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Performance Comparison of IRIS I and IRIS II Variable Aperture Collimators On A Cyberknife System

W Jiang

W Jiang1*, S Sharma1 , C Lin2 , Y Feng1,2 , (1)Department of Radiation Oncology, East Carolina University, Greenville, NC 27834 (2)Department of Physics, East Carolina University, Greenville, NC 27858


SU-I-GPD-T-531 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To compare radiation beam performance of IRIS I and IRIS II variable collimators mounted on a Cyberknife system using film measurements and water tank scanning with diode detectors.

Methods: Gafchromic EBT2 films were utilized for field size and center measurements for 8 apertures at 800-mm SAD. 16-bit film images were scanned at 300 dpi in an Epson V700 scanner. An 11x11 pixel median filter were used to remove noise from raw images. Beam profiles and output factors (OF) were measured using PTW60012 diode detector for 12 designed aperture(DA) sizes from 5mm to 60mm in a water tank. Beam profiles were measured at fixed SAD of 800mm and multiple depths. Profiles in different angles (0°,15°,90°and 105°) were scanned and an averaged profile was generated for each depth. The quality of profile was evaluated by FWHM. OFs were measured at 15mm depth and 3 SADs (650,800 and 1000mm). The OFs from 650 and 1000mm were normalized to 800mm using inverse-square law.

Results: Average difference for 8 radiation field sizes (AVG_F) between IRIS I and DA was -0.586mm, and AVG_F between IRIS II and DA was -0.086mm. Average off-center position(AVG_OC) of IRIS I was 0.1mm, and AVG_OC between IRIS II and DA was -0.04mm. The measurements from water scanning with diode for 12 sizes shows that the average difference of FWHM(AVG_W) between IRIS I and DA was 0.617mm, and AVG_W between IRIS II and DA was 0.043mm. Average percentage difference of OF(AVG_OF) between IRIS I and fixed collimator was -2.92%, and AVG_OF between IRIS II and fixed collimator was -2.76%.

Conclusion: Radiation field sizes from IRIS II was closer to the designed ones. Off-center distance of IRIS II was highly improved as compared with IRIS I. OFs measured from IRIS II is closer to the ones measured from fixed collimators.

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