seen on the 2h scan, 22 on 24h scan, 30 on 48h scan, 28 on 72h scan and 27 on 96h scan. Of note, significant esophageal uptake was seen on the 2h and less on the 24h scans. A total of 33/37 (89%) lesions were seen on either the 24h scan or the 48h scan, of which 19 were seen on both scans. The three additional lesions seen on the 24h scan were either isolated reads not seen at other time points or not seen on CT correlate; whereas the eleven more lesions seen on the 48h scan were distant metastasis with correlated on CT or seen at other time points. Four out of six patients with diffuse lung metastasis did not have 124I uptake at any time point; however, two patients had scattered 124I uptake at 48h, 72h and 96h with no specific CT correlate, except for the four largest ~1cm distinct lung nodules. There was significant statistical difference between 24h and 48h scans in the total number of lesions detected (P Conclusions124I PET/CT at 24h or 48h is superior to 2h, 72h or 96h images in metastatic lesion detection. 48h scans yielded greater number of distant metastatic uptake with very high visual intensity. SUVmax is not a reliable measure for 124I uptake on PET/CT. Additional quantitative analysis such as tumor-to-background ratio with greater number of foci is needed. Research support We are grateful for the generous donations from our patients.