Central Axis Depth Dose Distributions Research Articles

Monte Carlo-based dosimetry of proposed bi-radionuclide (125I and 106Ru/106Rh) eye plaque: A feasibility study.

Combining the sharp dose fall off feature of beta-emitting 106Ru/106Rh radionuclide with larger penetration depth feature of photon-emitting125I radionuclide in a bi-radionuclide plaque, prescribed dose to the tumor apex can be delivered while maintaining the tumor dose uniformity and sparing the organs at risk. The potential advantages of bi-radionuclide plaque could be of interest in context of ocular brachytherapy. The aim of the study is to evaluate the dosimetric advantages of a proposed bi-radionuclide plaque for two different designs, consisting of indigenous 125I seeds and 106Ru/106Rh plaque, using Monte Carlo technique. The study also explores the influence of other commercial 125I seed models and presence or absence of silastic/acrylic seed carrier on the calculated dose distributions. The study further included the calculation of depth dose distributions for the bi-radionuclide eye plaque for which experimental data are available. The proposed bi-radionuclide plaque consists of a 1.2-mm-thick silver (Ag) spherical shell with radius of curvature of 12.5mm, 20µm-thick-106Ru/106Rh encapsulated between 0.2mm Ag disk, and a 0.1-mm-thick Ag window, and water-equivalent gel containing 12 symmetrically arranged 125I seeds. Two bi-radionuclide plaque models investigated in the present study are designated as Design I and Design II. In Design I, 125I seeds are placed on the top of the plaque, while in Design II 106Ru/106Rh source is positioned on the top of the plaque. In Monte Carlo calculations, the plaque is positioned in a spherical water phantom of 30cm diameter. The proposed bi-radionuclide eye plaque demonstrated superior dose distributions as compared to 125I or 106Ru plaque for tumor thicknesses ranges from 5 to 10mm. Amongst the designs, dose at a given voxel for Design I is higher as compared to the corresponding voxel dose for Design II. This difference is attributed to the higher degree of attenuation of 125I photons in Ag as compared to beta particles. Influence of different 125I seed models on the normalized lateral dose profiles of Design I (in the absence of carrier) is negligible and within 5% on the central axis depth dose distribution as compared to the corresponding values of the plaque that has indigenous 125I seeds. In the presence of a silastic/acrylic seed carrier, the normalized central axis dose distributions of Design I are smaller by 3%-12% as compared to the corresponding values in the absence of a seed carrier. For the published bi-radionuclide plaque model, good agreement is observed between the Monte Carlo-calculated and published measured depth dose distributions for clinically relevant depths. Regardless of the type of 125I seed model utilized and whether silastic/acrylic seed carrier is present or not, Design I bi-radionuclide plaque offers superior dose distributions in terms of tumor dose uniformity, rapid dose fall off and lesser dose to nearby critical organs at risk over the Design II plaque. This shows that Design I bi-radionuclide plaque could be a promising alternative to 125I plaque for treatment of tumor sizes in the range 5 to 10mm.

Application of stoichiometric CT number calibration method for dose calculation of tissue heterogeneous volumes in boron neutron capture therapy.

