Efficiency improvements for ion chamber calculations in high energy photon beams

Wulff, J; Zink, Klemens; Kawrakow, I.

doi:10.1118/1.2874554

Cited by 198 publications

(279 citation statements)

References 33 publications

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“…A series of scoring volumes is created, with the chamber model modified for different, correlated calculations as described above. The series of cavity doses is calculated in single runs using the EGSnrc-based 27 user-code EGSគchamber, 28 in which absorbed doses are scored using correlated sampling, i.e., taking into account the covariance of cavity doses when computing the uncertainty in the absorbed dose ratios at the end of the simulation. In addition to other variance reduction techniques ͑Russian roulette, cross section enhancement͒ used in EGSគchamber, this technique increases the efficiency of the simulation.…”

Section: Iib Monte Carlo Methodsmentioning

confidence: 99%

Ionization chamber gradient effects in nonstandard beam configurations

et al. 2009

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Purpose: For the purpose of nonstandard beam reference dosimetry, the current concept of reporting absorbed dose at a point in water located at a representative position in the chamber volume is investigated in detail. As new nonstandard beam reference dosimetry protocols are under development, an evaluation of the role played by the definition of point of measurement could lead to conceptual improvements prior to establishing measurement procedures. Methods: The present study uses the current definition of reporting absorbed dose to calculate ionization chamber perturbation factors for two cylindrical chamber models ͑Exradin A12 and A14͒ using the Monte Carlo method. The EGSnrc based user-code EGSគchamber is used to calculate chamber dose responses of 14 IMRT beams chosen to cause considerable dose gradients over the chamber volume as previously used by Bouchard and Seuntjens ͓"Ionization chamber-based reference dosimetry of intensity modulated radiation beams," Med. Phys. 31͑9͒, 2454-5465 ͑2004͔͒. Results: The study shows conclusively the relative importance of each physical effect involved in the nonstandard beam correction factors of 14 IMRT beams. Of all correction factors involved in the dosimetry of the beams studied, the gradient perturbation correction factor has the highest magnitude, on average, 11% higher compared to reference conditions for the Exradin A12 chamber and about 5% higher for the Extradin A14 chamber. Other perturbation correction factors ͑i.e., P wall , P stem , and P cel ͒ are, on average, less than 0.8% different from reference conditions for the chambers and beams studied. The current approach of reporting measured absorbed dose at a point in water coinciding with the location of the centroid of the chamber is the main factor responsible for large correction factors in nonstandard beam deliveries ͑e.g., intensity modulated radiation therapy͒ reported in literature. Conclusions: To reduce or eliminate the magnitude of perturbation correction factors in nonstandard beam reference dosimetry, two possible ways to report absorbed dose are suggested: ͑1͒ Reporting average dose to the sensitive volume of the chamber filled with water, combined with removing the reference field implicit gradient effect when measuring output factors, and ͑2͒ reporting average dose to the chamber itself during output factor verifications. The first option could be adopted if clinical beam correction factors are negligible. The second option could simplify quality assurance procedures when correction factors are not negligible and have to be calculated using Monte Carlo simulations.

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Section: Iib Monte Carlo Methodsmentioning

confidence: 99%

Ionization chamber gradient effects in nonstandard beam configurations

et al. 2009

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“…The EGSnrc user-code EGS_CHAMBER 10,11 was used to simulate the dose deposition in water and the dose deposition in the active layer of the film for monoenergetic incident photon energies from 3 keV to 18 MeV. A cylindrical water phantom with a radius of 20 cm and a thickness of 30 cm was modeled.…”

