Modelling of electron contamination in clinical photon beams for Monte Carlo dose calculation

The purpose of this work is to model electron contamination in clinical photon beams and to commission the source model using measured data for Monte Carlo treatment planning. In this work, a planar source is used to represent the contaminant electrons at a plane above the upper jaws. The source size depends on the dimensions of the field size at the isocentre. The energy spectra of the contaminant electrons are predetermined using Monte Carlo simulations for photon beams from different clinical accelerators. A 'random creep' method is employed to derive the weight of the electron contamination source by matching Monte Carlo calculated monoenergetic photon and electron percent depth-dose (PDD) curves with measured PDD curves. We have integrated this electron contamination source into a previously developed multiple source model and validated the model for photon beams from Siemens PRIMUS accelerators. The EGS4 based Monte Carlo user code BEAM and MCSIM were used for linac head sinulation and dose calculation. The Monte Carlo calculated dose distributions were compared with measured data. Our results showed good agreement (less than 2% or 2 mm) for 6, 10 and 18 MV photon beams.     

Experimental investigation of a fast Monte Carlo photon beam dose calculation algorithm

An experimental verification of the recently developed XVMC code, a fast Monte Carlo algorithm to calculate dose distributions of photon beams in treatment planning, is presented. The treatment head is modelled by a point source with energy distribution (primary photons) and an additional head scatter contribution. Utility software is presented, allowing the determination of the parameters for this model using a single measured depth dose curve in water. The simple beam model is considered to be a starting point for more complex models being planned for future versions of the code. This paper is mainly focused on the influence of the different techniques on variance reduction and material property determination for dose distributions. It is demonstrated that XVMC and the simple beam model reproduce measured (by a diamond detector) relative dose distributions with an accuracy of better than +/-2% in various homogeneous and inhomogeneous phantoms. Furthermore, relative dose distributions in solid state phantoms have been measured by film. Also for these cases, measured and calculated dose distributions agree within experimental uncertainty. The short calculation time (depending on voxel resolution, statistical accuracy, field size and energy, a span of 1 min to 1 h using a present-day personal computer) and an interface to a commercial planning system will allow the implementation of the code for routine treatment planning of clinical electron and photon beams.     

Influence of initial electron beam parameters on Monte Carlo calculated absorbed dose distributions for radiotherapy photon beams

Our aim in the present study was to investigate the effects of initial electron beam characteristics on Monte Carlo calculated absorbed dose distribution for a linac 6 MV photon beam. Moreover, the range of values of these parameters was derived, so that the resulted differences between measured and calculated doses were less than 1%. Mean energy, radial intensity distribution and energy spread of the initial electron beam, were studied. The method is based on absorbed dose comparisons of measured and calculated depth-dose and dose-profile curves. All comparisons were performed at 10.0 cm depth, in the umbral region for dose-profile and for depths past maximum for depth-dose curves. Depth-dose and dose-profile curves were considerably affected by the mean energy of electron beam, with dose profiles to be more sensitive on that parameter. The depth-dose curves were unaffected by the radial intensity of electron beam. In contrast, dose-profile curves were affected by the radial intensity of initial electron beam for a large field size. No influence was observed in dose-profile or depth-dose curves with respect to energy spread variations of electron beam. Conclusively, simulating the radiation source of a photon beam, two of the examined parameters (mean energy and radial intensity) of the electron beam should be tuned accurately, so that the resulting absorbed doses are within acceptable precision. The suggested method of evaluating these crucial but often poorly specified parameters may be of value in the Monte Carlo simulation of linear accelerator photon beams.     

A virtual photon energy fluence model for Monte Carlo dose calculation

The presented virtual energy fluence (VEF) model of the patient-independent part of the medical linear accelerator heads, consists of two Gaussian-shaped photon sources and one uniform electron source. The planar photon sources are located close to the bremsstrahlung target (primary source) and to the flattening filter (secondary source), respectively. The electron contamination source is located in the plane defining the lower end of the filter. The standard deviations or widths and the relative weights of each source are free parameters. Five other parameters correct for fluence variations, i.e., the horn or central depression effect. If these parameters and the field widths in the X and Y directions are given, the corresponding energy fluence distribution can be calculated analytically and compared to measured dose distributions in air. This provides a method of fitting the free parameters using the measurements for various square and rectangular fields and a fixed number of monitor units. The next step in generating the whole set of base data is to calculate monoenergetic central axis depth dose distributions in water which are used to derive the energy spectrum by deconvolving the measured depth dose curves. This spectrum is also corrected to take the off-axis softening into account. The VEF model is implemented together with geometry modules for the patient specific part of the treatment head (jaws, multileaf collimator) into the XVMC dose calculation engine. The implementation into other Monte Carlo codes is possible based on the information in this paper. Experiments are performed to verify the model by comparing measured and calculated dose distributions and output factors in water. It is demonstrated that open photon beams of linear accelerators from two different vendors are accurately simulated using the VEF model. The commissioning procedure of the VEF model is clinically feasible because it is based on standard measurements in air and water. It is also useful for IMRT applications because a full Monte Carlo simulation of the treatment head would be too time-consuming for many small fields.     