Monte Carlo simulation code is commonly used for the dose calculation of boron neutron capture therapy. In the past, dose calculation was performed assuming a homogeneous mass density and elemental composition inside the tissue, regardless of the patient's age or sex. Studies have shown that the mass density varies with patient to patient, particularly for those that have undergone surgery or radiotherapy. A method to convert computed tomography numbers into mass density and elemental weights of tissues has been developed and applied in the dose calculation process using Monte Carlo codes. A recent study has shown the variation in the computed tomography number between different scanners for low- and high-density materials. The aim of this study is to investigate the effect of the elemental composition inside each calculation voxel on the dose calculation and the application of the stoichiometric CT number calibration method for boron neutron capture therapy planning. Monte Carlo simulation package Particle and Heavy Ion Transport code System was used for the dose calculation. Firstly, a homogeneous cubic phantom with the material set to ICRU soft tissue (four component), muscle, fat, and brain was modelled and the NeuCure BNCT system accelerator-based neutron source was used. The central axis depth dose distribution was simulated and compared between the four materials. Secondly, a treatment plan of the brain and the head and neck region was simulated using a dummy patient dataset. Three models were generated; (1) a model where only the fundamental materials were considered (simple model), a model where each voxel was assigned a mass density and elemental weight using (2) the Nakao20 model, and (3) the Schneider00 model. The irradiation conditions were kept the same between the different models (irradiation time and irradiation field size) and the near maximum (D1%) and mean dose to the organs at risk were calculated and compared. A maximum percentage difference of approximately 5% was observed between the different materials for the homogeneous phantom. With the dummy patient plan, a large dose difference in the bone (greater than 12%) and region near the low-density material (mucosal membrane, 7%-11%) was found between the different models. A stoichiometric CT number calibration method using the newly developed Nakao20 model was applied to BNCT dose calculation. The results indicate the importance of calibrating the CT number to elemental composition for each individual CT scanner for the purpose of BNCT dose calculation along with the consideration of heterogeneity of the material composition inside the defined region of interest.

Effect of cavity under bolus on the dose of superficial tissue

Objective To investigate the effect of cavity thickness, area and distance under the bolus upon the dose in the superficial tissues. Methods An accelerator model was constructed based on Geant4.The model accuracy was validated by the comparison of the calculated data with the measured data. A 30×30×30 cm3 water phantom with the upper surface located at the isocenter level and a 30×30×1 cm3 water film were constructed. Different models with the water film close to or different cavities with the water phantom were established. Under the 10×10 cm2 field with 6 MV X-ray beam, the central axis depth dose distribution and the lateral dose profiles at a depth of 0.1 cm (profile1) of the models with different cavities were calculated. The calculated data of different model with the water film close to or different cavities with the water phantom were statistically compared. Results When the cavity thickness was ≤ 0.5 cm, the cavity exerted slight effect upon the depth of maximum dose (Dmax) and superficial dose. As the cavity thickness was increased, the Dmax was also increased, the PDD at 0.1 cm (PDD1) was decreased rapidly and the profile1 was increased from the cavity center to the edge. Along with the increase of cavity area, the Dmax was initially increased and then decreased, whereas the PDD1 was first decreased followed by an increase. When the cavity area was small, the profile1 was gradually increased from the cavity center to the edge. When the cavity area was large, the profile1 was initially decreased and subsequently increased. When the distance was ≥ 0.2 cm, it was qualified for the clinical requirement and it exerted no effect when the distance was ≥ 1.0 cm. The profile1 distant from the cavity was not affected. Conclusion The cavity under the bolus should be minimized to reduce the cavity thickness, area and distance as possible. Key words: Monte Carlo; Bolus; Cavity; Superficial dose; Profile

Dose accuracy research on air cavity interface in treatment planning system

Objective To study the accuracy of collapsed cone convolution (CCC) and anisotropic analytical algorithm (AAA) in dosimetric calculation on the air cavity interface. Methods A BEAMnrc/EGSnrc Monte Carlo (MC) simulation was performed on a Varian Trilogy linear accelerator. The IBA Dosimetry blue 3D scanning system was used to verify the accuracy and reliability of the MC simulation. Central axis depth dose distribution and lateral dose profile in a water-equivalent phantom with variously sized air cavities were calculated by CCC and AAA. The obtained depth dose distribution and lateral dose profile were compared with those by MC simulation and EBT2 film, respectively. Results Both CCC and AAA overestimated the dose on the air cavity interface. In spite of some errors, CCC had a higher accuracy than AAA. The errors were mainly related to computational grid, field size, photon energy, cavity size, and the number of fields. Conclusion Electronic disequilibrium on the air cavity interface should be taken into account when CCC and AAA are used for dosimetric calculation in treatment planning system. Key words: Electron disequilibrium; Collapsed cone convolution; Anisotropic analytical algorithm; Monte Carlo algorithm; EBT2 film

ROPES eye plaque dosimetry: commissioning and verification of an ophthalmic brachytherapy treatment planning system

