Technical Note: Evaluation of plastic scintillator detector for small field stereotactic patient-specific quality assurance

Purpose To evaluate the performance of a commercial plastic scintillator detector (PSD) for small-field stereotactic patient-specific quality assurance (QA) measurements using flattening-filter-free beam. Methods A total of ten spherical targets (volume range: [0.03cc-2cc]) were planned with two techniques: (1) Dynamic Conformal Arc (DCA-10 plans), and (2) Volumetric Modulated Arc Therapy (VMAT-10 plans). All plans were generated using Varian Eclipse treatment planning system, and AcurosXB v.13 algorithm in 1.0 mm grid size. Additionally, 14 previously-treated cranial and spine SRS plans were evaluated (6 DCA, 8 VMAT, volume range: [0.04cc-119.02cc]). Plan modulation was quantified via two metrics: MU per prescription dose (MU/Rx) and Average Leaf Pair Opening (ALPO). QA was performed on the Varian Edge linear accelerator equipped with HDMLC. Three detectors were used: (1) PinPoint ion chamber (PTW; active volume 0.015cc), (2) Exradin W1 PSD (Standard Imaging; active volume 0.002cc), and (3) Gafchromic EBT3 film (Ashland). PinPoint chamber and PSD were positioned perpendicular to beam axis in a Lucy phantom (Standard Imaging); films were placed horizontally capturing the coronal plane. Results PSD, film and PinPoint chamber measured average differences of 1.00±1.54%, 1.30±1.69%, and -0.66±2.36% respectively, compared to AcurosXB dose calculation. As the target volume decreased, PinPoint chamber measured lower doses (maximum -5.07% at 0.07cc target), while PSD and film measured higher doses (2.87% and 2.54% at 0.03cc target) than AcurosXB. Film agreed with the benchmark detector PSD by an average difference of 0.31±1.20%, but suffered from larger uncertainty; PinPoint chamber underestimated dose by more than 4% for targets smaller than 0.2cc. Taking PSD as the measurement standard, DCA plans achieved good QA results across all volumes studied, with an average difference of -0.07±0.89%; for VMAT plans, PSD measured consistently higher dose (1.95±1.36%) than AcurosXB. Correlation study revealed that plan modulation quantified by both MU/Rx and ALPO correlated significantly with QA results. Conclusion Among all three detectors, PSD demonstrated superior performances in plans with small fields and heavy modulation. High consistency and low uncertainty made PSD a suitable detector for clinical routine SRS QA. PinPoint chamber should be avoided for targets smaller than 0.2cc; film dosimetry can be utilized with careful evaluation of its uncertainty bracket. Compared to PSD measurements, AcurosXB calculation demonstrated high accuracy for non-modulated small fields. The positive correlation between plan modulation and QA discrepancy calls for our attention for clinical SRS plans with high modulation. This article is protected by copyright. All rights reserved.