Usefulness of 124I PET/CT imaging to predict absorbed doses in patients affected by metastatic thyroid cancer and treated with 131I.

The aim of this work was the evaluation of the usefulness of 124I PET/CT sequential scans to predict absorbed doses to metastatic thyroid cancer in patients undergoing 131I therapy. METHODS: From July 2011 until April 2012 8 patients affected by metastatic thyroid cancer were enrolled. Each patient underwent 4 PET/CT scans at 4, 24, 48, 72 h after the administration of about 74 MBq of 124I. Blood samples and whole body exposure measurements were obtained to calculate blood and red marrow doses. Activity concentrations and lesion volumes obtained from PET/CT images were used to evaluate tumour doses with MIRD formalism and spheres model. The average administered 131I therapeutic activity was 6475 MBq (range: 3700-9250 MBq). RESULTS: 124I PET/CT images showed, with a very good resolution, all 131I avid lesions detected by post therapy whole body scans. The average dose rates for blood, red marrow and lesions were respectively: 6.58E-02 ± 1.64E-02 mGy/MBq, 5.73E-02 ± 1.57E-02 mGy/MBq, 2.22E+01 ± 1.62E+01 mGy/MBq. Three out of eight patients did not show any uptake of 124I in all PET/CT scans, despite high level of TSH and CT detectable lesions. Post-therapy 131I whole body scan confirmed the absence of focal iodine uptake. CONCLUSION: Negative 124I PET/CT images probably could be used as predictive of real absence of iodine avidity, avoiding all toxicity from useless 131I therapy. A higher number of patients is necessary to validate these preliminary results and a project is ongoing to compare MIRD results to voxel dosimetry based on Monte Carlo simulation.