In this study, the Plaque SimulatorTM eye plaque brachytherapy planning system was commissioned for ROPES eye plaques and Amersham Health model 6711 Iodine 125 seeds, using TG43-UI data. The brachytherapy module of the RADCALC® independent checking program was configured to allow verification of the accuracy of the dose calculated by Plaque SimulatorTM. Central axis depth dose distributions were compared and observed to agree to within 2% for all ROPES plaque models and depths of interest. Experimental measurements were performed with a customized PRESAGEm 3-D type dosimeter to validate the calculated depth dose distributions. Preliminary results have shown the effect of the stainless steel plaque backing decreases the measured fluorescence intensity by up to 25%, and 40% for the 15 mm and 10 mm diameter ROPES plaques respectively. This effect, once fully quantified should be accounted for in the Plaque SimulatorTM eye plaque brachytherapy planning system.

Radial Secondary Electron Dose Profiles and Biological Effects in Light-Ion Beams Based on Analytical and Monte Carlo Calculations using Distorted Wave Cross Sections

To speed up dose calculation, an analytical pencil-beam method has been developed to calculate the mean radial dose distributions due to secondary electrons that are set in motion by light ions in water. For comparison, radial dose profiles calculated using a Monte Carlo technique have also been determined. An accurate comparison of the resulting radial dose profiles of the Bragg peak for (1)H(+), (4)He(2+) and (6)Li(3+) ions has been performed. The double differential cross sections for secondary electron production were calculated using the continuous distorted wave-eikonal initial state method (CDW-EIS). For the secondary electrons that are generated, the radial dose distribution for the analytical case is based on the generalized Gaussian pencil-beam method and the central axis depth-dose distributions are calculated using the Monte Carlo code PENELOPE. In the Monte Carlo case, the PENELOPE code was used to calculate the whole radial dose profile based on CDW data. The present pencil-beam and Monte Carlo calculations agree well at all radii. A radial dose profile that is shallower at small radii and steeper at large radii than the conventional 1/r(2) is clearly seen with both the Monte Carlo and pencil-beam methods. As expected, since the projectile velocities are the same, the dose profiles of Bragg-peak ions of 0.5 MeV (1)H(+), 2 MeV (4)He(2+) and 3 MeV (6)Li(3+) are almost the same, with about 30% more delta electrons in the sub keV range from (4)He(2+)and (6)Li(3+) compared to (1)H(+). A similar behavior is also seen for 1 MeV (1)H(+), 4 MeV (4)He(2+) and 6 MeV (6)Li(3+), all classically expected to have the same secondary electron cross sections. The results are promising and indicate a fast and accurate way of calculating the mean radial dose profile.

An investigation of central axis depth dose distribution perturbation due to an air gap between patient and bolus for electron beams

Electron beams can be used for the radiotherapy treatment of superficial cancers. In many cases of electron beam radiotherapy, tissue equivalent bolus material placed on the skin is to be used to enhance skin dose. An air gap might be present between the bolus and the skin due to variation in the patient contour. The impact of semi-infinite air gaps under bolus material on central axis depth dose distributions for electron beams was investigated in this study. Semi-infinite air gaps were introduced between bolus and the surface of a water phantom for air gap sizes up to 20.0 mm and for bolus thicknesses of 5, 10 and 15 mm. The electron beams studied had nominal energies of 6, 10 and 14 MeV and circular fields of 3, 5, 7 and 9 cm diameter. Depth dose measurements were carried out in the water phantom with a Scanditronix p-Si electron diode. It was found that the impact of an air gap is dependent on beam energy, field size, air gap size and bolus thickness used. The impact of the air gap on central axis depth dose distribution increased with decreasing field size, increasing air gap size, decreasing electron beam energy and increasing bolus thickness. For 15 mm bolus, 3 cm diameter circular field, 6 MeV beam and the 20 mm air gap, the maximum dose and the surface dose was reduced by approximately 60% and the depth of dose maximum shifted 3.5 mm. An air gap between bolus and a patient should be avoided to ensure that there is no impact on the treatment. The measured data in this study can be used to determine the likely degree of impact on the treatment, of unavoidable air gaps between bolus and the patient.

