Program Information
Verification of the BJR-25 Method of KQ Determination for CyberKnife Absolute Dosimetry
J Gersh1,2*, B Willett3 , (1) Gibbs Cancer Center & Research Institute - Pelham, Greer , SC, (2) Spectrum Medical Physics, LLC - Greenville, SC, (3) Accuray, Inc., Sunnyvale, CA
Presentations
SU-E-T-804 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: Absolute calibration of the CyberKnife is performed using a 6cm-diameter cone defined at 80cm SAD. Since kQ is defined using PDD values determined using 10x10 cm fields at 100cm SSD, the PDD must be corrected in order to correctly apply the quality conversion factor. The accepted method is based on equivalent field-size conversions of PDD values using BJR25. Using the new InCise MLC system, the CK is capable of generating a rectangular field equivalent to 10x10 cm square field. In this study, a comparison is made between kQ values determined using the traditional BJR25 method and the MLC method introduced herein.
Methods: First, kQ(BJR) is determined: a PDD is acquired using a 6cm circular field at 100cm SSD, its field size converted to an equivalent square, and PDD converted to a 10x10cm field using the appropriate BJR25 table. Maintaining a consistent setup, the collimator is changed, and the MLC method is used. Finally, kQ is determined using PDDs acquired with a 9.71x10.31cm at 100cm SSD. This field is produced by setting the field to a size of 7.77x8.25cm (since it is defined at 80cm SAD). An exact 10x10cm field since field size is relegated to increments of its leaf width (0.25cm). This comparison is made using an Exradin A1SL, IBA CC08, IBA CC13, and an Exradin A19. For each detector and collimator type, the beam injector was adjusted to give 5 different beam qualities; representing a range of clinical systems.
Results: Averaging across all beam qualities, kQ(MLC) differed from kQ(BJR) by less than 0.15%. The difference between the values increased with detector volume.
Conclusion: For CK users with standard cone collimators, the BJR25 method has been verified. For CK users the MLC system, a technique is described to determine kQ.
Funding Support, Disclosures, and Conflict of Interest: Primary author is the President/Owner of Spectrum Medical Physics, LLC, a company which maintains contracts with Siemens Healthcare and Standard Imaging, Inc.
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