engleski

External radiation dosimetry aspects of patients receiving radioiodine for thyroid cancer

Administration of radioactive iodine (I-131) following surgical thyroidectomy is widely accepted procedure in patients with thyroid cancer, in order to ablate residual thyroid tissue and local or distant functional metastases. As such patients are potentially hazardous to hospital stuff and members of the family and public, the recommended radiation precautions usually take into account retained bodily activity (<1110 MBq), instantaneous absorbed dose rate (<50ľGy/h) or effective absorbed dose (1-5 mSv). The purpose of this work was to consider thyroid cancer patients as an external sources of radiation. Thirty seven patients were investigated after ablation I–131 activity was administered because of residual thyroid tissue after surgery. Collimated whole–body counter was used to determine whole body retention by means of geometric mean of net full energy spectrum supine and prone counts, in order to minimize effects of activity distribution, body size and statistical counting uncertainties. Absorbed dose rates were measured at one meter from patient's anterior mid trunk by calibrated beta–gamma survey meter. The initial measurement (2 h) with no excretion meanwhile served as the baseline standard for subsequent measurements. A correction for deadtime was made if it was necessary. The fitting of the curve of activity and dose rate against time was performed for each patient using typically six points data set and method of nonlinear regression. Biexponential effective radiation decay was found; whole body activity as A(t)/A0=0.86exp(–0.0858t)+0.14exp(–0.0238t) and absorbed dose rate as DR(t)/DR0=0.90exp(–0.0853t)+0.10exp(–0.0226t), where A0=2471ą830 MBq and DR0=137.8ą48.3 ľSv/h were initial meanąSD whole body activity and absorbed dose rate, and numerical parameters were mean relative intensity and effective decay constant for each exponential component. Mean effective half–times of short and long lived components were 9.1ą2.5 and 47.1ą30.9 h for retained whole body activity, and 9.4ą3.0 and 48.4ą32.2 h for absorbed dose rate. Significant correlation between retained whole body activity and absorbed dose rate was found (r=0.99, p<0.01). Mean absorbed dose rate per unit retained activity was 0.055ą0.022 ľSv/h/MBq during the first 168 hours after radioiodine administration. In regard to the radiation safety regulation and rapid effective radiation decay observed, thyroid cancer patients receiving ablation activity of up to 5 GBq I–131 may generally be discharged from the hospital, travel home and be on work on the first day after the treatment.

thyroid; cancer; radioiodine-131; whole-body counting; dosimetry; radiation protection

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