Monte Carlo Simulated Dose Research Articles

Poster - Thurs Eve-15: Comparison of Cobalt-60 and 6 MV linac based tomotherapy: A prostate case study.

Previous work reported by us has shown the potential for Cobalt-60 (Co-60) tomotherapy for sites with small separations such as in head and neck site. In this work we extend our investigations by comparing tomotherapy plans for the treatment of a typical prostate cancer obtained for 6 MV and Co-60 beams. Beam collimation was provided by the MIMiC® (NOMOS Corporation, Sewickly, PA) multi-leaf collimator (MLC). Both plans used 21 beam angles, each utilizing the central 10 leaf-pairs of the MLC for intensity modulation. An in-house inverse treatment planning program, based on the active-set conjugate gradient method, was used for dose-volume optimization. BEAMnrc and DOSXYZnrc Monte Carlo simulated beam and dose data, including inhomogeneity corrections, were used to calculate the optimized tomotherapy dose distributions. Prostate, rectum, and external body contours were outlined and dose-volume optimization objectives were set to deliver a minimum of 95% and a maximum of 105% of the 76 Gy dose prescription to the prostate and limiting only 20% of the rectum volume to receive ⩾ 70 Gy. A quantitative analysis of the dose distributions and dose-area histograms show that both Co-60 and 6 MV plans achieve the initial objectives for target (prostate) and organ at risk (rectum). Although the dose to the body and rectum for Co-60 is slightly higher than that for 6 MV, it satisfies the plan objectives based on the clinical dose tolerance. Our results demonstrate that Co-60 based tomotherapy can provide clinically competitive dose distributions for the treatment of prostate cancer.

Cylindrical coordinate‐based TG‐43U1 parameters for dose calculation around elongated brachytherapy sources

In 2001, the use of cylindrical coordinates was demonstrated to be more suitable than was the use of polar coordinates for accurate computer calculations during treatment planning for I192r intravascular brachytherapy sources. In the present work, we investigated the applicability of cylindrical coordinate–based TG‐43U1 parameters for dosimetric evaluation and dose calculations for RadioCoil 103Pd sources (RadioMed Corporation, Tyngsboro, MA) 1.0‐cm to 6.0‐cm long. For brevity, only the results for sources 1.0‐cm, 3.0‐cm, and 5.0‐cm long are presented here. Dosimetric characteristics of RadioCoil 103Pd sources were calculated in liquid water using the Monte Carlo simulation technique. To demonstrate the suitability of this methodology, the Monte Carlo–simulated dose profiles for a RadioCoil 103Pd source 5.0‐cm long at radial distances of 0.5 cm, 0.9 cm, and 1.25 cm were compared with calculated data using TG‐43U1 parameters in the polar and cylindrical coordinate systems. In addition, we also used a source 1.0‐cm long parameterized using cylindrical coordinates to investigate the application of a linear segmented source (LSS) model originally introduced by our group. The results indicate that, for dose calculation around elongated brachytherapy sources, cylindrical coordinate–based TG‐43U1 parameters more accurately represent the dose distribution around an elongated source than the polar coordinate–based parameters. In addition, the LSS model, in conjunction with the cylindrical coordinate–based parameters for a source 1.0‐cm long, can be used to replicate the dose distribution around any integral source length. This process eliminates the need to collect and enter data for multiple source lengths into treatment planning systems.PACS number: 87.66.Jj

Dosimetrische Evaluation von Augenlinsen-Protektoren in der Computertomographie – Messungen und Monte-Carlo-Simulationen

The present study aims at the investigation of eye lens shielding of different composition for the use in computed tomography examinations. Measurements with thermo-luminescent dosimeters and a simple cylindrical waterfilled phantom were performed as well as Monte Carlo simulations with an equivalent geometry. Besides conventional shielding made of Bismuth coated latex, a new shielding with a mixture of metallic components was analyzed. This new material leads to an increased dose reduction compared to the Bismuth shielding. Measured and Monte Carlo simulated dose reductions are in good agreement and amount to 34% for the Bismuth shielding and 46% for the new material. For simulations the EGSnrc code system was used and a new application CTDOSPP was developed for the simulation of the computed tomography examination. The investigations show that a satisfying agreement between simulation and measurement with the chosen geometries of this study could only be achieved, when transport of secondary electrons was accounted for in the simulation. The amount of scattered radiation due to the protector by fluorescent photons was analyzed and is larger for the new material due to the smaller atomic number of the metallic components.

