Carlo Based Treatment Planning System Research Articles

Evaluation of the material assignment method used by a Monte Carlo treatment planning system

An evaluation of the conversion process from Hounsfield units (HU) to material composition in computerised tomography (CT) images, employed by the Monte Carlo based treatment planning system ISOgray™ (DOSIsoft), is presented. A boundary in the HU for the material conversion between “air” and “lung” materials was determined based on a study using 22 patients. The dosimetric consequence of the new boundary was quantitatively evaluated for a lung patient plan.

WE‐C‐BRB‐06: Antiproton Radiotherapy: Development of Physically and Biologically Optimized Monte Carlo Treatment Planning Systems for Intensity and Energy Modulated Delivery

Purpose: Antiprotons have become of interest in radiotherapy due to their higher peak to plateau dose ratio relative to protons and carbon ions, and a beneficial increase in RBE towards the Bragg peak as recently verified by experimental investigations of the AD‐4 collaboration at CERN. An obstacle limiting further research is the lack of a treatment planning system capable of concurrently optimizing the necessary modulation of intensity and energy, while accounting for the variation in biological effectiveness. Here we develop a Monte Carlo based treatment planning system for this purpose and subsequently quantify its performance. Materials and Methods: Dose kernels corresponding to different energy and source configurations were calculated using MCNPX in phantom and voxelized patient CT scans, and then converted to biological equivalent dose using depth dependent RBE weighting factors derived from theory and experiment. Linear equations were formulated for each pixel representing superposition of different kernels weighted by unknown intensities. Algorithms using constrained least square and gradient descent optimization were developed to minimize objective functions measuring the geometric correlation of the planning target volume (PTV) with the calculated distribution, yielding an optimized intensity for beams as function of energy and direction. Results: Biologically optimized treatment plans implemented on a voxelized 38 year old human were in good agreement with the input PTVs, reproducing the PTVs with a mean error of less than 2.24%. Proof of principle demonstrations were successful in producing complicated structures, such resemblance of Einstein, in water phantom with a correlation greater than 93%. Conclusions: We have developed a Monte Carlo treatment planning system for energy and intensity modulated antiproton therapy capable of incorporating depth‐dependency of the RBE, and reproducing complicated PTVs with high accuracy. The work can be readily extended to incorporate more sophisticated objective functions such as NTCP and TCP functionals.

Evaluation of a commercial VMC++ Monte Carlo based treatment planning system for electron beams using EGSnrc/BEAMnrc simulations and measurements

In the present work, Monte Carlo (MC) models of electron beams (energies 4, 12 and 18 MeV) from an Elekta SL25 medical linear accelerator were simulated using EGSnrc/BEAMnrc user code. The calculated dose distributions were benchmarked by comparison with measurements made in a water phantom for a wide range of open field sizes and insert combinations, at a single source-to-surface distance (SSD) of 100 cm. These BEAMnrc models were used to evaluate the accuracy of a commercial MC dose calculation engine for electron beam treatment planning (Oncentra MasterPlan Treament Planning System (OMTPS) version 1.4, Nucletron) for two energies, 4 and 12 MeV. Output factors were furthermore measured in the water phantom and compared to BEAMnrc and OMTPS. The overall agreement between predicted and measured output factors was comparable for both BEAMnrc and OMTPS, except for a few asymmetric and/or small insert cutouts, where larger deviations between measurements and the values predicted from BEAMnrc as well as OMTPS computations were recorded. However, in the heterogeneous phantom, differences between BEAMnrc and measurements ranged from 0.5 to 2.0% between two ribs and 0.6–1.0% below the ribs, whereas the range difference between OMTPS and measurements was the same (0.5–4.0%) in both areas. With respect to output factors, the overall agreement between BEAMnrc and measurements was usually within 1.0% whereas differences up to nearly 3.0% were observed for OMTPS. This paper focuses on a comparison for clinical cases, including the effects of electron beam attenuations in a heterogeneous phantom. It, therefore, complements previously reported data (only based on measurements) in one other paper on commissioning of the VMC++ dose calculation engine. These results demonstrate that the VMC++ algorithm is more robust in predicting dose distribution than Pencil beam based algorithms for the electron beams investigated.

Sci-Fri PM: Planning-06: Verification of a Monte Carlo based treatment planning system in a homogeneous water phantom.