A Monte Carlo dose calculation algorithm for proton therapy

A Monte Carlo (MC) code (VMCpro) for treatment planning in proton beam therapy of cancer is introduced. It is based on ideas of the Voxel Monte Carlo algorithm for photons and electrons and is applicable to human tissue for clinical proton energies. In the present paper the implementation of electromagnetic and nuclear interactions is described. They are modeled by a Class II condensed history algorithm with continuous energy loss, ionization, multiple scattering, range straggling, delta-electron transport, nuclear elastic proton nucleus scattering and inelastic proton nucleus reactions. VMCpro is faster than the general purpose MC codes FLUKA by a factor of 13 and GEANT4 by a factor of 35 for simulations in a phantom with inhomogeneities. For dose calculations in patients the speed improvement is larger, because VMCpro has only a weak dependency on the heterogeneity of the calculation grid. Dose distributions produced with VMCpro are in agreement with GEANT4 results. Integrated or broad beam depth dose curves show maximum deviations not larger than 1% or 0.5 mm in regions with large dose gradients for the examples presented here.     

Description and dosimetric verification of the PEREGRINE Monte Carlo dose calculation system for photon beams incident on a water phantom

PEREGRINE is a three-dimensional Monte Carlo dose calculation system written specifically for radiotherapy. This paper describes the implementation and overall dosimetric accuracy of PEREGRINE physics algorithms, beam model, and beam commissioning procedure. Particle-interaction data, tracking geometries, scoring, variance reduction, and statistical analysis are described. The BEAM code system is used to model the treatment-independent accelerator head, resulting in the identification of primary and scattered photon sources and an electron contaminant source. The magnitude of the electron source is increased to improve agreement with measurements in the buildup region in the largest fields. Published measurements provide an estimate of backscatter on monitor chamber response. Commissioning consists of selecting the electron beam energy, determining the scale factor that defines dose per monitor unit, and describing treatment-dependent beam modifiers. We compare calculations with measurements in a water phantom for open fields, wedges, blocks, and a multileaf collimator for 6 and 18 MV Varian Clinac 2100C photon beams. All calculations are reported as dose per monitor unit. Aside from backscatter estimates, no additional, field-specific normalization is included in comparisons with measurements. Maximum discrepancies were less than either 2% of the maximum dose or 1.2 mm in isodose position for all field sizes and beam modifiers.     

A Monte Carlo model for out-of-field dose calculation from high-energy photon therapy

As cancer therapy becomes more efficacious and patients survive longer, the potential for late effects increases, including effects induced by radiation dose delivered away from the treatment site. This out-of-field radiation is of particular concern with high-energy radiotherapy, as neutrons are produced in the accelerator head. We recently developed an accurate Monte Carlo model of a Varian 2100 accelerator using MCNPX for calculating the dose away from the treatment field resulting from low-energy therapy. In this study, we expanded and validated our Monte Carlo model for high-energy (18 MV) photon therapy, including both photons and neutrons. Simulated out-of-field photon doses were compared with measurements made with thermoluminescent dosimeters in an acrylic phantom up to 55 cm from the central axis. Simulated neutron fluences and energy spectra were compared with measurements using moderated gold foil activation in moderators and data from the literature. The average local difference between the calculated and measured photon dose was 17%, including doses as low as 0.01% of the central axis dose. The out-of-field photon dose varied substantially with field size and distance from the edge of the field but varied little with depth in the phantom, except at depths shallower than 3 cm, where the dose sharply increased. On average, the difference between the simulated and measured neutron fluences was 19% and good agreement was observed with the neutron spectra. The neutron dose equivalent varied little with field size or distance from the central axis but decreased with depth in the phantom. Neutrons were the dominant component of the out-of-field dose equivalent for shallow depths and large distances from the edge of the treatment field. This Monte Carlo model is useful to both physicists and clinicians when evaluating out-of-field doses and associated potential risks.     

