Influence of ion chamber response on in‐air profile measurements in megavoltage photon beams

Tonkopi, Elena; McEwen, M; Walters, B. R. B.; Kawrakow, I.

doi:10.1118/1.2011090

Cited by 33 publications

(46 citation statements)

References 31 publications

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“…A similar approach has been used successfully in previous investigations. [28][29][30] The HVL comparison between measured HVL and calculated transmission data for the XVI unit at a nominal energy of 120 kV is depicted in Fig. 10͑b͒.…”

Section: Iiia3 Half Value Layer Calculationsmentioning

confidence: 99%

Monte Carlo simulation of an x‐ray volume imaging cone beam CT unit

et al. 2008

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In this work the authors characterized the radiation field produced by a kilovolt cone beam computed tomography (CBCT) unit integrated in the Elekta Synergy linear accelerator. The x-ray volume imaging (XVI) radiation unit was modeled in detail using the BEAMNRC Monte Carlo (MC) code system. The simulations of eight collimator cassettes and the neutral filter F0 were successfully carried out. MC calculations from the EGSNRC code DOSXYZNRC were benchmarked against measurements in water. A large set of depth dose and lateral profiles was acquired with the ionization chamber in water, with the x-ray tube in a stationary position, and with the beam energy set to 120 kV. Measurements for all the available collimator cassettes were compared with calculations, showing very good agreement (< 2% in most cases). Furthermore, half value layer measurements were carried out and used to validate the MC model of the XVI unit. In this case dose calculations were performed with the EGSNRC code cavity and these showed excellent agreement. In this manuscript the authors also report on the optimization work of the relevant parameters that influenced the development of the MC model. The dosimetric part of this work was very useful in characterizing the XVI radiation output for the energy of interest. The detailed simulation part of the work is the first step toward an accurate MC based assessment of the dose delivered to patients during routine CBCT scans for image and dose guided radiotherapy.

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Section: Iiia3 Half Value Layer Calculationsmentioning

confidence: 99%

Monte Carlo simulation of an x‐ray volume imaging cone beam CT unit

et al. 2008

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“…The Varian head model from our previously published description has been updated to include electron contributions from various treatment head shielding components, as indicated by other authors . The Elekta Precise head model has been validated and documented in previous publications …”

Section: Monte Carlo Modelmentioning

confidence: 99%

“…The ionization chamber simulations used the egs_chamber user code, a egspp C++ module specifically designed for accurate IC simulations. Full MC IC modeling has been successful in characterizing a wide variety of chamber response characteristics . The Exradin A18 cylindrical IC serves as the reference for the parameter variations.…”

Section: Monte Carlo Modelmentioning

confidence: 99%

Quantitative ionization chamber alignment to a water surface: Theory and simulation

2017

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The depth-ionization gradient has a discontinuity and is maximized when the outer-radius of a submerged IC coincides with the water surface. This feature can be used to auto-align ICs to the water surface at the time of scanning and/or be applied retrospectively to scan data to quantify absolute IC depth. Utilization of the gCAP should yield accurate and reproducible depth calibration for clinical depth-ionization measurements between setups and between users.

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“…In addition, in-air profiles measured with ionization chambers using a low atomic number buildup cap as well as beam profiles measured at shallow depths in water were found to be sensitive indicators of the linac focal spot size and shape. 3,4 In Monte Carlo simulations of PDD distributions and beam profiles, a FWHM of 1 -1.5 mm is often used as an initial focal spot size estimate and subsequently modified to match the calculated beam profiles with measured ones. 5,6 Thus, the measured beam profiles can serve as practical means for determination of the effective focal spot diameter for use in Monte Carlo simulations, provided that the beam profile data are measured accurately.…”

Section: Introductionmentioning

confidence: 99%

Influence of focal spot on characteristics of very small diameter radiosurgical beams

et al. 2008

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Percentage depth dose (PDD) distributions and beam profiles of very small diameter (1.5-5 mm) megavoltage radiosurgical beams calculated with Monte Carlo (MC) technique critically depend on the diameter of the circular focal spot used in the simulation: The smaller is the field diameter, the larger is the effect. Thus, in simulations of radiosurgical fields that have diameters of the order of the focal spot size, an accurate focal spot geometry should be used. We used a simplified moving slit technique in conjunction with a diode detector for evaluation of the focal spot size and shape of a megavoltage 6 MV linac as well as for determination of the equivalent focal spot diameter of the linac for use in MC simulations. The measured total diode signal contains three components: A direct focal spot signal, a background signal, and an extra-focal radiation signal. A single profile scan of the focal spot signal is Gaussian like in shape, and its full width at half maximum is used to define the focal spot dimension for this scan. The focal spot of our 6 MV linac is approximated with a Gaussian circle, and when the geometry of the effective focal spot circle is used in MC simulations, the agreement between MC-calculated and measured PDD distributions as well as beam profiles is good even for radiosurgical fields as small as 1.5 mm in diameter. Our results also confirm that matching the penumbral areas of accurately measured large-field beam profiles to the same areas of the MC-calculated beam profiles reliably leads to a realistic effective focal spot size for use in MC simulations of very small diameter beams.

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Influence of ion chamber response on in‐air profile measurements in megavoltage photon beams

Cited by 33 publications

References 31 publications

Monte Carlo simulation of an x‐ray volume imaging cone beam CT unit

Monte Carlo simulation of an x‐ray volume imaging cone beam CT unit

Quantitative ionization chamber alignment to a water surface: Theory and simulation

Influence of focal spot on characteristics of very small diameter radiosurgical beams

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