Modeling photon output caused by backscattered radiation into the monitor chamber from collimator jaws using a Monte Carlo technique

PENELOPE simulations and experiment for 6 MV clinac iX accelerator for standard and small static fields

The goal of this work was to produce accurate data for use as a 'gold standard' and a valid tool for measurements in reference dosimetry for standard/small static field sizes from 0.5 × 0.5 to 10 × 10 cm². It is based on the accuracy of the phase space files (PSFs) as a key quantity. Because the IAEA general public database provides few PSFs for the Varian iX, we simulated the head through Monte Carlo (MC) simulations and calculated validated PSFs for 12 square field sizes including seven for small static fields. The resulting dosimetric calculations allowed us to reach a good level of agreement in comparison to our relative and absolute dose measurements performed on a Varian iX in water phantom. Measured and MC calculated output factors were investigated for different detectors. Based on the TRS 483 formalism and MC (PENELOPE/penEasy), we calculated output correction factors for the unshielded Diode-E (T60017) and the PinPoint-3D (T31016) micro-chamber according to manufacturers' blueprints. Our MC results were in agreement with the recommended data; they compete with recent measurements and MC simulations and in particular the TRS 483 MC data obtained from similar simulations. Moreover, our MC results provide supplemental data in comparison to TRS 483 data in particular for the PinPoint-3D (T31016). We suggest our MC output correction factors as new datasets for future TRS compilations. The work was substantial, used different robust MC strategies depending on the scoring regions, and led in most cases to uncertainties of less than 1%.

Monte Carlo calculations of radiotherapy dose in "homogeneous" anatomy

Given the substantial literature on the use of Monte Carlo (MC) simulations to verify treatment planning system (TPS) calculations of radiotherapy dose in heterogeneous regions, such as head and neck and lung, this study investigated the potential value of running MC simulations of radiotherapy treatments of nominally homogeneous pelvic anatomy. A pre-existing in-house MC job submission and analysis system, built around BEAMnrc and DOSXYZnrc, was used to evaluate the dosimetric accuracy of a sample of 12 pelvic volumetric arc therapy (VMAT) treatments, planned using the Varian Eclipse TPS, where dose was calculated with both the Analytical Anisotropic Algorithm (AAA) and the Acuros (AXB) algorithm. In-house TADA (Treatment And Dose Assessor) software was used to evaluate treatment plan complexity, in terms of the small aperture score (SAS), modulation index (MI) and a novel exposed leaf score (ELS/ELA). Results showed that the TPS generally achieved closer agreement with the MC dose distribution when treatments were planned for smaller (single-organ) targets rather than larger targets that included nodes or metastases. Analysis of these MC results with reference to the complexity metrics indicated that while AXB was useful for reducing dosimetric uncertainties associated with density heterogeneity, the residual TPS dose calculation uncertainties resulted from treatment plan complexity and TPS model simplicity. The results of this study demonstrate the value of using MC methods to recalculate and check the dose calculations provided by commercial radiotherapy TPSs, even when the treated anatomy is assumed to be comparatively homogeneous, such as in the pelvic region.

Clinical implementation of a Monte Carlo based independent TPS dose checking system

The increase in complexity of treatment plans over time through modalities such as intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) has often not been met with an increase in capability of the secondary dose calculation checking systems typically used to verify the treatment planning system. Monte Carlo (MC) codes such as EGSnrc have become easily available and are capable of performing calculations of highly complex radiotherapy treatments. This educational note demonstrates a method for implementing and using a fully automated system for performing and analysing full MC calculations of conformal, IMRT and VMAT radiotherapy plans. Example calculations were based on BEAMnrc/DOSXYZnrc and are performed automatically after either uploading exported plan DICOM data through a Python-based web interface, or exporting DICOM data to a monitored network location. This note demonstrates how completed MC calculations can then be analysed using an automatically generated dose point comparison report, or easily re-imported back into the treatment planning system. Agreement between the TPS and MC calculation was an improvement on agreement between RadCalc and the TPS, with differences ranging from 1.2 to 5.5% between RadCalc and the treatment planning system (TPS), and 0.1-1.7% between MC and TPS. Comparison of the dose-volume histogram (DVH) parameters [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] for the example VMAT plans showed agreement for the mean planning target volume dose within [Formula: see text], [Formula: see text] and [Formula: see text] generally within [Formula: see text] with the exception of a brain case, and [Formula: see text] within [Formula: see text]. Overall, this note provides a demonstration of a system that has been integrated well into existing clinical workflow, and has been shown to be a valuable additional tool in the secondary checking of treatment plan calculations.

