Measurement of the radiation emitted by nuclear medicine patients for radiation shielding calculations

Objectives Regulatory bodies can require Nuclear Medicine facilities to provide calculations that show that radiation dose to staff and members of the public from exposure to patients administered radiopharmaceuticals are kept below specified levels. Conventional calculation methods often treat patients as "point" sources and do not take into account self-attenuation and biologic excretion of activity over time. These calculation methods can result in money spent on structural shielding that might not be required if more accurate data regarding the radiation emitted by patients were available.The goals of this project were (1) to measure the radiation dose emitted by patients administered typical radiotracers as a function of time post-administration, for a representative sample of patients and (2) to develop models of the radiation dose emitted over time as functions of radioisotope, pharmaceutical, and patient size.

Methods Marks were made on the floor 1 m to 4 m from areas where patients were administered radiopharmaceuticals and from the gamma cameras where they were scanned. The activity, time of administration, radiopharmaceutical and patient weight were recorded for a series of patients. Measurements of dose rate versus time and distance were recorded using a calibrated survey meter.

Results Biological clearance depended on the radiopharmaceutical administered. However, there was variation in clearance of the same tracer in different patients. Self-attenuation correlated with body weight. Measured dose rates were significantly lower than those calculated using a point source model which did not take into account self-attenuation and biologic clearance.

Conclusions When measured data is used instead of a point source model, lower estimates of radiation dose in areas near Nuclear Medicine patients result. Depending on local regulations, this could lead to significant savings by eliminating the need to add structural shielding