SU-FF-T-229: Fast Dose Calculation with Superposition Algorithm Using a Scaling Method

Purpose: To speed up the scatter dose calculation procedure in a superposition algorithm by using a scaling method where the superposition of scatter contributions can be carried out through instead of all sites in the medium but randomly selected scatter sites. Method and Materials: The scatter dose in a heterogeneous medium was obtained by Monte Carlo simulated scatter dose in water multiplied by the ratio of the scatter dose in the heterogeneous medium to the scatter dose in water calculated with superposition algorithm. Because this ratio is the scaling of the dose calculation in the medium by the same calculation in water the superposition could be carried out through in stead of all scatter sites but randomly selected, sufficiently enough numbers of scatter sites representing the structure of the medium. The 3‐D dose distributions for heterogeneous structures in a spherical water phantom such as spherical lung or bone materials were calculated for a point photon source of energies of 30 keV, 100 keV and 400 keV. Results: For the phantom considered here, the reduction of about 106 scatter sites to 104 random scatter sites speeds up the dose calculation by a factor of 100 while keeping the fluctuation caused by the random selection within 5%. Conclusion: Dose calculation with a superposition algorithm can be significantly speeded up by this scaling method with randomly selected scatter sites.

SU-FF-T-286: MARS - A Monte Carlo Assisted Research System for Radiotherapy

Purpose: To develop a dedicated and versatile research system for radiotherapy treatment planning that combines Monte Carlo dose calculation accuracy with inverse planning capability. Method and Materials: In this work, a research platform with interactive graphics user interface (GUI) has been developed for Monte Carlo based inverse planning for radiotherapy. It consists of a top-level module that integrates and coordinates the other four sub-modules in the system: a DICOM IMAGE/RT server module that receives all incoming DICOM images and DICOM RT objects, an iPort module that constructs a patient phantom in EGS4 format from DICOM image studies, a phant2optim module that performs ray-tracing in the phantom and prepares input for pre-optimization Monte Carlo simulations of the beamlets, and an optim2mcsim module that optimizes a treatment plan based on Monte Carlo simulated beamlets dose distributions and prepares input for post-optimization Monte Carlo simulations of the plan. Results: A Monte Carlo assisted research system (MARS) with five designed modules has been developed and integrated by an interactive graphics user interface. Interfacing of the MARS with the Varian Eclipse treatment planning system (TPS) via DICOM RT and with the EGS4/BEAM for Monte Carlo dose calculation has been successfully established and verified. Conclusions: A Monte Carlo based inverse planning system has been developed for radiotherapy treatment planning and research. This system is able to import DICOM IMAGE/RT objects from a clinical TPS, convert them into a patient CT phantom, and optimize a plan based on Monte Carlo simulations of beamlet dose distributions in the patient CT phantom. With the dose calculation accuracy of Monte Carlo, multiple inverse planning algorithms, and a user-friendly GUI, we hope this system will further enhance the application and research of Monte Carlo treatment planning in radiotherapy. We plan to have it available free to interested parties in the future.

SU-FF-T-304: Monte Carlo Investigation of Electron Beams Collimated by An Electron-Specific MLC for Modulated Electron Radiotherapy

Purpose: This work investigates clinical electron beams collimated by an electron multileaf collimator (eMLC) for modulated electron radiotherapy (MERT) treatment optimization and dose calculation. Material and Method: In this work we used MCBEAM and MCSIM codes for accelerator simulation and phantom/patient dose calculation, respectively. Particle phase space files at a plane directly above the eMLC were generated using MCBEAM. For each energy we first simulated with several monoenergetic electron beams. Then a previously developed optimization code was used to derive the energy spectra of the clinical electron beam to match the measured percent depth doses (PDD). The derived spectrum was used in the MCBEAM to generate the phase space file. Simulations were done to generate phase space files for all the energies available 6, 9, 12, 16, and 20MeV. The MCSIM code was used to simulate the eMLC using the generated phase space files as the source to calculate the dose distribution in a water phantom. Regular and irregular shapes were formed by positioning leaves of the eMLC. Selected shapes were then simulated. Monte Carlo simulated dose distributions were compared with measurements. Results: Good agreement (2%/1mm) was achieved between Monte Carlo simulations and measurements for PDDs collimated by both electron applicators/cutouts and by the eMLC for all electron energies and square fields. The Monte Carlo dose distributions resulting from multiple static electron fields collimated by the eMLC agreed to 3%/2mm with ion chamber and film measurements. Conclusion: In this study we have, through extensive verification, shown that our Monte Carlo code with the source represented by phase space files is capable of accurately modeling electron beam delivery with the eMLC.