In this abstract we present the results of the verification of the XVMC based treatment planning system "PrecisePLAN" (release 2.50; Elekta). We investigated a 6 MV photon beam from an Elekta Synergy linear accelerator in a homogeneous water phantom. Measurements of profiles, percentage depth dose curves (pdds) and relative output factors (ROFs) for square fields of sides 3, 5, 7, 10, 15, 20 and 30 cm were compared to calculations performed with PrecisePLAN and simulations with BEAMnrc and DOSXYZnrc. Following recommendations from the literature we investigated the deviations between measurements and calculations of the profiles and pdds in four regions and checked for the criterion of acceptability (given in parenthesis): the inner region (3 %, 2 % at the central axis), the penumbra (2 mm distance to agreement), the outer region (3 %) and the build up region (2 mm). The mean deviation +/- 1.5 times the standard deviation should lie within the confidence interval of the thresholds quoted above. For the ROFs we found very good agreement with the measurement. The deviations are lower than two and one percent for PrecisePLAN and BEAMnrc, respectively, for the investigated field sizes. Also most of the profiles and pdds fulfill the criteria of acceptability given above, but for the large field sizes some of the profiles and pdds fall outside the threshold values. The overall agreement between measurements and calculations, especially for the clinically relevant situations in modern conformal/IMRT techniques, is clinically acceptable.

Poster - Thurs Eve-05: An assessment of PDDs and outputs predicted by a Monte Carlo-based treatment planning system for electron beams.

Monte Carlo simulation is currently considered to be the most accurate method of calculating dose distributions for electron beam therapy, and commercial treatment planning software using simplified macro Monte Carlo is available for electron treatment planning. In this work, Eclipse V8.1.18 is being investigated in preparation for the clinical use of CT-based electron treatment planning. Water tank measurements of percentage depth doses (PDDs) and absolute outputs at depth of maximum dose (Zmax ) under different geometric conditions are compared to the results calculated by Eclipse. The measurements are carried out for a range of electron energies (6, 9, 12, and 16 MeV) for the standard open field (10×10 cm2 ) and for circular cutouts (2, 3, and 6 cm diameters) at SSD of 100 cm. In addition, extended SSDs (105 and 110 cm) and oblique beam incident (gantry 345 degree) for the open field and 3 cm diameter cutout are measured and compared to Eclipse. For PDDs, the results predicted by Eclipse are generally acceptable, falling mostly within 5% of those measured in water. For output, the results predicted by Eclipse are similar, falling mostly within 3% of those measured in water. We observed the greatest differences between Eclipse and measurements near the water surface and in high dose gradients for PDDs. A similar observation is noted for a small field in the case of outputs.

SU‐GG‐T‐322: Determination of the Optimum Statistical Uncertainty and Voxel Size to Perform Monte Carlo Based Dose Calculation in the Context of Radiotherapy

Purpose: To evaluate the influence of statistical uncertainty and voxel size on target dose volume histograms (DVH) calculated with a Monte Carlo based treatment planning system (TPS). To determine the maximum uncertainty level and voxel size acceptable in the context of external beam radiotherapy. Method and Materials: First, the effect of increasing voxel sizes was investigated by studying the display of material and mass density matrices related to patient CT images. Then, the dose calculations were performed using the TPS ISOgray™ (Dosisoft) based on a fast Monte Carlo simulation code called PENFAST. The dose deposition due to electron beams was calculated for one breast plan. Intramammary node DVH were calculated first for increasing statistical uncertainty (1σ) levels of 0.7%, 1%, 1.3%, 2%, 3% and 6%, and secondly by increasing voxel sizes from 2×4×2 mm3 to 8×8×8 mm3. The same study was conducted for one medulloblastoma radiotherapy plan. Spinal cord DVH were calculated for uncertainty levels (1σ) of 1.3%, 1.7%, 2%, 3%, 5%, 12% and voxel sizes from 2.1×3×2.1 mm3 to 5.7×6×5.7 mm3. Results: The consequence of increasing voxel sizes is the blurring of material and mass density matrices by averaging values contained in neighbouring voxels which must be avoided at the interface between heterogeneities. The study of the DVH for both breast and medulloblastoma plans showed that they were not modified for statistical uncertainty levels (1σ) below 2% and voxel sizes below 4×4×4 mm3. Conclusion: This work indicates that, in clinical context, to save computation time while keeping the reliability of the target DVH for clinical use, Monte Carlo based dose calculation can be performed with uncertainty level (1σ) below 2% and voxel size which must not be greater than 4×4×4 mm3.This work was performed in the framework of the Project MAESTRO (IP CE503564) granted by the European Commission.

