2136 - Enhancing Gamma Knife Radiosurgery Precision with Plastic Scintillator Detector-Based Quality Assurance

Purpose/Objective(s): Gamma Knife stereotactic radiosurgery (GK-SRS) demands exceptional precision, often requiring multiple measurement devices to ensure the necessary level of accuracy for quality assurance. Plastic scintillator detectors (PSDs) offer distinct advantages for small-field dosimetry, including a minimal active volume and stability comparable to ionization chambers, making them well-suited for high-accuracy measurements in GK-SRS. In this study, we demonstrate the use of a PSD for an end-to-end (E2E) quality assurance test of a GK-SRS system using an anthropomorphic SRS head phantom, implementing a customized in-house procedure.

Materials/Methods: An ion chamber was cross-calibrated on a GK dosimetry phantom using the nominal dose rate from the treatment planning system (TPS) with an 8-mm shot plan. To validate this calibration, measurements were compared against an ADCL-calibrated 0.125 cc semiflex ion chamber. A custom 3D-printed chamber holder was designed in-house for the PSD, with its structural accuracy verified using cone-beam computed tomography (CBCT) imaging and dose measurements. A GK treatment plan was created for the head phantom, consisting of 22 shots with various isocenter positions and collimator settings to deliver 4 Gy at 50% isodose to the target. The PSD was positioned in the central hole of the phantom, and five repeated measurements were performed, each time repositioning the PSD to account for setup variability. A correction factor was hand-derived to adjust for differences in total attenuation coefficient of the head phantom which is comprised of brain and skull equivalent materials vs. a water-only assumption.

Results: The PSD cross-calibration factor derived from the nominal dose rate was 111.5 pC/Gy, showing excellent agreement (-1.1% difference) with the semiflex ion chamber measurement. The five measurements on the head phantom yielded an average charge of 734.5 pC, corresponding to a measured dose of 6.59 Gy. The mean dose to the chamber’s active volume was 7.08 Gy as calculated by the TPS, which employs a tissue-maximum ratio (TMR) 10-based algorithm. The attenuation correction factor was estimated based on an average chamber-to-skin distance of 8.2 cm and a mean CT number of 115.8 HU, and applying this resulted in a -5.13% adjustment in relative dose, yielding a corrected TPS dose of 6.67 Gy. The difference between measured vs. TPS corrected dose was -1.19%, with a standard deviation of 0.17%, demonstrating an excellent agreement in the E2E test.

Conclusion: This study highlights the feasibility of using a PSD for GK SRS machine QA. The PSD demonstrated excellent stability that does not require correction, supporting its potential as a highly reliable alternative to various chambers for absolute dosimetry and other measurement QA tasks in small-field applications.