Section: Iib Simulationsmentioning

confidence: 99%

Monte Carlo calculated absorbed‐dose energy dependence of EBT and EBT2 film

2010

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Purpose:The absorbed-dose energy dependence of GAFCHROMIC EBT and EBT2 film irradiated in photon beams is studied to understand the shape of the curves and the physics behind them. Methods: The absorbed-dose energy dependence is calculated using the EGSnrc-based EGS_cham-ber and DOSRZnrc codes by calculating the ratio of dose to water to dose to active film layers at photon energies ranging from 3 keV to 18 MeV. These data are compared to the mass energy absorption coefficient ratios and the restricted stopping power ratios of water to active film materials as well as to previous experimental results. Results: In the photon energy range of 100 keV to 18 MeV the absorbed-dose energy dependence is found to be energy independent within Ϯ0.6%. However, below 100 keV, the absorbed-dose energy dependence of EBT varies by approximately 10% due to changes in mass energy absorption coefficient ratios of water to film materials, as well as an increase in the number of electrons being created and scattered in the central surface layer of the film. Results are found to disagree with previous experimental studies suggesting the possibility of an intrinsic energy dependence at lower photon energies. For EBT2 film the absorbed-dose energy dependence at low photon energies varies by 50% or 10% depending on the manufacturing lot due to changes in the ratio of mass energy absorption coefficients of the active emulsion layers to water. Conclusions: Caution is recommended when using GAFCHROMIC EBT/EBT2 films at photon energies below 100 keV. It is recommended that the effective atomic number of future films be produced as close to that of water and that thicker active layers are advantageous.

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“…The user code egs_chamber [22] was applied for the calculation of the dose Table 2. Characteristic data of the electron sources applied in this study.…”

Section: Methodsmentioning

confidence: 99%

On the Perturbation Correction Factor <i>p<sub>cav</sub></i> of the Markus Parallel-Plate Ion Chamber in Clinical Electron Beams

Voigts-Rhetz¹,

Vorwerk²,

Zink³

2017

IJMPCERO

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Purpose: All present dosimetry protocols recommend well-guarded parallelplate ion chambers for electron dosimetry. For the guard-less Markus chamber, an energy dependent fluence perturbation correction p cav is given. This perturbation correction was experimentally determined by van der Plaetsen by comparison of the read-out of a Markus and a NACP chamber, which was assumed to be "perturbation-free". Aim of the present study is a Monte Carlo based reiteration of this experiment. Methods: Detailed models of four parallel-plate chambers (Roos, Markus, NACP and Advanced Markus) were designed using the Monte Carlo code EGSnrc and placed in a water phantom. For all chambers, the dose to the active volume filled with low density water was calculated for 13 clinical electron spectra (E 0 = 6 -21 MeV) and three energies of an Electra linear accelerator at the depth of maximum and at the reference depth under reference conditions. In all cases, the chamber's reference point was positioned at the depth of measurement. Moreover, the dose to water D W was calculated in a small water voxel positioned at the same depth. Results: The calculated dose ratio D NACP /D Markus , which according to van der Plaetsen reflects the fluence perturbation correction of the Markus chamber, deviates less from unity than the values given by van der Plaetsen, but exhibits similar energy dependence. The same holds for the dose ratios of the other well-guarded chambers. But, in comparison to water, the Markus chamber reveals the smallest overall perturbation correction which is nearly energy independent at both investigated depths. 151all well-guarded chambers, the guard-less Markus chamber reveals the smallest overall perturbation correction and also the smallest energy dependence.

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Efficiency improvements for ion chamber calculations in high energy photon beams

Cited by 198 publications

References 33 publications

Ionization chamber gradient effects in nonstandard beam configurations

Ionization chamber gradient effects in nonstandard beam configurations

Monte Carlo calculated absorbed‐dose energy dependence of EBT and EBT2 film

On the Perturbation Correction Factor <i>p<sub>cav</sub></i> of the Markus Parallel-Plate Ion Chamber in Clinical Electron Beams

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Efficiency improvements for ion chamber calculations in high energy photon beams

Cited by 198 publications

References 33 publications

Ionization chamber gradient effects in nonstandard beam configurations

Ionization chamber gradient effects in nonstandard beam configurations

Monte Carlo calculated absorbed‐dose energy dependence of EBT and EBT2 film

On the Perturbation Correction Factor &lt;i&gt;p&lt;sub&gt;cav&lt;/sub&gt;&lt;/i&gt; of the Markus Parallel-Plate Ion Chamber in Clinical Electron Beams

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On the Perturbation Correction Factor <i>p<sub>cav</sub></i> of the Markus Parallel-Plate Ion Chamber in Clinical Electron Beams