Usefulness of 124I PET/CT for 131I treatment planning in patients affected by metastatic thyroid cancer: Preliminary results

2063 Objectives 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 patients undergoing 131I therapy. Methods We designed a research protocol, to enroll about 30 patients in 2 years: at present time we studied 3 patients affected by diffuse lung metastasis from thyroid cancer. 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 tumor doses with MIRD formalism. Results Patient 1 and Patient 2: Most of the lesions showed an uptake of 124I with a maximum concentration at the 24th hour. Patient 3: Despite high level of TSH and CT detectable lesions this patient didn’t show any uptake of 124I in all PET/CT scans. Post therapy I131 Whole body scan was negative. All patients showed mild early uptake in the salivary glands with an exponential washout. Patients received respectively 7400 MBq, 9250 MBq and 3700 MBq of 131I for therapy. The average dose rates for blood, red marrow, lesions, parotid and salivary glands were respectively: 8.13E-02 mGy/MBq, 7.10E-02 mGy/MBq, 9.11 mGy/MBq, 1.31 mGy/MBq and 7.48E-01. Conclusions Negative 124I PET/CT images probably could be used as predictive of real absence of iodine avidity. MIRD model could be used to estimate absorbed doses to small lesions but could not be able to predict lung dose due to the extreme heterogeneity of lung uptake and lung density when diffuse disease is present. Dose to the salivary glands was limited but all precautions to avoid their compromising must be applied to preserve their functionality. More patients are necessary to validate these very preliminary results and a project is ongoing to compare MIRD results to voxel dosimetry based on Monte Carlo simulation