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On the output factor measurements of the CyberKnife iris collimator small fields: Experimental determination of the k Q clin , Q msr f clin , f msr correction factors for microchamber and diode detectors
Author(s) -
Pantelis E.,
Moutsatsos A.,
Zourari K.,
Petrokokkinos L.,
Sakelliou L.,
Kilby W.,
Antypas C.,
Papagiannis P.,
Karaiskos P.,
Georgiou E.,
Seimenis I.
Publication year - 2012
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4736810
Subject(s) - collimator , dosimeter , optics , materials science , diode , dosimetry , thermoluminescent dosimeter , detector , physics , nuclear medicine , radiation , optoelectronics , medicine
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 k Q clin , Q msrf clin , f msrcorrection 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. k Q clin , Q msrf clin , f msrcorrection 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 k Q clin , Q msrf clin , f msrcorrection 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 k Q clin , Q msrf clin , f msrcorrection factor was also found to increase with decreasing SDD; k Q clin , Q msrf clin , f msrvalues 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 k Q clin , Q msrf clin , f msrcorrection factors with SDD was observed. Conclusions: This work demonstrates an experimental procedure for the determination of the k Q clin , Q msrf clin , f msrcorrection factors required to obtain small field OF results of increased accuracy.

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