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Evaluation of the Planar Ion Chamber and the Cylindrical Diode Arrays for Extracranial Cyberknife Patient Specific QA

M Lin

M Lin1*, I Veltchev1, J Li1, R Price1, J Fan1, L Jin1, S Koren2, C Ma1, (1) Fox Chase Cancer Center, Philadelphia, PA, (2) Beth Israel Comprehensive Cancer Center, New York, NY

MO-F-108-12 Monday 4:30PM - 6:00PM Room: 108

Purpose: The robotic-radiosurgery-system (Cyberknife) has been increasingly employed for extracranial treatments. The recent development of the MLC-equipped -Cyberknife emphasizes this application more, especially for larger treatment volumes. This work is aimed to study the feasibility of the ArcCHECK and the MatriXX systems for CyberKnife patient-specific QA and compare their performance.

Methods: Fiducial markers were implanted in both systems to enable image-based setup. The output factors measured by them were compared with commissioning data. The impact of angular dependency on the Cyberknife measurements and the sensitivity of detecting the translational (1-3mm) and the rotational (1-3 degree) delivery errors were also evaluated for both systems. Single-beam-plans, isocentric-plans and ten extracranial Cyberknife patient plans (PTV 5-136cm³) were measured and compared with calculated QA plans. The plan dose distributions were calculated with Ray-tracing and the Monte Carlo (MC) method, respectively. The gamma-passing-rates with different criteria were evaluated and compared.

Results: With the 0.8x0.8 mm² diodes, the output factors measured with the ArcCHECK compare better with commissioning data. The maximum angular-correction for a given beam is 8.2% for the MatriXX and 2.4% for the ArcCHECK. Both systems demonstrate a comparable sensitivity of detecting the translational targeting errors while the ArcCHECK is more sensitive to the rotational targeting error. MC calculations are necessary for the ArcCHECK calculations since the Ray-tracing-algorithm fails to handle the heterogeneity. For all the studied Cyberknife patient plans, the ArcCHECK system demonstrates 100% (3%/3mm) and >96% (2%/2mm) passing-rates. In contrast, the MatriXX system demonstrates >92% (3%/3mm) and ~80% (2%/2mm) passing-rates and the passing-rates decrease with PTV/cone sizes.

Conclusions: The feasibility of using the ArcCHECK or the MatriXX systems for Cyberknife patient-specific QA has been demonstrated. The gamma-passing-rate is higher for the ArcCHECK system. The custom angular correction is necessary as proven by the improved passing-rate, especially with the MatriXX system.

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