WE-B-T-6E-01: Source Modelling and Beam Commissioning for Monte Carlo Treatment Planning

Source modeling and beam commissioning are important components of a Monte Carlo based treatment planning system. In this presentation, we will review the current status of Monte Carlo simulations of clinical photon and electron beams and the theories and methodologies used in particle phase space representation and reconstruction for Monte Carlo dose calculation. We will discuss the sensitivity of beam characterization to simulation details, such as beam energy, angle, intensity, and details of the treatment head design. We will review different source models for photon and electron beam characterization and discuss the accuracy and efficiency tradeoffs between full phase space and simplified source models. We will describe the methods and software that have been developed for source modeling and beam commissioning for the clinical implementation of the Monte Carlo method for treatment planning and beam delivery verification. We will present different methods for source parameterization based on simulated phase space data and a standard set of measured beam data including in-air and in-phantom output factors and in-phantom dose distributions. We will discuss problems associated with CT scanning and phantoms that can affect Monte Carlo accuracy. We will describe requirements and procedures for commissioning and acceptance testing of Monte Carlo dose calculation software for radiotherapy treatment planning. Educational Objectives: 1. Introduce the Monte Carlo method for clinical photon and electron beam simulations 2. Review theories and methodologies for phase space representation and reconstruction 3. Present different source models for Monte Carlo dose calculation 4. Describe different methods for source parameterization and beam commissioning 5. Provide requirements and guidelines for the clinical testing of Monte Carlo dose calculation software for radiotherapy treatment planning

Clinical use of a commercial Monte Carlo treatment planning system for electron beams

In 2002 we fully implemented clinically a commercial Monte Carlo based treatment planning system for electron beams. The software, developed by MDS Nordion (presently Nucletron), is based on Kawrakow's VMC++ algorithm. The Monte Carlo module is integrated with our Theraplan Plus™ treatment planning system. An extensive commissioning process preceded clinical implementation of this software. Using a single virtual ‘machine’ for each electron beam energy, we can now calculate very accurately the dose distributions and the number of MU for any arbitrary field shape and SSD. This new treatment planning capability has significantly impacted our clinical practice. Since we are more confident of the actual dose delivered to a patient, we now calculate accurate three-dimensional (3D) dose distributions for a greater variety of techniques and anatomical sites than we have in the past. We use the Monte Carlo module to calculate dose for head and neck, breast, chest wall and abdominal treatments with electron beams applied either solo or in conjunction with photons. In some cases patient treatment decisions have been changed, as compared to how such patients would have been treated in the past. In this paper, we present the planning procedure and some clinical examples.

Optimization of combined electron and photon beams for breast cancer

Recently, intensity-modulated radiation therapy and modulated electron radiotherapy have gathered a growing interest for the treatment of breast and head and neck tumours. In this work, we carried out a study to combine electron and photon beams to achieve differential dose distributions for multiple target volumes simultaneously. A Monte Carlo based treatment planning system was investigated, which consists of a set of software tools to perform accurate dose calculation, treatment optimization, leaf sequencing and plan analysis. We compared breast treatment plans generated using this home-grown optimization and dose calculation software for different treatment techniques. Five different planning techniques have been developed for this study based on a standard photon beam whole breast treatment and an electron beam tumour bed cone down. Technique 1 includes two 6 MV tangential wedged photon beams followed by an anterior boost electron field. Technique 2 includes two 6 MV tangential intensity-modulated photon beams and the same boost electron field. Technique 3 optimizes two intensity-modulated photon beams based on a boost electron field. Technique 4 optimizes two intensity-modulated photon beams and the weight of the boost electron field. Technique 5 combines two intensity-modulated photon beams with an intensity-modulated electron field. Our results show that technique 2 can reduce hot spots both in the breast and the tumour bed compared to technique 1 (dose inhomogeneity is reduced from 34% to 28% for the target). Techniques 3, 4 and 5 can deliver a more homogeneous dose distribution to the target (with dose inhomogeneities for the target of 22%, 20% and 9%, respectively). In many cases techniques 3, 4 and 5 can reduce the dose to the lung and heart. It is concluded that combined photon and electron beam therapy may be advantageous for treating breast cancer compared to conventional treatment techniques using tangential wedged photon beams followed by a boost electron field.

