Recombinant Human Thyroid-Stimulating Hormone Versus Thyroid Hormone Withdrawal in I PET/CT-Based Dosimetry for I Therapy of Metastatic Differentiated Thyroid Cancer.

Patients with metastatic differentiated thyroid cancer (DTC) may be prepared using either thyroid-stimulating hormone withdrawal (THW) or recombinant human thyroid-stimulating hormone (rhTSH) injections before I administration for treatment. The objective of this study was to compare the absorbed dose to the critical organs and tumors determined by I PET/CT-based dosimetry for I therapy of metastatic DTC when the same patient was prepared with and imaged after both THW and rhTSH injections. Four DTC patients at MedStar Washington Hospital Center were first prepared using the rhTSH method and imaged by I PET/CT at 2, 24, 48, 72, and 96 h after administration of approximately 30-63 MBq of I. After 5-8 wk, the same patients were prepared using the THW method and imaged as before. The I PET/CT images acquired as part of a prospective study were used to perform retrospective dosimetric calculations for I therapy for the normal organs with the dosimetry package 3D-RD. The absorbed doses from I for the lungs, liver, heart, kidneys, and bone marrow were obtained for each study (rhTSH and THW). Twenty-two lesions in 3 patients were identified. The contours were drawn on each PET image of each study. Time-integrated activity coefficients were calculated and used as input in OLINDA/EXM sphere dose calculator to obtain the absorbed dose to tumors. The THW-to-rhTSH organ absorbed dose ratio averaged over 5 organs for the first 3 patients was 1.5, 2.5, and 0.64, respectively, and averaged over 3 organs for the fourth patient was 1.1. The absorbed dose per unit administered activity to the bone marrow was 0.13, 0.086, 0.33, and 0.068 mGy/MBq after rhTSH and 0.11, 0.14, 0.22, and 0.080 mGy/MBq after THW for each patient, respectively. With the exception of 3 lesions of 1 patient, the absorbed dose per unit administered activity of I was higher in the THW study than in the rhTSH study. The ratio of the average tumor absorbed dose after stimulation by THW compared with stimulation by rhTSH injections was 3.9, 27, and 1.4 for patient 1, patient 2, and patient 3, respectively. The ratio of mean tumor to bone marrow absorbed dose per unit administered activity of I, after THW and rhTSH, was 232 and 62 (patient 1), 12 and 0.78 (patient 2), and 22 and 11 (patient 3), respectively. The results suggest a high patient variability in the overall absorbed dose to the normal organs per MBq of I administered, between the 2 TSH stimulation methods. The tumor-to-dose-limiting-organ (bone marrow) absorbed dose ratio, that is, the therapeutic index, was higher in the THW-aided than rhTSH-aided administrations. Additional comparison for tumor and normal organ absorbed dose in patients prepared using both methods is needed before definitive conclusions may be drawn regarding rhTSH versus THW patient preparation methods for I therapy of metastatic DTC.