The determination of calibration and correction factors for dose calibrators on 90y activity measurement

Abstract

Yttrium-90 (90Y) is a high energy beta emitter which has been widely used in nuclear medicine for therapeutic purposes. 90Y activity can be measured by dose calibrator from its consequently produced bremsstrahlung that might alter accuracy of activity measurement. However, the activity especially for treatment is crucially considered. The aims of this study were to investigate the effect of geometry on the 90Y activity measurement and to determine calibration factors and correction factors for the specific conditions of 90Y measurement in the dose calibrators. Four dose calibrators from two institutes, Ramathibodi Hospital and Thailand Institute of Nuclear Technology (TINT), were investigated. The commercial 90Y from Polatom was used as the reference of 90Y. Firstly, the activity of 90Y was measured from the shipment vial using the individual calibration factors for each dose calibrator. The calibration factors were adjusted when the measured activity was different from the actual activity and then used in the following experiments. Consequently, the geometry effects were investigated in a 3-mL plastic syringe and a 10-mL glass vial. Approximately 3mCi of the reference 90Y was transferred into the containers and then the final volume was adjusted to 0.5, 1.0, 1.5, 2.0 and 2.5mL to obtain the constant activity of 90Y solution with the varied volumes. The activities of 90Y were measured with their calibration factors. Then calibration factor was adjusted when the measured activity was unequal to the actual activity. The percentages of error between the measured activities and actual activities for each specific condition were calculated. Additionally, the correction factors were calculated based on the actual activity and the measured activity of 90Y from the measurement experiment. The percentage error results showed that 90Y activity measurements in the shipment container, 3-mL plastic syringe and 10-mL glass vial were mostly inaccurate when the containers used for initial calibrating dose calibrator were different to those of containers used in this experiment. In addition, the results exhibited that the percentage error was higher with the increasing of 90Y volume in the 10-mL glass vial measurements. The differences of the calculated correction factors were shown for the particular geometry. In conclusion, this study demonstrated that geometry certainly affected the 90Y activity measurement with different degrees of inaccuracy among the dose calibrators. Therefore, to obtain accuracy of the 90Y activity measurement with dose calibrator, calibration factor or correction factor should be individually determined and applied to the specific conditions of measurement. Implication of Thesis: The effect of geometry on the 90Y activity measurement in dose calibrators was revealed in this study. The 90Y activity measurements in several types of containers were mostly inaccurate when the containers used for the initial calibrating dose calibrator were different from those of used in the measurement. In this work, calibration factors and correction factors were determined and applied to the specific conditions of measurement to obtain an accuracy of the 90Y activity measurement with the dose calibrator. Therefore, this study should be beneficial for the routine clinical practice of the 90Y treatment to obtain accurate activity measurement of 90Y because currently the patient’ administrated doses should be within ±5% of the prescribed doses according to national recommendation.