¹³¹I Effective Half-Life and Dosimetry in Thyroid Cancer Patients

Abstract

131 I treatment in thyroid cancer patients may induce side effects, including extrathyroidal cancer and leukemia. There are still some uncertainties concerning parameters that may influence the effective half-life of 131 I and the absorbed doses by extrathyroidal organs. Methods: Whole-body retention of radioiodine was measured in 254 patients, and repeated quantitative whole-body scans and measurements of the urinary excretion of 131 I were performed on 30 of these patients. Results: The mean effective half-life (10.5 h) was shorter by 31%, with little difference between patients, in the 36 patients who received recombinant human thyroid-stimulating hormone than in the 218 patients who underwent thyroid hormone withdrawal (15.7 h). The residence times in the stomach and in the rest of the body were significantly shorter in patients who received recombinant human thyroid-stimulating hormone than in patients who underwent withdrawal, but the residence times were similar in the colon and bladder. Conclusion: In patients who undergo thyroid hormone withdrawal, the longer mean effective half-life is mainly due to delayed renal excretion of 131 I and results in dose estimates higher than the data in report 53 of the International Commission on Radiological Protection, which were obtained from healthy, euthyroid subjects.

Estimation of the Release Time from Isolation for Patients with Differentiated Thyroid Cancer Treated with High-dose I-131

“…Remy et al [10] reported that the mean T eff was 10.5 h in rhTSH patients and was 15.7 h in thyroid-hormone-withdrawal patients. The T eff of our patients (thyroid-hormone-withdrawal patients) was 16.0 h, which was similar to that reported by Pacilio et al [11].…”

Comparison of the Influence on the Liver Function Between Thyroid Hormone Withdrawal and rh-TSH Before High-Dose Radioiodine Therapy in Patients with Well-Differentiated Thyroid Cancer

“…In these patients, the efficacy of radioiodine treatment is related to the radiation dose delivered to remnant and neoplastic thyroid tissue [19]. However, the significant radiation doses delivered to extrathyroidal tissues may induce side effects such as nausea and vomiting, sialadenitis and xerostomia, loss of taste, and bone marrow insufficiency [19].…”

ALARA in rhTSH-stimulated post-surgical thyroid remnant ablation: what is the lowest reasonably achievable activity?

“…Some important differences exist in this regard between LT 4 withdrawal and rhTSH stimulation. The iodine clearance in euthyroid state is considerably higher than in hypothyroidism [8,[13][14][15][16] for example. Therefore the biological whole-body half-life of 131 I is shorter under rhTSH stimulation than it is under withdrawal.…”

“…Therefore the biological whole-body half-life of 131 I is shorter under rhTSH stimulation than it is under withdrawal. In contrast it is also known that the intrathyroidal 131 I half-life is longer during rhTSH stimulation than during LT 4 withdrawal [8,16,17]. This can be explained by the relatively short duration of the rhTSH stimulus, which only results in a modest secretion of thyroid hormone-bound radioiodine.…”

“…This can be explained by the relatively short duration of the rhTSH stimulus, which only results in a modest secretion of thyroid hormone-bound radioiodine. The 131 I residence time between rhTSH stimulation and LT 4 withdrawal in normal cells does not differ however, as the thyrocytes' 131 I uptake tends to be lower after rhTSH stimulation than during LT 4 withdrawal [8,16].…”