Prediction of absorbed dose to normal organs in thyroid cancer patients treated with 131I by use of 124I PET and 3-dimensional internal dosimetry software.

UNLABELLED: The objective of this work was to determine normal organ (131)I dosimetry in patients undergoing radioiodide therapy for thyroid cancer by use of serial scanning with (124)I PET.

METHODS: A total of 26 patients who had papillary and follicular metastatic thyroid cancer and who were already enrolled in a Memorial Sloan-Kettering Cancer Center (131)I thyroid cancer protocol were selected for this study. Imaging before (131)I therapy consisted of multiple, whole-body (124)I PET studies over a period of 2-8 d, an (18)F-FDG PET scan and, for some, a diagnostic CT scan. With a set of in-house-developed software tools (3-dimensional internal dosimetry [3D-ID] and Multiple Image Analysis Utility [MIAU]), the following procedures were performed: all PET emission and transmission and CT image sets were aligned; half-life-corrected tomographic images of (131)I activity were integrated voxel by voxel to produce cumulated (131)I activity images; and the latter images were, in turn, convolved with a (131)I electron-photon point kernel to produce images of (131)I dose distribution. Cumulated activity values and calculated residence times obtained from our patient-specific dosimetry software (3D-ID) were used as inputs to OLINDA, and volume difference-adjusted comparisons were made between the mean dose estimates.

RESULTS: With 3D-ID, dose volume histograms and mean doses were calculated for 14 organs, and results were expressed in Gy/GBq. The highest mean dose, 0.26 Gy/GBq, was seen in the right submandibular gland, whereas the lowest mean dose, 0.029 Gy/GBq, was seen in the brain.

CONCLUSION: This is the first comprehensive study of normal organ dosimetry in patients by use of a quantitative tomographic imaging modality.