Function Of Off-axis Distance Research Articles

Technical Note: Small field dose correction factors for radiochromic film in lung phantoms.

Radiochromic film has been established as a detector that can be used without the need for perturbation correction factors for small field dosimetry in water. However, perturbation factors in low density media such as lung have yet to be published. This study calculated the factors required to account for the perturbation of radiochromic film when used for small field dosimetry in lung equivalent material. Monte Carlo simulations were used to calculate dose to Gafchromic EBT3 film when placed inside a lung phantom. The beam simulated had a nominal energy of 6MV and the field sizes simulated ranged from 10×10mm2 to 30×30mm2 . The lung density simulated was varied between 0.2 and 0.3g/cm3 . Each simulation was repeated with the film replaced by lung material (the same as the surrounding medium), and the required correction factors for film dosimetry in lung ( ) were calculated by dividing the dose in lung by the dose in film. For field sizes 30×30mm2 and larger, no correction factors were required. At a 20×20mm2 field size, small corrections were required, but were within the approximate accuracy of film dosimetry (~2%). For a 10×10mm2 field size, significant correction factors need to be applied (0.935 for lung density of 0.20g/cm3 to 0.963 for lung density of 0.30g/cm3 ). The values lower than one mean that the film is over-responding. At the "upstream" lung-water interface the correction factors were close to unity; while at the downstream interface the corrections required were marginally smaller to those at the center of lung. One centimeter or more away from the interfaces, the correction factor did not vary as a function distance from the interface (in the beam direction). Away from the central axis (perpendicular to the beam direction), the correction factors increased slightly (away from unity) as a function of off-axis distance, before abruptly changing direction at the penumbra, with the film actually under-responding by ~10% outside the field edges. Accurate dosimetry of very small fields (15×15mm2 or smaller) using radiochromic film requires correction factors for the perturbation of the film on the surrounding lung material. This correction factor was as high as 6.5% for a 10×10mm2 field size and a density of 0.2g/cm3 . This will increase if either the density or the field size decrease further. This correction factor does not vary as a function of depth in lung once charged particle equilibrium is established.

Analysis of Secondary Photons Emergent from Combined Material Slab as a Function of Slab Thickness

Material science is very important for developing the linear accelerator. Determination and understanding of material behavior face to X-rays is a basic study for photon beam modifiers improvements. In this study, the 6 MV photon beams produced by Varian Clinac 2100 was modelled by Monte Carlo simulation using BEAMnrc code and thereafter the flattening filter was replaced by a slab of aluminum and copper separately and by slab of both materials combined together with different thickness of 2.5, 5, 7.5, and 10 mm. The purpose of this study is to investigate the scattered photons with thickness of combined material slab as a function of off-axis distance. The scattered photons increased with thickness of copper alone slab, combined aluminum-copper slab and copper-aluminum slab, but for aluminum alone slab they decreased with slab thickness. The stacking order of these two materials affects the characterization of scattered photons emergent from material slab with thickness. The combination of materials and the manner that the stacking was done affects the scattered photons production. The material combination could improve the radiotherapy efficiency in beam modifier development using more than two materials.

Relative Attenuation and Beam Softening Study with Flattening Filter Volume Reduction: Monte Carlo Study

The flattening filter (FF) volume reduction increases the clinical photons for deep tumor treatment. The beam softening determination is crucial for flattening filter improvement in geometry and materials. Determination and understanding the photon beam properties using material and geometry of a beam modifier is very important for dosimetry improvement in radiotherapy department and also for patient life quality development. This Monte Carlo study aims to investigate the relative attenuation and associated beam softening due to flattening filter volume reduction. The FF volume was reduced by 10, 20, and 30% of the initial volume data provided by the manufacturer. The relative attenuation and beam softening coefficients increased with FF volume reduction more near the beam central axis than the beam edge. We have illustrated that relative photon beam softening coefficient v was more stable than the coefficient u as a function of offaxis distance and with FF volume reduction. For increasing the photon fluence and dose delivered inside the phantom volume as mentioned in IAEA protocols, the FF volume should be reduced more near the FF top region than the FF edge region. Our work can be a basic investigation that will be used in improvement for the future linac configuration in terms of photon beam softening for material, geometry, and volume that were used in beam modifiers as a flattening filter.

