Recombinant Human Thyroid-Stimulating Hormone Versus Thyroid Hormone Withdrawal in¹²⁴I PET/CT–Based Dosimetry for¹³¹I Therapy of Metastatic Differentiated Thyroid Cancer

Abstract

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 131 I administration for treatment. The objective of this study was to compare the absorbed dose to the critical organs and tumors determined by 124 I PET/CT-based dosimetry for 131 I therapy of metastatic DTC when the same patient was prepared with and imaged after both THW and rhTSH injections. Methods: Four DTC patients at MedStar Washington Hospital Center were first prepared using the rhTSH method and imaged by 124 I PET/CT at 2, 24, 48, 72, and 96 h after administration of approximately 30-63 MBq of 124 I. After 5-8 wk, the same patients were prepared using the THW method and imaged as before. The 124 I PET/CT images acquired as part of a prospective study were used to perform retrospective dosimetric calculations for 131 I therapy for the normal organs with the dosimetry package 3D-RD. The absorbed doses from 131 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. Results: 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 131 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 131 I, after THW and rhTSH, was 232 and 62 (patient 1), 12 and 0.78 (patient 2), and 22 and 11 (patient 3), respectively. Conclusion: The results suggest a high patient variability in the overall absorbed dose to the normal organs per MBq of 131 I administered, between the 2 TSH stimulation methods. The tumorto-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 131 I therapy of metastatic DTC.

Combined model-based and patient-specific dosimetry for 18F-DCFPyL, a PSMA-targeted PET agent

“…The final activity in the LG was determined by subtraction of background activity following the small volume activity measurement methodology proposed by Plyku et al [23]. For the recovery coefficient method, a NEMA phantom containing six spheres with diameters 10, 13, 17, 22, 28 and 37 mm was filled with 18 F and water with a sphere-to-background activity ratio of 4:1 and imaged using a GE Discovery DRX PET/CT scanner, following which resolution recovery coefficients for 18 F were calculated.…”

“…The TIA was determined by integrating the fitted time–activity curves for each patient plotted using the time–activity data from each VOI. The activity value for each time-point was obtained after subtraction of the background following methodology described previously [23]. Integration was performed using a hybrid fit: a trapezoidal fit in the uptake phase and an exponential functional fit in the clearance phase.…”

Tratamiento con radioyodo endovenoso y estimulación con TSH recombinante humana en paciente con cáncer de tiroides extenso. Caso clínico

“…Asimismo, se ha estudiado su uso pacientes con metástasis a distancia con similares tasas de sobrevivida global a 5 años que en aquellos en los cuales se preparan mediante suspensión de la hormona tiroidea 11 . Sin embargo, aún faltan estudios que aseguren que las dosis absorbidas por las lesiones son similares con preparación con TSHrh versus suspensión de hormona tiroidea 12 . En la literatura reciente se encontró el caso de un paciente con gastrostomía y administración endovenosa, sin TSHrh 2 .…”

Accuracy in dosimetry of diagnostic agents: impact of the number of source tissues used in whole organ S value-based calculations

“…The remainder-of-body TIAC was calculated by subtracting from the whole-body TIAC with the individual TIAC of the respective tissues that were progressively included. To account for the partial volume effects in small organs/tissues (e.g., pituitary gland), a method developed by Plyku et al [9] for small volumes (e.g., tumors) was used. Regarding larger tissues/organs, the measured activity concentration within the VOIs was used with the respective phantoms tissue/organ to calculate their respective TIAC.…”