I-124 PET/CT image-based dosimetry in patients with differentiated thyroid cancer treated with I-131: correlation of patient-specific lesional dosimetry to treatment response

Purpose The objective of this study is to evaluate the lesion absorbed dose (AD), biological effective dose (BED), and equivalent uniform dose (EUD) to clinical–response relationship in lesional dosimetry for 131 I therapy. Methods Nineteen lesions in four patients with metastatic differentiated thyroid cancer (DTC) were evaluated. The patients underwent PET/CT imaging at 2 h, 24 h, 48 h, 72 h, and 96 h post administration of ~ 33–65 MBq (0.89–1.76 mCi) of 124 I before undergoing 131 I therapy. The 124 I PET/CT images were used to perform dosimetry calculations for 131 I therapy. Lesion dose–rate values were calculated using the time–activity data and integrated over the measured time points to obtain AD and BED. The Geant4 toolkit was used to run Monte Carlo on spheres the same size as the lesions to estimate EUD. The lesion AD, BED, and EUD values were correlated with response data (i.e. change in lesion size pre- and post-therapy): complete response (CR, i.e. disappearance of the lesion), partial response (PR, i.e. any decrease in lesion length), stable disease (SD, i.e., no change in length), and progressive disease (PD, i.e., any increase in length). Results The lesion responses were CR and PR (58%, 11/19 lesions), SD (21%, 4/19), and PD (21%, 4/19). For CR and PR lesions, the ADs, BEDs and EUDs were > 75 Gy for 82% (9/11) and < 75 Gy for 18% (2/11). The ADs and BEDs were < 75 Gy for SD and PD lesions. Conclusion By performing retrospective dosimetry calculations for 131 I therapy based on 124 I PET/CT imaging, we evaluated the correlation of three dosimetric quantities to lesional response. When lesion AD, BED, and EUD values were > 75 Gy, 47% (9/19) of the lesions had a CR or PR. The AD, BED, and EUD values for SD and PD lesions were < 75 Gy. The data presented herein suggest that the greater the lesion AD, BED, and/or EUD, the higher the probability of a therapeutic response to 131 I therapy.