Monte Carlo simulated correction factors for output factor measurement with the CyberKnife system—results for new detectors and correction factor dependence on measurement distance and detector orientation

Abstract

A previous study of the corrections needed for output factor measurements with the CyberKnife system has been extended to include new diode detectors (IBA SFD and Exradin D1V), an air filled microchamber (Exradin CC01) and a scintillation detector (Exradin W1). The dependence of the corrections on detector orientation (detector long axis parallel versus perpendicular to the beam axis) and source to detector distance (SDD) was evaluated for these new detectors and for those in our previous study. The new diodes are found to over-respond at the smallest (5 mm) field size by 2.5% (D1V) and 3.3% (SFD) at 800 mm SDD, while the CC01 under-responds by 7.4% at the same distance when oriented parallel to the beam. Corrections for all detectors tend to unity as field size increases. The W1 corrections are <0.5% at all field sizes. Microchamber correction factors increase substantially if the detector is oriented perpendicular to the beam (by up to 23% for the PTW 31014). Corrections also vary with SDD, with the largest variations seen for microchambers in the perpendicular orientation (up to 13% change at 650 mm SDD versus 800 mm) and smallest for diodes (~1% change at 650 mm versus 800 mm). The smallest and most stable corrections are found for diodes, liquid filled microchambers and scintillation detectors, therefore these should be preferred for small field output factor measurements. If air filled microchambers are used, then the parallel orientation should be preferred to the perpendicular, and care should be taken to use corrections appropriate to the measurement SDD.

Absorbed dose distribution in liquid water for a CyberKnife VSI using radiochromic EBT3 film

“…! "# , discrepancy up 16% is obtained between the EBT3 film and MC calculation data reported in ref [5]. Comparing our data for EBT3 film with that reported and displayed in Table 1, within measurement uncertainties, there exists quite good agreement for all the field sizes, except for the 7.5 mm and 5 mm diameters where variations of 11% and 16 % are obtained, respectively.…”

“…"# , as defined through equation 1, should only depend on the field size but not if it is evaluated through MC simulation or experimentally. Nevertheless, the data reported in the literature for several detectors [5,8,15] and shown in Table 1 indicates no consensus on the ! !…”

“…Consequently, no consensus data has been provided so far. In the particular case of the CyberKnife system composed of a linear accelerator (Linac) with circular small fields mounted on a robotic arm [4], data based on Monte Carlo simulation as well as on experiments have been published for 1-dimensional (1D) dosimeters like diodes and small ionization chambers [3,5]. Whereas for 2D dosimeters, data for dose profile in a 60 mm diameter field measured with EBT2 has been reported [6] while S sf , PDD and OAR measured with EBT have been published for 6 CyberKnife system fields [7].…”

Small field dosimetry: What have we learnt?

“…Francescon et al (33,35) recently published two studies for the CyberKnife system, on the corrections needed for various detectors for use in small field dosimetry. This study provided an updated set of kΩ values for a series of detectors used for measurements in the CyberKnife system at an SSD of 800 mm.…”

Spectral distribution of particle fluence in small field detectors and its implication on small field dosimetry

“…The output correction factor takes into account all the small‐field effects that have an impact on the output factor, such as partial volume averaging, atomic number, density and detector shielding. The new formalism was followed by a series of scientific papers providing numerical values of the correction factors for several beam quality, field size and detector combinations . Correction factors for most available radiotherapy and radiosurgery techniques (e.g., linear accelerators, CyberKnifes and Gamma Knifes) and modern radiation detectors (e.g., ionization chambers and solid‐state detectors) can be found in the cited papers.…”