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 ¹³¹I dosimetry in patients undergoing radioiodide therapy for thyroid cancer by use of serial scanning with ¹²⁴I 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 ¹³¹I thyroid cancer protocol were selected for this study. Imaging before ¹³¹I therapy consisted of multiple, whole-body ¹²⁴I PET studies over a period of 2–8 d, an ¹⁸F-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 ¹³¹I activity were integrated voxel by voxel to produce cumulated ¹³¹I activity images; and the latter images were, in turn, convolved with a ¹³¹I electron–photon point kernel to produce images of ¹³¹I 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.