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.

Abstract

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