TU‐C‐224C‐06: Determination of TG‐43U1 Recommended Parameters for Elongated RadioCoil™ Brachytherapy Sources 1.0 to 6.0 Cm in Length Using Experimental and Monte Carlo Simulation Techniques

Purpose: In this project TG‐43U1 recommended dosimetric characteristics of newly designed elongated RadioCoil™ brachytherapy sources(1 to 6)cm in length have been determined following experimental and Monte Carlo simulation techniques. Monte Carlo simulated dose profiles have also been calculated for sources 1 to 6cm in length. Materials and Methods: TG‐43U1 recommended dosimetric characteristics (Λ, g(r), F(r,θ), φan) of RadioCoil™ sources have been determined using experimental(TLD) and Monte Carlo(MCNP5) simulation techniques. MCNP5 simulations were performed in spherical Solid Water™ and water phantoms 20cm in diameter for 108 histories. F(r,θ) of RadioCoil™ were determined for angles 0° to 90° for r⩾L/2(where L=active length), and angles 5°⩽θ⩽90° for r⩾L/2, with 5° increment. TLD measurements were performed in solid Water™ using same geometric arrangement as that of Monte Carlo simulations. Measured and calculated dose profiles along the longitudinal axis of these sources were utilized to validate the dose calculation with commercially available treatment planning systems. Results: Results of these investigations indicate good agreement between MCNP5 simulated and TLD determined values for RadioCoil™ sources. Upon the good agreement between these two methodologies, confirming the accuracy of our simulation geometry, dosimetric parameters of these sources have been determined in liquid water for their clinical applications. It has been found that in order to achieve a good agreement between the treatment planning dose distribution, the F(r,θ) of these sources have to be determined at radial distances ranging from 0.5 to 5.0cm with 0.5cm increment and L/2 ±0.2cm. Conclusion: Dosimetric characteristics of newly designed RadioCoil™ have been determined following TG‐43U1 recommendations using experimental and Monte Carlo simulation technique. In these determinations it has been found that the dose profile can be closely reproduced if the 2D anisotropy function is determined at 0.5 cm increments for radial distance ranging from 0.5 cm to 5cm, and L/2 ± 0.2cm.

Absolute depth-dose-rate measurements for an Ir192 HDR brachytherapy source in water using MOSFET detectors

Reported MOSFET measurements concern mostly external radiotherapy and in vivo dosimetry. In this paper, we apply the technique for absolute dosimetry in the context of HDR brachytherapy using an 192Ir source. Measured radial dose rate distributions in water for different planes perpendicular to the source axis are presented and special attention is paid to the calibration of the R and K type detectors, and to the determination of appropriate correction factors for the sensitivity variation with the increase of the threshold voltage and the energy dependence. The experimental results are compared with Monte Carlo simulated dose rate distributions. The experimental results show a good agreement with the Monte Carlo simulations: the discrepancy between experimental and Monte Carlo results being within 5% for 82% of the points and within 10% for 95% of the points. Moreover, all points except two are found to lie within the experimental uncertainties, confirming thereby the quality of the results obtained.

144Ce as a potential candidate for interstitial and intravascular brachytherapy

To investigate the suitability of (144)Ce for both interstitial and intravascular brachytherapy applications. Monte Carlo calculations of radial dose rate distributions in water were performed for (144)Ce in a spring-shaped source and compared with two commonly used interstitial and intravascular sources, (192)Ir and (32)P. The numeric simulations were checked experimentally with a calibrated ionization chamber in a water phantom. Other source characteristics, such as half-life and specific activity, were also compared. For interstitial brachytherapy, (144)Ce presents dosimetric advantages over (192)Ir in terms of higher dose rate at shorter distances and lower irradiation of organs outside the tumor. The source size and shape reduce the anisotropy and the number of dwell positions necessary. The longer half-life of (144)Ce might also be advantageous over (192)Ir. For intravascular brachytherapy, (144)Ce permits the treatment of larger arteries as compared with (32)P, compensates centering errors more effectively, and has a more suitable half-life. The experimental validation showed good agreement (within 10%) with the Monte Carlo simulated dose rate distributions. There are certain potential advantages of using (144)Ce as a source for both interstitial and intravascular brachytherapy. The basis for this finding is provided by the Monte Carlo radial dose rate comparisons with (192)Ir and (32)P, as well as by such characteristics as half-life and specific activity.

