Measurement of Surface Absorbed Dose Rate from Liberty Vision LV-90 Y-90 Ophthalmic Brachytherapy Applicator Using Extrapolation Ionization Chamber
Abstract
Purpose
The Liberty Vision Y-90 ophthalmic applicator (LV-90) is an FDA approved HDR source for episcleral brachytherapy to treat ocular tumors and benign growths. It is an attractive option for patients because it does not require episcleral sutures, muscle relocation, outpatient dwell time, or second surgery. Accurate dose determination is critical for clinical efficacy; this requires precise source calibration and the identification of reliable transfer instruments. The goal of this work is to establish accurate dose to water calibration methods using extrapolation ionization chambers.
Methods
Using an extrapolation chamber, the primary physical standard for beta-emitting brachytherapy devices, the absorbed dose to water at the surface of the source was able to be determined by measuring the ionization current at varying airgaps and extrapolating to an airgap of zero. The surface dose rate was then calculated using a combination of measured and EGSnrc-simulated correction factors. Additionally, the source was measured using a Standard Imaging A20 ionization chamber with a custom jig to evaluate its use as a clinical transfer instrument.
Results
Surface dose rates [Gy/s] were determined for several LV-90 sources using the extrapolation chamber. By correlating these dose rates with the ionization current measured in the A20 chamber, a calibration coefficient [Gy/nA] was established for the transfer instrument.
Conclusion
Source calibration for the Liberty Vision LV-90 applicator was successfully performed using an extrapolation chamber and EGSnrc simulations. The establishment of a calibration coefficient for the A20 chamber provides a reliable clinical transfer standard. This work enhances the accuracy of dose delivery for Y-90 ophthalmic treatments, ultimately improving patient outcomes.