On the output factor measurements of the CyberKnife iris collimator small fields: Experimental determination of the correction factors for microchamber and diode detectors

Medical Physics - Tập 39 Số 8 - Trang 4875-4885 - 2012
E. Pantelis1, A. Moutsatsos2, K. Zourari2, L. Petrokokkinos3, L. Sakelliou3, W Kilby4, Christos Antypas5, P. Papagiannis6, P. Karaiskos6, Evangelos Georgiou6, Ioannis Seimenis7
1Medical Physics Laboratory, Medical School, University of Athens, 75 Mikras Asias, 115 27 Athens, Greece and CyberKnife Center, Iatropolis‐MagnitikiTomografia, 54‐56 Ethnikis‐Antistaseos, 152 31 Athens, Greece
2Medical Physics Laboratory, Medical School, University of Athens, 75 Mikras Asias, 115 27 Athen, Greece
3Nuclear and Particle Physics Section, Physics Department, University of Athens, Panepistimioupolis, Ilisia, 157 71 Athens, Greece
4Accuray Incorporated, Sunnyvale, California 94089
51st Department of Radiology, Aretaieion Hospital, Medical School, University of Athens, Vas. Sophias, 115 28 Athens, Greece and CyberKnife Center, Iatropolis-MagnitikiTomografia, 54-56 Ethnikis-Antistaseos, 152 31 Athens, Greece
6Medical Physics Laboratory, Medical School, University of Athens, 75 Mikras Asias, 115 27 Athens (Greece)
7Medical Physics Laboratory, Medical School, Democritus University of Thrace, 2nd Building of Preclinical Section, University Campus, 68100 Alexandroupolis, Greece

Tóm tắt

Purpose: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 polymer gels 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 polymer gel dosimeters. correction factors for the PinPoint microchamber and the diode dosimeters were calculated through comparison against corresponding polymer gel, 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.

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Medical Physics

Tập 39 Số 8

4875-4885

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