107 oral Validation of monitor unit calculations at arbitrary source-to-surface distance for the new commercial Monte Carlo based treatment planning system for electron beams

107 oral Validation of monitor unit calculations at arbitrary source-to-surface distance for the new commercial Monte Carlo based treatment planning system for electron beams

A comparative dosimetric study on tangential photon beams, intensity-modulated radiation therapy (IMRT) and modulated electron radiotherapy (MERT) for breast cancer treatment

Recently, energy- and intensity-modulated electron radiotherapy (MERT) has garnered a growing interest for the treatment of superficial targets. In this work, we carried out a comparative dosimetry study to evaluate MERT, photon beam intensity-modulated radiation therapy (IMRT) and conventional tangential photon beams for the treatment of breast cancer. A Monte Carlo based treatment planning system has been investigated, which consists of a set of software tools to perform accurate dose calculation, treatment optimization, leaf sequencing and plan analysis. We have compared breast treatment plans generated using this home-grown treatment optimization and dose calculation software for these treatment techniques. The MERT plans were planned with up to two gantry angles and four nominal energies (6, 9, 12 and 16 MeV). The tangential photon treatment plans were planned with 6 MV wedged photon beams. The IMRT plans were planned using both multiple-gantry 6 MV photon beams or two 6 MV tangential beams. Our results show that tangential IMRT can reduce the dose to the lung, heart and contralateral breast compared to conventional tangential wedged beams (up to 50% reduction in high dose volume or 5 Gy in the maximum dose). MERT can reduce the maximum dose to the lung by up to 20 Gy and to the heart by up to 35 Gy compared to conventional tangential wedged beams. Multiple beam angle IMRT can significantly reduce the maximum dose to the lung and heart (up to 20 Gy) but it induces low and medium doses to a large volume of normal tissues including lung, heart and contralateral breast. It is concluded that MERT has superior capabilities to achieve dose conformity both laterally and in the depth direction, which will be well suited for treating superficial targets such as breast cancer.

Monte Carlo based treatment planning for modulated electron beamradiation therapy

A Monte Carlo based treatment planning system for modulated electronradiation therapy (MERT) is presented. This new variation ofintensity modulated radiation therapy (IMRT) utilizes an electron multileafcollimator (eMLC) to deliver non-uniform intensity maps at several electron energies.In this way, conformal dose distributions are delivered to irregulartargets located a few centimetres below the surface while sparingdeeper-lying normal anatomy. Planning for MERT begins with Monte Carlo generationof electron beamlets. Electrons are transported with proper in-airscattering and the dose is tallied in the phantom for each beamlet.An optimized beamlet plan may be calculated using inverse-planning methods.Step-and-shoot leaf sequences are generated for the intensity maps anddose distributions recalculated using Monte Carlo simulations. Here,scatter and leakage from the leaves are properly accounted for bytransporting electrons through the eMLC geometry. The weights for thesegments of the plan are re-optimized with the leaf positions fixed andbremsstrahlung leakage and electron scatter doses included. Thisoptimization gives the final optimized plan. It is shown that asignificant portion of the calculation time is spent transportingparticles in the leaves. However, this is necessary since optimizingsegment weights based on a model in which leaf transport is ignoredresults in an improperly optimized plan with overdosing of target andcritical structures. A method of rapidly calculating the bremsstrahlungcontribution is presented and shown to be an efficient solution to thisproblem. A homogeneous model target and a 2D breast plan are presented.The potential use of this tool in clinical planning is discussed.

Optimal electron-beam treatment planning for retinoblastoma using a new three-dimensional Monte Carlo-based treatment planning system.

Electron-beam treatment planning for retinoblastoma was investigated and an optimal treatment plan was devised for a particular case using a new three-dimensional Monte Carlo-based treatment planning system known to be capable of correctly predicting dose perturbations caused by body surface obliquities and tissue heterogeneities. Computed tomography (CT) data files were used to construct a three-dimensional eye phantom representing the anatomy of a child's orbit. Dose distributions in sagittal, transverse, and coronal planes were predicted with 1-mm resolution. Study of these distributions led to an optimal treatment plan consisting of an anterior-lateral pair, with the anterior field being a 10-MeV, 30-mm-diam circular field, centrally blocked by a 10-mm-diam lucite lens shield and the lateral field being a 16-MeV, 30 x 25-mm D-shaped field. The anterior field delivers a therapeutic dose to the ora serrata, but it underdoses the posterior retinal surface behind the lens shield; the lateral field provides the necessary boost dose to the posterior retinal surface. An equally weighted combination of the two fields produces a dose distribution in which the entire retinal surface receives a therapeutic dose, with less than 10% of that dose being delivered to the lens, brain, and the contralateral orbit.