Continuous monitoring of radiation emissions from 131I thyroid cancer ablation subjects: development of a novel radiation detector system and measurement of effective retention half-time in 250 subjects

Objectives(s): To report methodology that has been developed to provide real-time monitoring of radiation emissions from subjects treated with radionuclide therapies and summarise the radioiodine retention profiles of 250 subjects treated for differentiated thyroid cancer with¹³¹I. Methods: A small ceiling-mounted radiation detector for continuously monitoring the exposure rate in the radiation isolation rooms has been developed. Measurements were made every minute after administration of 1-6 GBq of ¹³¹I over the one to three days typical inpatient admission. The data are saved in text format and have been fitted with a mono-exponential curve to measure retention half time. Results: The average effective retention half time (t½ (Eff)) for all subjects was 11.9±3.2 hrs (range: 5.0–23.1 hrs; n=250). Over 90% of the subjects had their serum TSH levels increased by injection of recombinant human TSH prior to treatment. Average retention half-time was found to be less in subjects lower than 55 year of age (t½ (Eff)=11.5 hrs) compared to those 55 or older (t½ (Eff)=14.4 hrs) (P=0.0007). Conclusions: Despite the subjects, being free to move around the isolation room during admission and thus changing the source-detector geometry markedly, the system has been able to characterise their retention profiles after radioiodine treatment. These real-time measurements have applications in planning therapy and monitoring the subjects during their admission to the hospital and can be used for “live” updates for all staff as well as providing insights into the fate of radioiodine in the body.