Dose perturbation in the presence of metallic implants: treatment planning system versus Monte Carlo simulations

An increasing number of patients receiving radiation therapy have metallic implants such as hip prostheses. Therefore, beams are normally set up to avoid irradiation through the implant; however, this cannot always be accomplished. In such situations, knowledge of the accuracy of the used treatment planning system (TPS) is required. Two algorithms, the pencil beam (PB) and the collapsed cone (CC), are implemented in the studied TPS. Comparisons are made with Monte Carlo simulations for 6 and 18 MV. The studied materials are steel, CoCrMo, Orthinox® (a stainless steel alloy and registered trademark of Stryker Corporation), TiAlV and Ti. Monte Carlo simulated depth dose curves and dose profiles are compared to CC and PB calculated data. The CC algorithm shows overall a better agreement with Monte Carlo than the PB algorithm. Thus, it is recommended to use the CC algorithm to get the most accurate dose calculation both for the planning target volume and for tissues adjacent to the implants when beams are set up to pass through implants.

Monte Carlo dose simulation for intracoronary radiation therapy with a rhenium 188 solution-filled balloon with contrast medium

Background. The therapeutic efficacy of percutaneous transluminal coronary angioplasty is limited by the incidence of restenosis. Intracoronary irradiation has shown to be effective in restenosis control by inhibiting the neointimal proliferation. Methods and Results. Monte Carlo simulation has been performed to calculate the dose to the vessel wall for intracoronary irradiation with a rhenium 188 solution-filled balloon for restenosis inhibition. With a 3-mm-diameter and 30-mm-long balloon, the radiation dose at 1 mm from the balloon surface was 5.3% lower when the balloon structure was included in geometric modeling of the angioplasty catheter, as compared with that obtained by ignoring the structure. The additional dose reduction due to Hexabrix 320 contrast medium added in 30% of volume ratio was 4.7%. With regard to axial dose distribution, the dose was uniform over the balloon length except near the balloon end, at which the dose was reduced by 35% at a 1-mm-deep layer in the vessel wall. With the Re-188 solution mixed with 30% of Hexabrix 320 in volume ratio, the Re-188 activity to be injected for delivery of 15 Gy to the 1-mm-deep layer by 1-minute irradiation was 27.3 GBq/mL. Conclusions. Dose estimates produced in this study should be helpful in determining the Re-188 activity to be injected or the irradiation time for a varying situation in terms of length and diameter of the irradiated arterial segment and depth of the target layer. (J Nucl Cardiol 2002;9:312-18.)

Phenomenological dose model for therapeutic photon beams: basic concepts and definitions.

A model for central-axis absorbed dose in therapeutic photon beams is developed. An expression for absorbed dose in a unit density material, including that in the regions of longitudinal and lateral electronic disequilibrium, is derived. The model is based on the concept of primary and scatter. Primary and scatter dose components are approximated using two identical analytical functions. Monte Carlo simulated dose data for 60Co gamma rays and 15 MV x rays in water are used to test the model. The accuracy of the model is demonstrated.

Range precision of therapeutic proton beams

The precision of proton radiotherapy treatment planning is limited mainly by the precision achieved in the determination of relative stopping power values of human tissue and the accuracy of the analytical dose calculation model. A stoichiometric calibration curve for the conversion from CT values into relative proton stopping power values was defined theoretically and its accuracy was checked by measurements on biological tissue samples. Including the effects of beam hardening on the CT values an upper limit of 2–4 mm for the uncertainty in the position of the distal falloff of the dose distribution was estimated. Different dose calculation algorithms for the spot‐scanning technique have been developed. Their accuracy was evaluated by a comparison to Monte Carlo simulated dose distributions. Large errors due to inaccuracies of the planning algorithm are encountered mainly in the presence of density heterogeneities where the range of the protons is poorly defined. A quick estimation of the expected range dilution as a function of the beam incidence angle can be used to identify critical areas. Besides uncertainties in the treatment planning itself one has to consider the effects of setup uncertainties on the dose distribution. A suggestion is presented on how to deal with uncertainties during the treatment planning process.

An approximation of central‐axis absorbed dose in narrow photon beams

In narrow photon beams of therapeutic energy range, the absorbed dose derived from experimental measurements is subject to a significant error. The error stems from high dose gradients characteristic to small radiation fields and from finite probe dimensions. In this study, a simple model for the narrow-beam absorbed dose is described. It is shown that broad-beam dose data are sufficient to predict a narrow-beam dose. The dose is calculated as a sum of primary and scatter components given in the form of respective analytical functions. For both functions, numerical coefficients are determined in broad-beam geometry. The model is evaluated by comparing calculated dose values with the Monte Carlo simulated narrow-beam dose data for 6 and 15 MV x rays.