Photon beam softening coefficient determination with slab thickness in small filed size: Monte Carlo study

Determination and understanding of photon beam softening using material soften photon beam for clinical usage is important for material study for attenuation and for beam modifier enhancements and linac improvements. Monte Carlo model was used to simulate 6 MeV photon beams produced by Varian Clinac 2100 accelerator with flattening filter thereafter the flattening filter was replaced by a slab of aluminum and copper with different of 0.5, 1, 1.5 and 2 mm. The Monte Carlo geometry was validated by a gamma index acceptance rate of 99% in PDD and 98% in dose profiles, the gamma criteria was 3% for dose difference and 3mm for distance to agreement. The purpose was to investigate the beam softening for small size and beam attenuation as a function of inserted slab thickness of copper and aluminum and also as a function of off-axis distance. For beam softening evaluation, variation amplitude of beam softening coefficient a1 was very high near the beam central axis and decreased with off-axis distance and also it was high for aluminum slab compared to copper slab. For aluminum slab, variation amplitude of beam softening coefficient a1 have a minimum at–0.5 cm–1 and a maximum at 0.5 cm–1 and for copper slab, variation amplitude of beam softening a1 have a minimum at–0.15 cm–1 and a maximum at 0.11 cm–1. Variation amplitude of beam softening coefficient a2 was very high near the beam central axis and decreased with off-axis distance and it was very high for aluminum slab compared to copper slab. For aluminum slab, variation amplitude of beam softening coefficient a2 have a minimum at–0.54 cm–2 and a maximum at 0.44 cm–2 and for copper slab, variation amplitude of beam softening coefficient a2 have a minimum at–0.111 cm–2 and a maximum at 0.0825 cm–2.

Study of Possibility to Reduce Flattening Filter Volume for Increasing Energetic Photons for High Radiotherapy Efficiency

Increasing dose for improved radiotherapy efficiency is essential for linear accelerator development and also for flattening filter geometry and material enhancement. Determination and understanding the photon beam properties using material and geometry of a beam modifier is very important in radiotherapy department and also for high patient life quality. This Monte Carlo study aims to improve the FF in a linac for increasing the photon number at the entrance of treatment volume tumor and the dose delivered inside the treatment volume as recommended by the IAEA protocols. The aim of this study was to check out the possibility to reduce the flattening filter volume for improving the clinical photons at phantom surface. We have studied photon attenuation coefficients and beam softening coefficients with FF volume reduction. The FF volume was reduced by 10, 20, and 30% of initial FF volume. The photon fluence increased with FF volume reduction near the beam central axis than the beam edge and the beam softening coefficients remained apparently invariable with FF volume reduction as a function of off-axis distance. Our work can be a basic study that will be used in research and improvement for future linac configuration in terms of photon attenuation and beam softening for a material, geometry and volume that were used for finding out good flattening and good softening to produce an optimal delivered dose as recommended by IAEA.

10-MV X-ray dose calculation in water for MLC and wedge fields using a convolution method with X-ray spectra reconstructed as a function of off-axis distance

10-MV X-ray dose calculation in water for MLC and wedge fields using a convolution method with X-ray spectra reconstructed as a function of off-axis distance

Experimental assessment of peripheral dose in high-energy electron beams used in external radiotherapy

Peripheral dose from bremsstrahlung and scattered electrons in high-energy electron beams in external radiotherapy remains poorly studied. The purpose of this work is to assess peripheral dose outside applicators at the patient level. Commissioning was performed for 6, 9, 12 and 18 MeV electron beams on three linear accelerators Varian 2300 C/D (Varian applicator), Siemens Primus KD2 (Deva applicator) and Siemens Oncor (applicator EA3). The peripheral dose was measured in a water phantom as a function of off-axis distance from 5 cm to 65 cm from the field edge for SSD = 100 cm. Thermoluminescent TLD-700 powder dosimeters were used. Measurements were made at both 1 cm and 10 cm depths, using applicator sizes from 6 × 6cm 2 to 20 × 20 cm 2 , the results assuming that the TLD signal is proportional to the dose. Whatever the field size, a peak dose spot appeared at 15 cm from the field edge for Siemens applicators. For Siemens Primus with an applicator size of 10 × 10 cm 2 , this peak reached 2,1%, 1%, 0,9% and 1,3% of Dmax for 6, 9, 12 and 18 MeV electron beams, respectively, doubled for 6 × 6 cm 2 field size. For Siemens Oncor, with the above applicator size, this peak dose reached 0,8%, 1,2% and 1,3% of Dmax for 6, 9 and 12MeV, respectively, doubled for 20 × 20cm 2 . In contrast for Varian 2300 C/D, the doses at 15 cm from field edge were 0,3%, 0,6% and 1,1% of Dmax for 6,12 and 18 MeV, respectively. No peak dose spot was evidenced for Varian applicator. The doses measured at 10 cm depth were 2–3 times lower than doses at 1 cm depth for out off- field distances from 5 cm to 35 cm, and were 4–7 times lower for larger distances. This work analyzes peripheral doses for different electron beams energies and three different applicator types. It shows that depending on beam energy, applicator size and type, the peak dose at 15 cm from field edge ranges from 0,3–2,7% of the Dmax. Measurements made at 10 cm depth show that, depending on beam energy and applicator size and collimator size, the dose at 15 cm from field edge is 0,07–0,2% of Dmax. Although the debate on signal to dose conversion coefficients for mixed fields is still ongoing, our results should be considered because, to date, peripheral dose with electron beams are not fully accounted for by TPS.

