title: Determination of kQ using MLC-collimated rectangular fields for absolute dosimetry of the CyberKnife.
abstract: Traditional CyberKnife (CK) calibration uses TG-51, which requires kQ to be defined using the standard reference condition of 100 cm SSD in a 10 cm × 10 cm field. Since the CK is calibrated using a 6 cm fixed-aperture collimating cone at 80 cm SAD, the BJR-25 method is commonly used to relate circular-field PDDs to square-field PDDs for kQ determination. Using the InCise MLC system, the CK is able to deliver rectangular fields, allowing a more direct measurement of %dd(10 cm) using conventional reference conditions. We define the PDD correction factor (CPDD) as the ratio of %dd(10 cm) measured using CK reference conditions to that measured using standard TG-51 reference conditions. Using four ionization chambers (A1SL, CC08, CC13, and A19), %dd(10 cm) is measured using a 6 cm fixed cone at 80 cm SSD and at 100 cm SSD using an effective 10 cm × 10 cm MLC-collimated field. These values are used to calculate CPDD, while the latter is used to directly calculate a kQ value. This direct kQ value is then compared to values determined using the BJR-25 method. Using the MLC system, this study demonstrates conversion between the %dd(10 cm) measured using CyberKnife reference conditions and TG-51 reference conditions. These values provide the means for derivation of a kQ curve as a function of direct measurements of %dd(10 cm) using a 6 cm fixed-aperture collimating cone at 80 cm SSD.
authors.affiliation: Gibbs Cancer Center and Research Institute and Spectrum Medical Physics, LLC. jgersh@gibbscc.org.
authors.forename: Jacob A
authors.initials: JA
authors.lastname: Gersh
authors.forename: Benjamin
authors.initials: B
authors.lastname: Willett
mesh_terms: D000465:Algorithms; D002138:Calibration; D006801:Humans; D009010:Monte Carlo Method; D011874:Radiometry / Q000295:instrumentation; D016634:Radiosurgery / Q000379:methods* / Q000592:standards / Q000706:statistics & numerical data; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods* / Q000592:standards / Q000706:statistics & numerical data