Comparison of dose calculation algorithms with Monte Carlo methods for photon arcs

The objective of this study is to seek an accurate and efficient method to calculate the dose distribution of a photon arc. The algorithms tested include Monte Carlo, pencil beam kernel (PK), and collapsed cone convolution (CCC). For the Monte Carlo dose calculation, EGS4/DOSXYZ was used. The SRCXYZ source code associated with the DOSXYZ was modified so that the gantry angle of a photon beam would be sampled uniformly within the arc range about an isocenter to simulate a photon arc. Specifically, photon beams (6/18 MV, 4 x 4 and 10 x 10 cm2) described by a phase space file generated by BEAM (MCPHS), or by two point sources with different photon energy spectra (MCDIV) were used. These methods were used to calculate three-dimensional (3-D) distributions in a PMMA phantom, a cylindrical water phantom, and a phantom with lung inhomogeneity. A commercial treatment planning system was also used to calculate dose distributions in these phantoms using equivalent tissue air ratio (ETAR), PK and CCC algorithms for inhomogeneity corrections. Dose distributions for a photon arc in these phantoms were measured using a RK ion chamber and radiographic films. For homogeneous phantoms, the measured results agreed well (approximately 2% error) with predictions by the Monte Carlo simulations (MCPHS and MCDIV) and the treatment planning system for the 180 degrees and 360 degrees photon arcs. For the dose distribution in the phantom with lung inhomogeneity with a 90 degrees photon arc, the Monte Carlo calculations agreed with the measurements within 2%, while the treatment planning system using ETAR, PK and CCC underestimated or overestimated the dose inside the lung inhomogeneity from 6% to 12%.     

Investigation of variance reduction techniques for Monte Carlo photon dose calculation using XVMC

Several variance reduction techniques, such as photon splitting, electron history repetition, Russian roulette and the use of quasi-random numbers are investigated and shown to significantly improve the efficiency of the recently developed XVMC Monte Carlo code for photon beams in radiation therapy. It is demonstrated that it is possible to further improve the efficiency by optimizing transpon parameters such as electron energy cut-off, maximum electron energy step size, photon energy cut-off and a cut-off for kerma approximation, without loss of calculation accuracy. These methods increase the efficiency by a factor of up to 10 compared with the initial XVMC ray-tracing technique or a factor of 50 to 80 compared with EGS4/PRESTA. Therefore, a common treatment plan (6 MV photons, 10 x 10 cm2 field size, 5 mm voxel resolution, 1% statistical uncertainty) can be calculated within 7 min using a single CPU 500 MHz personal computer. If the requirement on the statistical uncertainty is relaxed to 2%, the calculation time will be less than 2 min. In addition, a technique is presented which allows for the quantitative comparison of Monte Carlo calculated dose distributions and the separation of systematic and statistical errors. Employing this technique it is shown that XVMC calculations agree with EGSnrc on a sub-per cent level for simulations in the energy and material range of interest for radiation therapy.     

Fast Monte Carlo simulation for patient-specific CT/CBCT imaging dose calculation

X-ray imaging dose from computed tomography (CT) or cone beam CT (CBCT) scans has become a serious concern. Patient-specific imaging dose calculation has been proposed for the purpose of dose management. While Monte Carlo (MC) dose calculation can be quite accurate for this purpose, it suffers from low computational efficiency. In response to this problem, we have successfully developed a MC dose calculation code, gCTD, on GPU architecture under the NVIDIA CUDA platform for fast and accurate estimation of the x-ray imaging dose received by a patient during a CT or CBCT scan. Techniques have been developed particularly for the GPU architecture to achieve high computational efficiency. Dose calculations using CBCT scanning geometry in a homogeneous water phantom and a heterogeneous Zubal head phantom have shown good agreement between gCTD and EGSnrc, indicating the accuracy of our code. In terms of improved efficiency, it is found that gCTD attains a speed-up of ～400 times in the homogeneous water phantom and ～76.6 times in the Zubal phantom compared to EGSnrc. As for absolute computation time, imaging dose calculation for the Zubal phantom can be accomplished in ～17 s with the average relative standard deviation of 0.4%. Though our gCTD code has been developed and tested in the context of CBCT scans, with simple modification of geometry it can be used for assessing imaging dose in CT scans as well.     