Moving GPU-OpenCL-based Monte Carlo dose calculation toward clinical use: Automatic beam commissioning and source sampling for treatment plan dose calculation

We have previously developed a GPU‐based Monte Carlo (MC) dose engine on the OpenCL platform, named goMC, with a built‐in analytical linear accelerator (linac) beam model. In this paper, we report our recent improvement on goMC to move it toward clinical use. First, we have adapted a previously developed automatic beam commissioning approach to our beam model. The commissioning was conducted through an optimization process, minimizing the discrepancies between calculated dose and measurement. We successfully commissioned six beam models built for Varian TrueBeam linac photon beams, including four beams of different energies (6 MV, 10 MV, 15 MV, and 18 MV) and two flattening‐filter‐free (FFF) beams of 6 MV and 10 MV. Second, to facilitate the use of goMC for treatment plan dose calculations, we have developed an efficient source particle sampling strategy. It uses the pre‐generated fluence maps (FMs) to bias the sampling of the control point for source particles already sampled from our beam model. It could effectively reduce the number of source particles required to reach a statistical uncertainty level in the calculated dose, as compared to the conventional FM weighting method. For a head‐and‐neck patient treated with volumetric modulated arc therapy (VMAT), a reduction factor of ~2.8 was achieved, accelerating dose calculation from 150.9 s to 51.5 s. The overall accuracy of goMC was investigated on a VMAT prostate patient case treated with 10 MV FFF beam. 3D gamma index test was conducted to evaluate the discrepancy between our calculated dose and the dose calculated in Varian Eclipse treatment planning system. The passing rate was 99.82% for 2%/2 mm criterion and 95.71% for 1%/1 mm criterion. Our studies have demonstrated the effectiveness and feasibility of our auto‐commissioning approach and new source sampling strategy for fast and accurate MC dose calculations for treatment plans.

Rounded leaf end modeling in Pinnacle VMAT treatment planning for fixed jaw linacs

During volume-modulated arc therapies (VMAT), dosimetric errors are introduced by multiple open dynamic leaf gaps that are present in fixed diaphragm linear accelerators. The purpose of this work was to develop a methodology for adjusting the rounded leaf end modeling parameters to improve out-of-field dose agreement in SmartArc VMAT treatment plans delivered by fixed jaw linacs where leaf gap dose is not negligible. Leaf gap doses were measured for an Elekta beam modulator linac with 0.4 cm micro-multileaf collimators (MLC) using an A16 micro-ionization chamber, a MatriXX ion chamber detector array, and Kodak EDR2 film dosimetry in a solid water phantom. The MLC offset and rounded end tip radius were adjusted in the Pinnacle treatment planning system (TPS) to iteratively arrive at the optimal configuration for 6 MV and 10 MV photon energies. Improvements in gamma index with a 3%/3 mm acceptance criteria and an inclusion threshold of 5% of maximum dose were measured, analyzed, and validated using an ArcCHECK diode detector array for field sizes ranging from 1.6 to 14 cm square field arcs and Task Group (TG) 119 VMAT test cases. The best results were achieved for a rounded leaf tip radius of 13 cm with a 0.1 cm MLC offset. With the optimized MLC model, measured gamma indices ranged between 99.9% and 91.7% for square field arcs with sizes between 3.6 cm and 1.6 cm, with a maximum improvement of 42.7% for the 1.6 cm square field size. Gamma indices improved up to 2.8% in TG-119 VMAT treatment plans. Imaging and Radiation Oncology Core (IROC) credentialing of a VMAT plan with the head and neck phantom passed with a gamma index of 100%. Fine-tune adjustments to MLC rounded leaf ends may improve patient-specific QA pass rates and provide more accurate predictions of dose deposition to avoidance structures.

