On the implementation of a recently proposed dosimetric formalism to a robotic radiosurgery system

Journal Article · Sat May 15 00:00:00 EDT 2010 · Medical Physics

DOI:https://doi.org/10.1118/1.3404289· OSTI ID:22096706

Pantelis, E.; Moutsatsos, A.; Zourari, K.; Kilby, W.; Antypas, C.; Papagiannis, P.; Karaiskos, P.; Georgiou, E.; Sakelliou, L. ^[1]

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1. Medical Physics Laboratory, Medical School, University of Athens, 75 Mikras Asias, 115 27 Athens (Greece) and CyberKnife Center, Iatropolis-Magnitiki Tomografia, 54-56 Ethnikis-Antistaseos, 152 31, Athens (Greece)

Purpose: The aim of this work is to implement a recently proposed dosimetric formalism for nonstandard fields to the calibration and small field output factor measurement of a robotic stereotactic radiosurgery system. Methods: Reference dosimetry measurements were performed in the nonstandard, 60 mm diameter machine specific reference (msr) field using a Farmer ion chamber, five other cylindrical chambers with cavity lengths ranging from 16.25 down to 2.7 mm, and alanine dosimeters. Output factor measurements were performed for the 5, 7.5, 10, and 15 mm field sizes using microchambers, diode detectors, alanine dosimeters, TLD microcubes, and EBT Gafchromic films. Measurement correction factors as described in the proposed formalism were calculated for the ion chamber and diode detector output factor measurements based on published Monte Carlo data. Corresponding volume averaging correction factors were calculated for the alanine output factor measurements using 3D dose distributions, measured with polymer gel dosimeters. Results: Farmer chamber and alanine reference dosimetry results were found in close agreement, yielding a correction factor of k{sub Q{sub m{sub s{sub r,Q}{sup f{sub m}{sub s}{sub r},f{sub r}{sub e}{sub f}}}}}=0.999{+-}0.016 for the chamber readings. These results were also found to be in agreement within experimental uncertainties with corresponding results obtained using the shorter cavity length ionization chambers. The mean measured dose values of the latter, however, were found to be consistently greater than that of the Farmer chamber. This finding, combined with an observed inverse relationship between the mean measured dose and chamber cavity length that follows the trend predicted by theoretical volume averaging calculations in the msr field, implies that the Farmer k{sub Q{sub m{sub s{sub r,Q}{sup f{sub m}{sub s}{sub r},f{sub r}{sub e}{sub f}}}}} correction is greater than unity. Regarding the output factor results, deviations as large as 33% were observed between the different dosimeters used. These deviations were substantially decreased when appropriate correction factors were applied to the measured microchamber, diode, and alanine values. After correction, all diode and microchamber measured output factors agreed within 1.6% with the corresponding alanine measurements, and within 3.1% with the TLD measurements. The weighted mean output factors were 0.681{+-}0.001, 0.824{+-}0.001, 0.875{+-}0.001, and 0.954{+-}0.001 for the 5, 7.5, 10, and 15 mm beams, respectively. Conclusions: The comparison of Farmer chamber measurements versus alanine reference dosimetry validates the use of the former for dosimetry in the msr field of this treatment delivery system. The corresponding results of this work obtained using chambers with different cavity lengths, combined with previous literature findings, suggest that a k{sub Q{sub m{sub s{sub r,Q}{sup f{sub m}{sub s}{sub r},f{sub r}{sub e}{sub f}}}}} Farmer chamber dose response correction factor of 1.01 may improve calibration measurement accuracy when using the proposed dosimetric formalism. The k{sub Q{sub m{sub s{sub r,Q}{sup f{sub m}{sub s}{sub r},f{sub r}{sub e}{sub f}}}}} correction factor is within 0.5% from unity for ion chambers with cavity lengths less than 10 mm. Substantial improvements in small field output factor measurement accuracy can be obtained when using microchambers and diodes by applying appropriately calculated correction factors to the detector measurements according to the proposed dosimetric formalism, and their routine use is therefore recommended.

• OSTI ID: 22096706
• Journal Information: Medical Physics, Vol. 37, Issue 5; Other Information: (c) 2010 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
• Country of Publication: United States
• Language: English

Related Subjects

61 RADIATION PROTECTION AND DOSIMETRY
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY
62 RADIOLOGY AND NUCLEAR MEDICINE
ACCURACY
ALANINES
CALIBRATION
DOSEMETERS
DOSIMETRY
FILMS
IMPLEMENTATION
IONIZATION CHAMBERS
MONTE CARLO METHOD
RADIATION DOSE DISTRIBUTIONS
RADIATION DOSES
RADIOTHERAPY
SURGERY