Room: AAPM ePoster Library
The output-factors in small SRS fields decrease with field size and output measurements need to be benchmarked to the treatment planning system as part of the annual QA. This study compares measurements of output-factors and sector-uniformities using the W2-scintillator with results from two routinely used detector types as well as manufacturer suggested values.
The Exradin W2-scintillator was verified for constancy and linearity at the Gamma-Knife Icon and then used for output measurements with combined and single sectors for the 4mm, 8mm and 16mm collimator settings. The measurements were repeated with the Capintec PR05 ion-chamber and the PTW microdiamond detector that are used for monthly and annual QA, respectively. The detectors were placed in custom inserts in the center of the Elekta solid-water phantom at isocenter (x=y=z=100mm) and connected to the SNC PC-electrometer.
The W2 short-term repeatability was within 0.04% and the dose-response linearity was 1.00. The Gamma-Plan relative output factors are 1, 0.9005 and 0.814 for 16mm, 8mm and 4mm collimators and have been measure to be 1, 0.90 and 0.79 with the W2-scintillator and 1, 0.89 and 0.84 with the microdiamond. The single-sector output factor was in average 1.4% higher for all detector types. The sector uniformity (one standard deviation) was within 0.45% for the microdiamond, 1.31 % for the ion-chamber, and 0.84% for the scintillator, and it was increasing with decreasing field size for the latter two detectors.
Both small field detectors are appropriate and within the 3% tolerance for output measurements at the Gamma Knife Icon at 16mm, 8mm and 4mm when corrections are applied. The W2-scintillator, however, showed larger fluctuations than the microdiamond especially for the smallest collimator size.