Pantak Therapax SXT 150: performance assessment and dose determination using IAEA TRS-398 protocol

The performance assessment and beam characteristics of the Therapax SXT 150 unit, which encompass both low and medium-energy beams, were evaluated. Dose determination was carried out by implementing the International Atomic Energy Agency (IAEA) TRS-398 protocol and measuring all the dosimetric parameters in order to have a solid, consistent and reliable data set for the unit. Mechanical movements, interlocks and applicator characteristics agreed with specifications. The timer exhibited good accuracy and linearity. The output was very stable, with good repeatability, long-term reproducibility and no dependence on tube head orientation. The measured dosimetric parameters included beam first and second half-value layers (HVLs), absorbed dose rate to water under reference conditions, central axis depth dose distributions, output factors and beam profiles. Measured first HVLs agreed with comparable published data, but the homogeneity coefficients were low in comparison with typical values found in the literature. The timer error was significant for all filters and should be taken into consideration for the absorbed dose rate determination under reference conditions as well as for the calculation of treatment times. Percentage depth-dose (PDD) measurements are strongly recommended for each filter-applicator combination. The output factor definition of the IAEA TRS-398 protocol for medium-energy X-ray qualities involves the use of data that is difficult to measure. Beam profiles had small penumbras and good symmetry and flatness except for the lowest energy beam, for which a heel effect was observed.

Sensitivity of large-field electron beams to variations in a Monte Carlo accelerator model

Adjustments made to Monte Carlo models during the commissioning of the simulation should be physically realistic and correspond to actual machine characteristics. Large electron fields, with the jaws fully open and the applicator removed, are sensitive to important source and geometry parameters and may provide the most accurate beam models, including those collimated by an applicator. We report on the results of a comprehensive Monte Carlo sensitivity study documenting the response of these large fields to changes in the configuration of a Siemens Primus linear accelerator. The study was performed for 6, 9 12, 15, 18 and 21 MeV configurations, and included variations of thickness, position and lateral alignment of all treatment head components. Variations of electron beam characteristics were also included in the study. Results were classified by their impact on central-axis depth dose distributions, including the bremsstrahlung tail, and on beam profiles near Dmax and in the bremsstrahlung region. Low-energy results show an increased sensitivity to electron beam properties. High-energy bremsstrahlung profiles are shown to be useful in determining misalignments between the beam axis and mechanical isocentre. For all energies, the alignment of the secondary scattering foil and monitor chamber are shown to be critical for correctly modelling beam asymmetries. The results suggest a methodology for commissioning of electron beams using Monte Carlo treatment head simulation.

On the initial angular variances of clinical electron beams

Electron beam radiotherapy treatment planning systems need to be fed with the characteristics of the high-energy electron beams (4-50 MeV) from the specifically applied accelerator. Beams can be characterized by their mean initial energy, effective initial angular variance, virtual source position and the resulting central axis depth dose distribution in water. This information is the only input to pencil beam dose calculation models. Newer calculation models like macro Monte Carlo, voxel Monte Carlo and phase space evolution require as input the full initial phase space or a parametrization of that initial phase space, generally consisting of a primary beam component and one or more scatter components. This primary beam component is often characterized by initial energy, primary beam initial angular variance and virtual source distance. The purpose of the present investigation was to investigate to what extent standard values can be used both for the effective initial angular variance as input to pencil beam models and for the primary beam initial angular variance. Comprehensive benchmark data were obtained on the initial angular variance of various types of accelerator, for various energies and field sizes. The initial angular variance has been derived from penumbra measurements in air by means of film dosimetry at various distances from the lower collimator. For the types of accelerator used in radiotherapy nowadays the measurements show values for of around 13 cm where T(E) is the ICRU-35 linear angular scattering power in air. This value can be chosen as standard value for the primary beam initial angular variance, only slightly compromising the dose calculation accuracy. As input to pencil beam models, an effective should be used incorporating the scatter from the lower collimator. For the case that the air gaps between lower collimator and patient are small (5-10 cm) an effective of 20 cm has been found and is recommended as the standard input for pencil beam models. Of the accelerators investigated, a different value was found only for the Elekta SL15, i.e. 50% higher for the effective .

