Radioactive Body Burden Measurements in 131 Iodine Therapy for Differentiated Thyroid Cancer: Effect of Recombinant Thyroid Stimulating Hormone in Whole Body 131 Iodine Clearance

Abstract

Protocols in the management of differentiated thyroid cancer, recommend adequate thyroid stimulating hormone (TSH) stimulation for radioactive 131I administrations, both for imaging and subsequent ablations. Commonly followed method is to achieve this by endogenous TSH stimulation by withdrawal of thyroxine. Numerous studies worldwide have reported comparable results with recombinant human thyroid stimulating hormone (rhTSH) intervention as conventional thyroxine hormone withdrawal. Radiation safety applications call for the need to understand radioactive 131I (RA131I) clearance pattern to estimate whole body doses when this new methodology is used in our institution. A study of radiation body burden estimation was undertaken in two groups of patients treated with RA131I; (a) one group of patients having thyroxine medication suspended for 5 weeks prior to therapy and (b) in the other group retaining thyroxine support with two rhTSH injections prior to therapy with RA131I. Sequential exposure rates at 1 m in the air were measured in these patients using a digital auto-ranging beta gamma survey instrument calibrated for measurement of exposure rates. The mean measured exposure rates at 1 m in μSv/h immediately after administration and at 24 h intervals until 3 days are used for calculating of effective ½ time of clearance of administered activity in both groups of patients, 81 patients in conventionally treated group (stop thyroxine) and 22 patients with rhTSH administration. The 131I activities ranged from 2.6 to 7.9 GBq. The mean administered 131I activities were 4.24 ± 0.95 GBq (n = 81) in “stop hormone” group and 5.11 ± 1.40 GBq (n = 22) in rhTSH group. The fall of radioactive body burden showed two clearance patterns within observed 72 h. Calculated T½eff values were 16.45 h (stop hormone group) 12.35 h (rhTSH group) for elapsed period of 48 h. Beyond 48 h post administration, clearance of RA131I takes place with T½eff> 20 h in both groups. Neck and stomach exposure rate measurements showed reduced uptakes in the neck for rhTSH patients compared with “stop thyroxine” group and results are comparable with other studies. Whole body clearance is faster for patients with rhTSH injection, resulting in less whole body absorbed doses, and dose to blood. These patients clear circulatory radioactivity faster, enabling them to be discharged sooner, thus reduce costs of the hospitalization. Reduction in background whole body count rate may improve the residual thyroid images in whole body scan. rhTSH provides TSH stimulation without withdrawal of thyroid hormone and hence can help patients to take up therapy without hormone deficient problems in the withdrawn period prior to RA131I therapy. This also will help in reducing the restriction time periods for patients to mix up with the general population and children.

Thyroid Uptake and Effective Half-Life of Radioiodine in Thyroid Cancer Patients at Radioiodine Therapy and Follow-Up Whole-Body Scintigraphy Either in Hypothyroidism or Under rhTSH

“…Absolute values for whole-body half-life differ in the literature. Ravichandran et al reported about 16.45 h in the hypothyroidism group versus 12.35 h in the rhTSH group (11,13). Our values for whole-body half-life were longer with 22.7 6 11.6 h for hypothyroid male and 18.1 6 5.1 h for hypothyroid female patients whereas in the rhTSH group values were 18.0 6 6.6 h for male and 15.9 6 3.7 h for female patients.…”

“…Second, whole-body scintigraphy for response assessment is associated with low radiation exposure and currently in our clinic done almost exclusively with rhTSH. Table 7 compiles publications comparing the effect of hypothyroidism and rhTSH on whole-body dose, half-life, or residence time (6)(7)(8)(9)(10)(11)(12)(13)(14)(15). Most publications are based on studies with a limited number of patients.…”

“…Most studies report lower whole-body radiation exposure in the rhTSH versus hypothyroid group (6)(7)(8)(9)(10)(11)(12)(13)(14). Comparison of these publications reveals considerable differences in methodology, especially in the methods for measuring thyroid uptake and effective half-life.…”

“…It is well established that thyroid half-life is longer than wholebody half-life, as the thyroid stores thyroid hormones. In male hypothyroid patients, thyroid half-life was 38.1 6 19.9 h whereas in the rhTSH group it was 32.3 6 15.5 h. Ravichandran et al measured 48.2 h in 8 rhTSH patients (11,13). Current developments in radioiodine therapy for differentiated thyroid cancer decrease the administered activity.…”

“…Currently, RITh is often given 2-3 wk after surgery (1). Previous studies found differences in radiation exposure between patients treated in hypothyroidism versus rhTSH (6)(7)(8)(9)(10)(11)(12)(13)(14)(15). The aim was to retrospectively examine thyroid uptake and effective half-life in patients with a shorter time interval between surgery and RITh, thus reflecting current therapeutic standards.…”