Abstract
2230
Objectives To estimate an Y-90 microspheres liver-directed brachytherapy external dose rate constant and thus maximum administered activity for release without radiation precaution instructions (≤1 mSv total effective dose equivalent, TEDE, to the most exposed person), based on patient measurements.
Methods Post-treatment dose rates (Dt, μSv/h) from twenty patients were measured anterior to the liver, at the two index distances (d) specified for radioactive patient release external dose calculations (1 m and 0.3 m). Dose rate was divided by net administered activity (A, MBq) decay-corrected to the time of measurement, to obtain a bremsstrahlung dose rate constant (Γ, μSv/MBq-h). Linear correlation of μSv/h vs. A (GBq), statistical analysis of Γ, and a power function fit of Γ vs. d, were performed.
Results Administered activities ranged from 810 MBq to 7960 MBq. Dose rate correlated well with activity at both 1 m (Dt=4.76E-1×GBq, r=0.88) and 0.3 m (Dt=2.64×GBq, r=0.76). The calculated Γ (μSv/MBq-h) varied substantially (moreso at 0.3 m). The mean±σ (range; coefficient of variation) was 4.96E-4±1.43E-4 (2.06E-4 to 7.19E-4; 29%) at 1 m, and 2.69E-3±1.18E-3 (0.90E-3 to 4.91E-3; 44%) at 0.3 m. A least-squares power function fit of Γ vs. d indicated an increase in Γ as distance decreases from 1 m proportional to 1/d1.34 rather than the conventionally assumed 1/d2. The 0.3 m-to-1 m Γ ratio was 5.59±2.19 (1.73 - 10.33; 39%). If the increase followed the inverse-square law, the expected mean ratio would be 11.11.
Conclusions Assuming a very conservative Γ at 1 m of 7.82E-04 (mean+2σ, < 2.5% probability it would be greater than that value), Y-90 microspheres liver-directed brachytherapy patients can be released with neither an external dose rate measurement nor specific radiation precaution instructions at up to 55 GBq administered activity