Analysis of Y-90 Dose Delivery Uncertainties
Abstract
Purpose
In the process to calculate the delivered Y-90 (Yttrium-90) dosage, there is measurement uncertainty associated with each step. All these uncertainties can be calculated with the knowledge of the dose calibrator and survey meter accuracy and the estimated operator’s reading uncertainties. The purpose of this project is to analyze the contribution of all the uncertainty sources to calculate the final delivery accuracy.
Methods
The first step in Y90 delivery is to assay the activity of the prescribed unit dose. The assay uncertainty is determined by the dose calibrator accuracy. The second step is to measure the exposure rate of the unit dose before delivery. The third step is to measure the exposure rate of the waste after delivery. The exposure rate ratio of the waste to the unit dose and the assayed activity are used to calculated the final dose delivered. We derived the formula to calculate the uncertainties at each step. The calculation was applied to analyze the uncertainties of the Y-90 delivery for our institute’s most recent 100 cases.
Results
The assay and the exposure rate ratio percentage uncertainties were proved to be independent to each other. There were 17 cases with initial exposure rate below 1mR/h. They all had exposure rate ratio uncertainties equal or above 5% (the highest being 14%). All other cases with initial exposure rate readings above 1mR/h had uncertainties below 5%. 43 high dose cases had uncertainties below 1%. The final dose percentage uncertainty would be the sum of the above percentage and the dose calibrator accuracy.
Conclusion
For a successful delivery, the waste exposure rate was usually close to the background. The exposure rate ratio would be higher for low dose cases whose initial exposure rate was low. This was the major reason for high delivery uncertainties for low dose cases.