Thyroid Remnant Dose: ¹²⁴I-PET/CT Dosimetric Comparison of rhTSH versus Thyroid Hormone Withholding Before Radioiodine Remnant Ablation in Differentiated Thyroid Cancer

Abstract

Aim: Recombinant human thyroid-stimulating hormone (rhTSH) recently was approved as an alternative to thyroid hormone withholding (THW) to elevate TSH for thyroid remnant ablation in differentiated thyroid carcinoma patients. High ablation success rates are reported with diverse rhTSH-aided ¹³¹I activities. Improved renal function causes ∼50% faster radioiodine clearance under euthyroidism versus hypothyroidism. Knowledge of comparative remnant radioiodine kinetics, particularly the remnant radiation dose in Gy/GBq of administered ¹³¹I activity (RDpA), could assist in choosing rhTSH-aided ablative activities.

Material and Methods: To compare the RDpA, determined through ¹²⁴I-positron emission tomography/computed tomography (PET/CT), under the two stimulation methods, we retrospectively divided into two groups 55 consecutive totally-thyroidectomized, radioiodine-naïve patients. The rhTSH group (n=16) received ¹²⁴I on thyroid hormone, 24 h after two consecutive daily intramuscular injections of rhTSH, 0.9 mg. The THW group (n=39) received ¹²⁴I after weeks-long THW, when serum TSH first measured ≥25 mIU/L. We performed PET investigations 4 h, 24 h, 48 h, 72 h and 96 h and PET/CT 25 h after ¹²⁴I administration.

Results: Median stimulated serum thyroglobulin was 15 times higher (p=0.023) and M1 disease almost twice as prevalent (p=0.05) in rhTSH versus THW patients. Mean±standard deviation RDpA was statistically equivalent between the groups: rhTSH, 461±600 Gy/GBq, THW, 302±329 Gy/GBq, two-sided p=0.258.

Conclusions: rhTSH or THW deliver statistically equivalent radiation doses to thyroid remnant and may be chosen based on safety, quality-of-life, convenience and pharmacoeconomic factors. Institutional fixed radioiodine activities formulated for use with THW need not be adjusted for rhTSH-aided ablation.

differentiated thyroid carcinoma - thyroid remnant - thyroid remnant dose - thyroid remnant ablation - 124-iodine - 131-iodine - radioiodine - positron emission tomography - computed tomography - dosimetry - recombinant human thyroid-stimulating hormone - thyroid hormone withholding - remnant dose per administered activity

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