SU‐G‐TeP2‐08: 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 using flattening‐filter‐free (FFF) beams. Methods: A total of ten spherical targets (volume range:[0.03cc–2cc]) were planned using Dynamic Conformal Arc(DCA‐10 plans) and Volumetric Modulated Arc Therapy(VMAT‐10 plans) techniques in Eclipse(AAA v.11, 1mm dose calculation grid size). Additionally, 15 previously‐treated cranial and spine SRS plans were evaluated (6 DCA, 9 VMAT, volume range:[0.04cc–119.02cc]). All measurements were acquired using Varian Edge equipped with HDMLC. Three detectors were used: PinPoint ion chamber (PTW;active volume 0.015cc), Exradin W1 PSD (Standard Imaging;active volume 0.002cc), and Gafchromic EBT3 film (Ashland). PinPoint and PSD were positioned perpendicular to beam axis in a Lucy phantom (Standard Imaging). Films were placed at isocenter in solid water. Calibration films were delivered for absolute dose analysis. Results: For large spherical targets(>1.5cc) with DCA, all detectors agreed within 1% of AAA calculations. As target volume decreased, PSD measured higher doses than AAA (maximum difference: 3.3% at 0.03cc target), while PinPoint chamber measured lower doses (maximum difference:‐3.8% at 0.03cc target). Inter‐detector differences between pinpoint and PSD increased with decreasing target size; differences>5% were observed for targets<0.09cc. Similar trends for inter‐detector behavior were observed for clinical plans. For target sizes<0.08cc, PSD measured>5% higher dose than PinPoint chamber (maximum difference: 9.25% at 0.04cc target). Film demonstrated agreement of −0.19±1.47% with PSD for all spherical targets, and agreement within −0.98±2.25% for all 15 clinical targets. Unlike DCA, VMAT plans did not show improved AAA‐to‐detector agreements for large targets. Conclusion: For all targets, the PSD measurements agreed with film within 1.0%, on average. For small volume targets (<0.10cc), PSD agreed with film but measured significantly higher doses (>5%) compared with the pin point ion chamber. The plastic scintillator detector appears to be suitable for accurate measurements of small SRS targets.