Estimation of effective half life of clearance of radioactive Iodine ( ¹³¹ I) in patients treated for hyperthyroidism and carcinoma thyroid

Abstract

BackgroundIn medical applications of radioisotopes, for calculations of whole body doses and radiation safety applications, there is a need to estimate radioactive body burden. Local recommendations in Oman stipulate the need for hospitalization of patients treated for radioactive-iodine (131 I) with activities above 400 MBq.Materials & Methods:A study of body burden estimation from sequentially measured exposure rates from patients treated for carcinoma thyroid and hyperthyroidism was undertaken. A digital auto-ranging beta gamma survey instrument calibrated for measurement of exposure rates is used in this study.Results:The mean measured exposure rates at 1 m in μSv/h immediately after administration and at 24 h intervals are used for estimation of effective half time of clearance of administered activity. For patients with post-operative thyroid carcinoma, the variation of body burden with time post-administration indicated tri-exponential clearance pattern, with T½eff values 14.4 h, 22 h, and 41.3 h. For patients treated for thyrotoxicosis, the body burden showed slow delayed clearance with a T½eff – 111.4 h, and exposure rates did not show appreciable fall off after 48 h.

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

“…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.…”

“…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.…”

Releasing thyroid cancer patients from the hospital based on dose rate measurement after 131I activity administration

“…Surgical resection of the thyroid gland followed by 131 I therapy has long been the standard treatment for differentiated thyroid cancer. The 131 I T eff in total body of the patients with intact thyroid gland is generally estimated as 5.5 d (132 h), but in thyroidectomized patients this value is much shorter (Ravichandran et al, 2010).…”

“…In a similar study by Barrington et al (1996) with 86 patients, a bi-exponential pattern was found and the correspondent values for T eff in the first and second phase were 0.50 (± 0.09) and 4.28 (± 1.55) days, respectively. In the study by Ravichandran et al (2010), the reported values were 14.4 and 22.0 hours, respectively. The results from the present work, i.e.…”