Experimental verification and clinical implementation of a commercial Monte Carlo electron beam dose calculation algorithm

This study describes the modeling and the experimental verification and clinical implementation of the alpha release of Pinnacle3 Monte Carlo (MC) electron beam dose calculation algorithm for patient-specific treatment planning. The MC electron beam modeling was performed for beam energies ranging from 6 to 18 MeV from a Siemens (Primus) linear accelerator using standard-shaped electron applicators and 100 cm source-to-surface distance (SSD). The agreement between MC calculations and measurements was, on average, within 2% and 2 mm for all applicator sizes. However, differences of the order of 3%-4% were noted in the off-axis dose profiles for the largest applicator modeled and for all energies. Output factors were calculated for standard electron cones and square cutouts inserted in the 10 x 10 cm2 applicator for different SSDs and were found to be within 4% of measured data. Experimental verification of the MC electron beam model was carried out using an ionization chamber and film in solid-water slab and anthropomorphic phantoms containing bone and lung materials. Agreement between calculated and measured dose distributions was within +/-3%. Clinical comparison was performed in four patient treatment plans with lesions in highly irregular anatomies, such as the ear, face, and breast, where custom-designed bolus and field shaping blocks were used in the patient treatments. For comparison purposes, treatment planning was also performed using the conventional pencil beam (PB) algorithm with the Pinnacle3 treatment planning system. Differences between MC and PB dose calculations for the patient treatment plans were significant, particularly in anatomies where the target was in close proximity to low density tissues, such as lung and air cavities. Concerning monitor unit calculations, the largest differences obtained between MC and PB algorithms were between 4.0% and 5.0% for two patients treated with oblique beams and involving highly irregular surfaces, i.e., breast and cheek. Clinical results are reported for overall uncertainty values (averaged over voxels with doses >50% dosemax) ranging from 2% to 0.3% and calculations were performed using cubic voxels with side 0.3 cm. Timing values ranged from 2 min to 24.5 h, depending on the field size, beam energy, number, and thickness of computed tomography slices used to define the patient's anatomy for the overall uncertainty values mentioned above.     

Monte Carlo modelling of external radiotherapy photon beams

An essential requirement for successful radiation therapy is that the discrepancies between dose distributions calculated at the treatment planning stage and those delivered to the patient are minimized. An important component in the treatment planning process is the accurate calculation of dose distributions. The most accurate way to do this is by Monte Carlo calculation of particle transport, first in the geometry of the external or internal source followed by tracking the transport and energy deposition in the tissues of interest. Additionally, Monte Carlo simulations allow one to investigate the influence of source components on beams of a particular type and their contaminant particles. Since the mid 1990s, there has been an enormous increase in Monte Carlo studies dealing specifically with the subject of the present review, i.e., external photon beam Monte Carlo calculations, aided by the advent of new codes and fast computers. The foundations for this work were laid from the late 1970s until the early 1990s. In this paper we will review the progress made in this field over the last 25 years. The review will be focused mainly on Monte Carlo modelling of linear accelerator treatment heads but sections will also be devoted to kilovoltage x-ray units and 60Co teletherapy sources.     

Magnetic fields with photon beams: dose calculation using electron multiple-scattering theory

Strong transverse magnetic fields can produce large dose enhancements and reductions in localized regions of a patient under irradiation by a photon beam. We have developed a new equation of motion for the transport of charged particles in an arbitrary magnetic field, incorporating both energy loss and multiple scattering. Key to modeling the latter process is a new concept, that of "typical scattered particles." The formulas which we have arrived at are particularly applicable to the transport of, and deposition of energy by, Compton electrons and pair-production electrons and positrons generated within a medium by a photon beam, and we have shown qualitatively how large dose enhancements and reductions can occur. A companion article examines this dose modification effect through systematic Monte Carlo simulations.     

A virtual-accelerator-based verification of a Monte Carlo dose calculation algorithm for electron beam treatment planning in homogeneous phantoms

By introducing Monte Carlo (MC) techniques to the verification procedure of dose calculation algorithms in treatment planning systems (TPSs), problems associated with conventional measurements can be avoided and properties that are considered unmeasurable can be studied. The aim of the study is to implement a virtual accelerator, based on MC simulations, to evaluate the performance of a dose calculation algorithm for electron beams in a commercial TPS. The TPS algorithm is MC based and the virtual accelerator is used to study the accuracy of the algorithm in water phantoms. The basic test of the implementation of the virtual accelerator is successful for 6 and 12 MeV (gamma < 1.0, 0.02 Gy/2 mm). For 18 MeV, there are problems in the profile data for some of the applicators, where the TPS underestimates the dose. For fields equipped with patient-specific inserts, the agreement is generally good. The exception is 6 MeV where there are slightly larger deviations. The concept of the virtual accelerator is shown to be feasible and has the potential to be a powerful tool for vendors and users.     

