To measure the output factors (OFs) of the small fields formed by the variable aperture collimator system (iris) of a CyberKnife (CK) robotic radiosurgery system, and determine the correction factors for a microchamber and four diode detectors.Methods:
OF measurements were performed using a PTW PinPoint 31014 microchamber, four diode detectors (PTW-60017, −60012, −60008, and the SunNuclear EDGE detector), TLD-100 microcubes, alanine dosimeters, EBT films, and polymergels for the 5 mm, 7.5 mm, 10 mm, 12.5 mm, and 15 mm iris collimators at 650 mm, 800 mm, and 1000 mm source to detector distance (SDD). The alanine OF measurements were corrected for volume averaging effects using the 3D dose distributions registered in polymergeldosimeters. correction factors for the PinPoint microchamber and the diode dosimeters were calculated through comparison against corresponding polymergel, EBT, alanine, and TLD results.Results:
Experimental OF results are presented for the array of dosimetric systems used. The PinPoint microchamber was found to underestimate small field OFs, and a correction factor ranging from 1.127 ± 0.022 (for the 5 mm iris collimator) to 1.004 ± 0.010 (for the 15 mm iris collimator) was determined at the reference SDD of 800 mm. The PinPoint correction factor was also found to increase with decreasing SDD; values equal to 1.220 ± 0.028 and 1.077 ± 0.016 were obtained for the 5 mm iris collimator at 650 mm and 1000 mm SDD, respectively. On the contrary, diode detectors were found to overestimate small field OFs and a correction factor equal to 0.973 ± 0.006, 0.954 ± 0.006, 0.937 ± 0.007, and 0.964 ± 0.006 was measured for the PTW-60017, −60012, −60008 and the EDGE diode detectors, respectively, for the 5 mm iris collimator at 800 mm SDD. The corresponding correction factors for the 15 mm iris collimator were found equal to 0.997 ± 0.010, 0.994 ± 0.009, 0.988 ± 0.010, and 0.986 ± 0.010, respectively. No correlation of the diode correction factors with SDD was observed.Conclusions:
This work demonstrates an experimental procedure for the determination of the correction factors required to obtain small field OF results of increased accuracy.
The work was partly supported by Accuray Inc. A. Moutsatsos was exclusively supported by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF) - Research Funding Program: Heracleitus II. Investing in knowledge society through the European Social Fund. The authors would also like to acknowledge the contribution of Dr. Francescon for sharing the MC calculated correction factors for the iris small fields and of Mu Young Lee, Accuray Inc., for lending the PTW 60017 diode and the EDGE detector, as well as for the helpful discussions.
II. MATERIALS AND METHODS
II.A. Chamber and diode measurements
II.B. Alanine and TLD measurements
II.C. EBT film measurements
II.D. Polymergeldosimetry measurements
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