External dose rates in radioiodine treatment of benign goitre: Estimation versus direct measurement

Abstract

Objective. According to European recommendations, the external dose rate (ED) in radioiodine‐treated goitre patients can be determined by estimating from calculation of the residual activity (RA) in the patient based on radioiodine uptake measurements or by measuring ED directly. In the European guidelines, “Radiation Protection 97”, it is assumed that an RA of 600 MBq ¹³¹I causes an ED of 30 μSv/h at a distance of 1 m. This implies a slope of 0.05 µSv/h/MBq for the ratio ED/RA relationship, but, theoretically, this ratio is higher, at 0.07, a difference that is due to measurement in air versus in a scattering medium. We sought to investigate what the true ratio might be. Material and methods. Sixty‐six patients scheduled for radioiodine treatment of benign goitre (mean size 102 mL, range 20–440), who received 131I orally (mean 984 MBq, range 173–3700) were examined. After 24 h and 96 h iodine uptake percentage we examined 7269 patients scheduled for radioiodine treatment of benign goitre (mean size 1042 mL, range 20–440) who received ¹³¹I orally (mean 101,100 MBq, range 180–3700). After 24 h and 96 h, the iodine uptake was determined, RA calculated and ED measured using a hand‐held dosimeter. Results. At 24 and 96 h, we observed a slope ratio of 0.103 µSv/h/MBq (95 % CI: 0.09564–0.111) and 0.101 µSv/h/MBq (95 % CI: 0.0915–0.11107), respectively, for the ED/RA relationship. None of the confidence intervals included the value 0.05 µSv/h/MBq, reflecting that the observed slopes differed significantly from the expected slope (p<0.001). Consequently, an RA of 600 MBq typically causes an ED of 60 and not 30 μSv/h, and therefore dose rates based on radioiodine uptake measurements and established assumptions were only about half as high as the directly measured values. We noticed that with an RA roughly below 450 MBq, the anticipated slope of 0.05 µSv/h/MBq is within the prediction interval of our claimed ratio, therefore we cannot rebut the anticipated slope for lower doses. Conclusions. Dose‐rate estimates based on radioiodine uptake measurements and established assumptions were only about half as high as the directly measured values in patients receiving doses higher than the widely accepted limits for outpatient treatment. This finding may have substantial implications for us, in that it makes a considerable difference whether the radiation precautions are taken to limit doses to the patient's surroundings and for deciding if a patient may or may not be regarded as an outpatient, as well as for the safe discharge of an inpatient from hospital.

• Dose–response relationship
• goitre
• iodine isotopes
• radiation
• radiometry radiotherapy