Full forward Monte Carlo calculation of portal dose from MLC collimated treatment beams

This work deals with a full Monte Carlo (MC) simulation of a radiotherapy treatment facility including a multi-leaf collimator (MLC) and electronic portal imaging device (EPID). A method for a planar calibration of the EPID response in terms of dose using the MC technique is presented. Calibration measurements and simulations with several blocks of attenuating material are carried out down to approximatively 5% of the open field transmitted dose. A linear relationship is shown between the squared EPID signal and the MC calculated dose. The calibrated EPID was used as a dosimetric system to validate a MC model for the MLC. Computations and measurements agreed within 2% of dose difference (or 2 mm in regions of high dose gradient). The technique described herein is not significantly limited by physics transport model constraints. Therefore it can potentially provide a more accurate verification of dose delivery to inhomogeneous anatomical regions in patients undergoing complex multi-field conformal or intensity-modulated radiation therapy.     

First macro Monte Carlo based commercial dose calculation module for electron beam treatment planning--new issues for clinical consideration

The purpose of this study is to present our experience of commissioning, testing and use of the first commercial macro Monte Carlo based dose calculation algorithm for electron beam treatment planning and to investigate new issues regarding dose reporting (dose-to-water versus dose-to-medium) as well as statistical uncertainties for the calculations arising when Monte Carlo based systems are used in patient dose calculations. All phantoms studied were obtained by CT scan. The calculated dose distributions and monitor units were validated against measurements with film and ionization chambers in phantoms containing two-dimensional (2D) and three-dimensional (3D) type low- and high-density inhomogeneities at different source-to-surface distances. Beam energies ranged from 6 to 18 MeV. New required experimental input data for commissioning are presented. The result of validation shows an excellent agreement between calculated and measured dose distributions. The calculated monitor units were within 2% of measured values except in the case of a 6 MeV beam and small cutout fields at extended SSDs (>110 cm). The investigation on the new issue of dose reporting demonstrates the differences up to 4% for lung and 12% for bone when 'dose-to-medium' is calculated and reported instead of 'dose-to-water' as done in a conventional system. The accuracy of the Monte Carlo calculation is shown to be clinically acceptable even for very complex 3D-type inhomogeneities. As Monte Carlo based treatment planning systems begin to enter clinical practice, new issues, such as dose reporting and statistical variations, may be clinically significant. Therefore it is imperative that a consistent approach to dose reporting is used.     

Evaluation of the first commercial Monte Carlo dose calculation engine for electron beam treatment planning

The purpose of this study is to perform a clinical evaluation of the first commercial (MDS Nordion, now Nucletron) treatment planning system for electron beams incorporating Monte Carlo dose calculation module. This software implements Kawrakow's VMC++ voxel-based Monte Carlo calculation algorithm. The accuracy of the dose distribution calculations is evaluated by direct comparisons with extensive sets of measured data in homogeneous and heterogeneous phantoms at different source-to-surface distances (SSDs) and gantry angles. We also verify the accuracy of the Monte Carlo module for monitor unit calculations in comparison with independent hand calculations for homogeneous water phantom at two different SSDs. All electron beams in the range 6-20 MeV are from a Siemens KD-2 linear accelerator. We used 10,000 or 50,000 histories/cm2 in our Monte Carlo calculations, which led to about 2.5% and 1% relative standard error of the mean of the calculated dose. The dose calculation time depends on the number of histories, the number of voxels used to map the patient anatomy, the field size, and the beam energy. The typical run time of the Monte Carlo calculations (10,000 histories/cm2) is 1.02 min on a 2.2 GHz Pentium 4 Xeon computer for a 9 MeV beam, 10 x 10 cm2 field size, incident on the phantom 15 x 15 x 10 cm3 consisting of 31 CT slices and voxels size of 3 x 3 x 3 mm3 (total of 486,720 voxels). We find good agreement (discrepancies smaller than 5%) for most of the tested dose distributions. We also find excellent agreement (discrepancies of 2.5% or less) for the monitor unit calculations relative to the independent manual calculations. The accuracy of monitor unit calculations does not depend on the SSD used, which allows the use of one virtual machine for each beam energy for all arbitrary SSDs. In some cases the test results are found to be sensitive to the voxel size applied such that bigger systematic errors (>5%) occur when large voxel sizes interfere with the extensions of heterogeneities or dose gradients because of differences between the experimental and calculated geometries. Therefore, user control over voxelization is important for high accuracy electron dose calculations.     

