Recombinant human thyroid-stimulating hormone vs thyroid hormone withdrawal in ¹²⁴I-PET/CT based 3D radiobiological dosimetry (3D-RD) for ¹³¹I therapy of metastatic differentiated thyroid cancer

Objectives Patients with metastatic differentiated thyroid cancer (DTC) can be prepared either using recombinant human thyroid-stimulating hormone (rhTSH) or thyroid hormone withdrawal (THW) prior to radioiodine administration for treatment. The objective of this study was to compare rhTSH and THW in terms of the absorbed dose (AD) to the critical organs for ¹³¹I therapy of metastatic DTC, where the same patients were prepared and imaged following both methods.

Methods Three DTC patients at MedStar Wash Hosp Ctr were first prepared using the rhTSH method and imaged by ¹²⁴I-PET/CT at 2, 24, 48, 72 and 96 hrs post administration of about 0.8 mCi Na¹²⁴I. After 1-2 months the same patients were prepared using the THW method and imaged as before. The diagnostic PET/CT images were used to perform dosimetric calculations for ¹³¹I therapy, using the dosimetry package 3D-RD. The AD in Gy/mCi of ¹³¹I, for lungs, liver, heart, kidneys and bone marrow was obtained for each study.

Results The AD per administered activity of ¹³¹I (in Gy/mCi) to the normal organs is higher in the THW compared to the rhTSH study for 2 patients. In the third patient, a slightly higher organ AD following rhTSH compared to THW was obtained. The THW to rhTSH ratio for each patient, averaged over 5 organs was 1.5, 2.7 and 0.5. The bone marrow AD for each patient following rhTSH was 5.1, 3.6, 24.0 mGy/mCi and 4.6, 6.1, 19.0 mGy/mCi following THW.

Conclusions The availability of ¹²⁴I diagnostic images following preparation using rhTSH and THW for the same patients, allows for a head-to-head comparison of the two methods. The organ AD in Gy/mCi is higher for the THW case vs rhTSH case for 2/3 patients. More patient data that have followed both rhTSH and THW diagnostic studies, will be analyzed in order to understand the impact of rhTSH vs THW on organ AD.