An analytical expression for electron beam central axis depth doses

An analytical expression based on four fitting parameters is proposed for a mathematical description of electron beam central axis depth dose distributions. The expression approximates well the measured electron beam data in the field size range from 4 x 4 cm2 to 25 x 25 cm2 and in the energy range from 6 to 20 MeV in all four regions of the electron depth dose curve: build-up, dose maximum, dose fall-off, and bremsstrahlung contamination.

Determination of the Dose Distribution of an Ophthalmic 106Ru Irradiator with TLDs and an Eye Phantom

Abstract Two 106Ru applicators of the type CCB as manufactured by the Zentralinstitut für Isotopen- und Strahlenforschung (Berlin, GDR) were investigated. The homogeneity of the relative dose distribution at the surface of the applicators was determined. For the evaluation of the data extremely small TLD crystals (2x1x1 mm3) had to be manufactured as well as an almost tissue-equivalent acrylic eye phantom. In order to ensure reproducibility of the applicator positioning, the plaque suture eyelets had to be placed on locating pins. 120 measurement points located within the active plaque area of 20 mm diameter were evaluated. The results show that the area to be used in tumour treatment is about 20 to 25% less than expected. One plaque shows a considerable inhomogeneity with respect to dose distribution (up to 40%). A modified eye phantom was used for the determination of the central axis depth dose distribution.

Parametrisation of depth dose for electron beams

Knowledge of the central axis depth dose curve is essential for the development of an accurate electron beam treatment planning system. The equation introduced by Kawachi (1975) and modified by Steben et al. (1976) gave general agreement with experimental data but it required a complex set of equations to extract the central axis depth dose. Even though interpolation of experimental data is one of the most common methods used in the available commercial treatment planning systems, an analytical expression might have the advantage of reducing both computer time and memory space. Unfortunately the electron beam central axis depth dose distribution has not yet been expressed accurately in an analytical form. Monte Carlo calculations are reasonably accurate, so they can be used as the standard for verifying complex experimental and theoretical work. However, they are impractical in day-to-day clinical work because of their demanding computational time. The authors have tried to reproduce the results of Monte Carlo calculations and experiments with an analytical expression. The analytical form may prove useful to centres which are now working on three-dimensional treatment planning codes for electron beam treatment planning.

Model for calculating depth dose distributions for broad electron beams.

Central axis electron beam depth dose distributions can be transformed by replacing dose by fluence and depth by a measure of angular dispersion. This transformation was applied to a set of broad beam central axis depth dose distributions calculated by Monte Carlo code for beams with initial energies ranging from 1 to 60 MeV in homogeneous media of water, aluminum, and copper. The resulting fluence distributions belong to a family of curves that can be parametrized by a single-valued function of initial beam energy and medium and can be used to calculate fluence distributions for accelerators. Fluence curves can be easily transformed to depth dose curves.

Use of the FBX dosemeter for the calibration of cobalt-60 and high-energy teletherapy machines

The doses from the cobalt-60 teletherapy machines were measured using the FBX and secondary-standard dosemeters of Farmer-Baldwin type. The FBX dosemeter contained 0.20 mM ferrous ammonium sulphate, 5.0 mM benzoic acid and 0.20 mM xylenol orange in 0.5 N sulphuric acid. The values were compared with the values from a Fricke dosemeter and a graphite chamber used as primary standards. The values of the FBX, Fricke and graphite chambers agreed. There were, however wide differences among the different secondary-standard dosemeters themselves and with the FBX dosemeter. The FBX dosemeter was used for the measurement of central axis depth dose distributions for 5, 8, 10, 20 and 30 MeV electron and 42 MV X-ray beams.

An analytic expression for central axis electron depth dose distributions

A new analytic expression is given for the central axis depth dose distribution for electron beams; its accuracy is verified by comparison with experimental data from a linear accelerator and a betatron for a variety of incident energies and field sizes. In addition, comparisons are provided with Laughlin et al's 10 exponential model for the electron depth dose distribution.