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Development of a High-Resolution EPID Based Dosimetry Strategy for Radiosurgery QA

B Han

B Han*, A Ding , L Xing , L Wang , Stanford University, Stanford, CA


TU-F-BRE-4 Tuesday 4:30PM - 6:00PM Room: Ballroom E

Purpose: To systematically investigate a high spatial-resolution (0.2mm) electronic portal imaging device (EPID) for CyberKnife (CK) based radiosurgery system quality assurance (QA).

Methods: An EPID-based dosimetric measurement technique is applied to CK output measurement and field size verification. A Monte Carlo (MC) simulated pixel-to-pixel EPID response specific to CK is used to convert a raw EPID-measured image of a radiosurgery field into water-based dose distribution. The output factors are measured using EPID for radiosurgery fields formed by fixed and variable aperture (Iris) cones. Circular fields of 5, 7.5, 10, 15, 30 and 60mm diameters are measured and compared with diode measurements. The equivalent diameters are determined by analyzing the area received dose greater than half maximum.

Results: For both fixed and Iris cones, the EPID measured output factors of circular fields of 5mm to 60mm diameters are in good agreement with the radiosurgery diode measurements. The mean output differences are 1.0% and 1.5% for fixed and Iris cone respectively. The max differences are 2.2% for the 15mm fixed cone, and 1.8% for the 10mm Iris field. The equivalent diameters derived from the EPID measurements are in good agreement comparing to the water scan result with mean differences of 0.21±0.09mm and 0.02±0.22mm for fixed and Iris cone, respectively. The high detector density EPID is able to measure the whole radiation field and identify the field edge and center. Therefore, there is no need to align the detector center perfectly at field center and the setup time is greatly reduced for QA.

Conclusion: The high spatial-resolution EPID is proved to be an accurate and efficient dosimetric tool for radiosurgery QA and especially useful in Cyberknife QA for variable aperture collimators.

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