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Correction factors for ionization chamber dosimetry in CyberKnife: machine-specific, plan-class and clinical fields.

Gago-Arias, A*; Antolin, E*; Fayos-Ferrer, F*; Simon, R*; Gonzalez-Castano, D M*; Palmans, H; Sharpe, P; Gomez, F*; Pardo-Montero, J* (2013) Correction factors for ionization chamber dosimetry in CyberKnife: machine-specific, plan-class and clinical fields. Med. Phys., 40 (1). 011721

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Purpose: The aim of this work is the application of the formalism for ionization chamber reference dosimetry of small and non-standard fields [R. Alfonso et al., Med. Phys. 35, 5179-5186 (2008)] to the CyberKnife robotic radiosurgery system. Correction factors for intermediate calibration fields including the machine specific reference field and two plan class specific reference fields have been studied. Additionally, the applicability of the new formalism to clinical dosimetry has been analyzed through the investigation of two clinical treatments.
Methods: PTW31014 and Scanditronix-Wellhofer CC13 ionization chamber measurements were performed for the fields under investigation. Absorbed dose to water was determined through alanine reference dosimetry, and experimental correction factors were calculated from alanine to ionization chamber readings ratios. In addition, Monte Carlo correction factors were calculated
for the intermediate calibration fields and one of the clinical treatments, and compared to the experimental values.
Results: Overall correction factors departing from unity by approximately 2% were obtained both from measurements and simulations, with values below and above unity for the studied intermediate calibration fields and clinical treatments. Monte Carlo simulations yielded correction factors compatible with measurements for the machine specific reference field, although differences
between a 1% and a 2.3% were obtained between measured and calculated correction factors for the composite intermediate calibration fields. Dose distributions inhomogeneities are thought to be responsible for such discrepancies.
Conclusions: Differences encountered between overall correction factors associated to the proposed intermediate calibration fields and the clinical treatments under investigation have revealed that the suitability of intermediate calibration fields for the performance of relative clinical dosimetry is treatment dependent, and thus CyberKnife machine would require a detailed investigation since clinical field factors cannot be simply computed as detector reading ratios.

Item Type: Article
Keywords: Non-standard fields, CyberKnife, Reference Dosimetry, Correction Factor
Subjects: Ionising Radiation
Ionising Radiation > Dosimetry
Identification number/DOI: 10.1118/1.4773047
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/5705

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