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

Protocols in the management of differentiated thyroid cancer, recommend adequate thyroid stimulating hormone (TSH) stimulation for radioactive 131 I 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 131 I (RA 131 I) 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 RA 131 I; (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 RA 131 I. 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 131 I activities ranged from 2.6 to 7.9 GBq. The mean administered 131 I 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 T ½eff 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 RA 131 I takes place with T ½eff > 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 RA 131 I therapy. This also will help in reducing the restriction time periods for patients to mix up with the general population and children.