基于GAMOS的蒙特卡罗方法模拟放疗电子线照射的剂量学研究

目的:探讨基于GAMOS的蒙特卡罗（MC）方法模拟电子线放疗的剂量精确性。方法:运用GAMOS MC程序,建立Varian Rapidarc加速器3档能量（6、9和12 MeV）及3种限光筒［（6×6）、（10×10）和（15×15） cm2］的束流模型,模拟束流在水模体中的剂量分布,并与测量得到的百分深度剂量和等平面剂量分布比较,评估GAMOS软件模拟电子线照射的精确性和运算效率。结果:模拟的粒子数越多,模拟与测量结果的误差越小;当模拟粒子的数量达到5×108时,各个能量的电子线射程（Rp）和50%剂量深度（R50）的模拟结果与测量结果一致;除建成区外,百分深度剂量模拟和测量的结果误差在2%以内;等平面剂量分布模拟和测量的结果误差也在2%以内,模拟的照射野大小与测量结果一致。运算效率中,能量越大,限光筒尺寸越大,并行同步模拟所用的时间越多,模拟时间的变化越大。结论:基于GAMOS的MC方法可准确地模拟放疗电子线照射剂量的分布,粒子数的增加可提高模拟的精确性,并行同步计算可提高模拟的效率。     

Validation of Monte Carlo calculated surface doses for megavoltage photon beams

Recent work has shown that there is significant uncertainty in measuring build-up doses in mega-voltage photon beams especially at high energies. In this present investigation we used a phantom-embedded extrapolation chamber (PEEC) made of Solid Water to validate Monte Carlo (MC)-calculated doses in the dose build-up region for 6 and 18 MV x-ray beams. The study showed that the percentage depth ionizations (PDIs) obtained from measurements are higher than the percentage depth doses (PDDs) obtained with Monte Carlo techniques. To validate the MC-calculated PDDs, the design of the PEEC was incorporated into the simulations. While the MC-calculated and measured PDIs in the dose build-up region agree with one another for the 6 MV beam, a non-negligible difference is observed for the 18 MV x-ray beam. A number of experiments and theoretical studies of various possible effects that could be the source of this discrepancy were performed. The contribution of contaminating neutrons and protons to the build-up dose region in the 18 MV x-ray beam is negligible. Moreover, the MC calculations using the XCOM photon cross-section database and the NIST bremsstrahlung differential cross section do not explain the discrepancy between the MC calculations and measurement in the dose build-up region for the 18 MV. A simple incorporation of triplet production events into the MC dose calculation increases the calculated doses in the build-up region but does not fully account for the discrepancy between measurement and calculations for the 18 MV x-ray beam.     

GPU-based fast Monte Carlo simulation for radiotherapy dose calculation

Monte Carlo (MC) simulation is commonly considered to be the most accurate dose calculation method in radiotherapy. However, its efficiency still requires improvement for many routine clinical applications. In this paper, we present our recent progress toward the development of a graphics processing unit (GPU)-based MC dose calculation package, gDPM v2.0. It utilizes the parallel computation ability of a GPU to achieve high efficiency, while maintaining the same particle transport physics as in the original dose planning method (DPM) code and hence the same level of simulation accuracy. In GPU computing, divergence of execution paths between threads can considerably reduce the efficiency. Since photons and electrons undergo different physics and hence attain different execution paths, we use a simulation scheme where photon transport and electron transport are separated to partially relieve the thread divergence issue. A high-performance random number generator and a hardware linear interpolation are also utilized. We have also developed various components to handle the fluence map and linac geometry, so that gDPM can be used to compute dose distributions for realistic IMRT or VMAT treatment plans. Our gDPM package is tested for its accuracy and efficiency in both phantoms and realistic patient cases. In all cases, the average relative uncertainties are less than 1%. A statistical t-test is performed and the dose difference between the CPU and the GPU results is not found to be statistically significant in over 96% of the high dose region and over 97% of the entire region. Speed-up factors of 69.1 ～ 87.2 have been observed using an NVIDIA Tesla C2050 GPU card against a 2.27 GHz Intel Xeon CPU processor. For realistic IMRT and VMAT plans, MC dose calculation can be completed with less than 1% standard deviation in 36.1 ～ 39.6 s using gDPM.