A convolution/superposition method using primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance: comparison of calculated and measured 10-MV X-ray doses in thorax-like phantoms

We performed experimental studies on the convolution/superposition method reported in the former companion paper (Iwasaki in Radiol Phys Technol 4, 2011) using 10-MV X-ray beams from open-jaw-collimated fields. The method uses primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance. We made a comparison of calculations and measurements in water phantoms and thorax-like phantoms with respect to percentage depth dose curves, tissue-phantom ratio curves, and dose profiles. We made the dose calculation by taking into account the beam-hardening effect with depth and the off-axis radiation-softening effect. We found that the method could be used, in general, for performing accurate dose calculations.

A convolution/superposition method using primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance: a theoretical study on 10-MV X-ray dose calculations in thorax-like phantoms

A convolution/superposition method is proposed for use with primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance. It should be noted that the number of energy bins is usually about ten, and that the reconstructed X-ray spectra can reasonably be applied to media with a wide range of effective Z numbers, ranging from water to lead. The study was carried out for 10-MV X-ray doses in water and thorax-like phantoms with the use of open-jaw-collimated fields. The dose calculations were made separately for primary, scatter, and electron contamination dose components, for which we used two extended radiation sources: one was on the X-ray target and the other on the flattening filter. To calculate the in-air beam intensities at points on the isocenter plane for a given jaw-collimated field, we introduced an in-air output factor (OPF(in-air)) expressed as the product of the off-center jaw-collimator scatter factor (off-center S (c)), the source off-center ratio factor (OCR(source)), and the jaw-collimator radiation reflection factor (RRF(c)). For more accurate dose calculations, we introduce an electron spread fluctuation factor (F (fwd)) to take into account the angular and spatial spread fluctuation for electrons traveling through different media.

Siemens Multileaf Collimator Characterization and Quality Assurance Approaches for Intensity-Modulated Radiotherapy

Application of the multileaf collimator (MLC) has evolved from replacing blocks to create treatment fields to creating photon fluence modulation for intensity-modulated radiotherapy (IMRT). Multileaf collimator system performance requirements are far more stringent for such applications and will require increased performance for future applications, such as motion tracking. This article reviews Siemens MLC systems, including a technical description and dosimetric characteristics of 56-, 82-, and 160-leaf designs. Routine quality assurance of MLC for IMRT necessitates frequent and critical assessment of MLC leaf position calibration errors that can present in many different ways (e.g., accuracy, reproducibility, longevity, hysteresis, and collimator/gantry angle dependencies). Several techniques for measuring these errors are presented, along with qualitative and quantitative techniques for analyzing results. In particular, increased accuracy of leaf position measurement at variable gantry angles is enabled by spatial transformations to electronic portal imaging device position quantified by calibration protocols introduced with megavoltage cone beam. Measured values of X-ray transmission (intra-leaf, inter-leaf, and through abutting leaf pairs) and penumbra (leaf end, leaf tongue, leaf groove) are presented with an evaluation of their characterization by a treatment-planning system. The dosimetric impact of planning system model inadequacies is demonstrated for collimator scatter, dose profile values within 30 mm of the field edge, and the resultant effect demonstrated on clinical cases. Finally, a description of automated quality assurance delivery, analysis, and calibration protocols applicable for the specific vendor's system is provided.

Formulation of spectra-based attenuation coefficients in water as a function of depth and off-axis distance for 4, 10 and 15 MV X-ray beams

On the basis of X-ray spectra information for 4, 10 and 15 MV X-rays from a linear accelerator, we propose a method for formulating the attenuation coefficient in water as a function of depth and off-axis distance. It was confirmed by transmission measurement that the formulation provides a good estimation of the attenuation coefficient in water. Using the formulation, we further assessed the effective attenuation coefficient in water as a function of off-axis distance.