Deriving detector-specific correction factors for rectangular small fields using a scintillator detector

The goal of this study was to investigate small field output factors (OFs) for flattening filter‐free (FFF) beams on a dedicated stereotactic linear accelerator‐based system. From this data, the collimator exchange effect was quantified, and detector‐specific correction factors were generated. Output factors for 16 jaw‐collimated small fields (from 0.5 to 2 cm) were measured using five different detectors including an ion chamber (CC01), a stereotactic field diode (SFD), a diode detector (Edge), Gafchromic film (EBT3), and a plastic scintillator detector (PSD, W1). Chamber, diodes, and PSD measurements were performed in a Wellhofer water tank, while films were irradiated in solid water at 100 cm source‐to‐surface distance and 10 cm depth. The collimator exchange effect was quantified for rectangular fields. Monte Carlo (MC) simulations of the measured configurations were also performed using the EGSnrc/DOSXYZnrc code. Output factors measured by the PSD and verified against film and MC calculations were chosen as the benchmark measurements. Compared with plastic scintillator detector (PSD), the small volume ion chamber (CC01) underestimated output factors by an average of ‐1.0%±4.9%(max.=‐11.7% for 0.5×0.5cm2 square field). The stereotactic diode (SFD) overestimated output factors by 2.5%±0.4%(max.=3.3% for 0.5×1cm2 rectangular field). The other diode detector (Edge) also overestimated the OFs by an average of 4.2%±0.9%(max.=6.0% for 1×1cm2 square field). Gafchromic film (EBT3) measurements and MC calculations agreed with the scintillator detector measurements within 0.6%±1.8% and 1.2%±1.5%, respectively. Across all the X and Y jaw combinations, the average collimator exchange effect was computed: 1.4%±1.1% (CC01), 5.8%±5.4% (SFD), 5.1%±4.8% (Edge diode), 3.5%±5.0% (Monte Carlo), 3.8%±4.7% (film), and 5.5%±5.1% (PSD). Small field detectors should be used with caution with a clear understanding of their behaviors, especially for FFF beams and small, elongated fields. The scintillator detector exhibited good agreement against Gafchromic film measurements and MC simulations over the range of field sizes studied. The collimator exchange effect was found to be important at these small field sizes. Detector‐specific correction factors were computed using the scintillator measurements as the benchmark.

PACS number(s): 87.56.Fc

An MLC-based version for the ecliptic method for the determination of backscatter into the beam monitor chambers in photon beams of medical accelerators

A very simple method to measure the effect of the backscatter from secondary collimators into the beam monitor chambers in linear accelerators equipped with multi-leaf collimators (MLC) is presented here. The backscatter to the monitor chambers from the upper jaws of the secondary collimator was measured on three beam-matched linacs by means of three methods: this new methodology, the ecliptic method, and assessing the variation of the beam-on time per monitor unit with dose rate feedback disabled. This new methodology was used to assess the backscatter characteristics of asymmetric over-traveling jaws. Excellent agreement between the backscatter values measured using the new methodology introduced here and the ones obtained using the other two methods was established. The experimental values reported here differ by less than 1% from published data. The sensitivity of this novel technique allowed differences in backscatter due to the same opening of the jaws, when placed at different positions on the beam path, to be resolved. The introduction of the ecliptic method has made the determination of the backscatter to the monitor chambers an easy procedure. The method presented here for machines equipped with MLCs makes the determination of backscatter to the beam monitor chambers even easier, and suitable to characterize linacs equipped with over-traveling asymmetric secondary collimators. This experimental procedure could be simply implemented to fully characterize the backscatter output factor constituent when detailed dosimetric modeling of the machine's head is required. The methodology proved to be uncomplicated, accurate and suitable for clinical or experimental environments.

Development and validation of MCNPX-based Monte Carlo treatment plan verification system

A Monte Carlo treatment plan verification (MCTPV) system was developed for clinical treatment plan verification (TPV), especially for the conformal and intensity-modulated radiotherapy (IMRT) plans. In the MCTPV, the MCNPX code was used for particle transport through the accelerator head and the patient body. MCTPV has an interface with TiGRT planning system and reads the information which is needed for Monte Carlo calculation transferred in digital image communications in medicine-radiation therapy (DICOM-RT) format. In MCTPV several methods were applied in order to reduce the simulation time. The relative dose distribution of a clinical prostate conformal plan calculated by the MCTPV was compared with that of TiGRT planning system. The results showed well implementation of the beams configuration and patient information in this system. For quantitative evaluation of MCTPV a two-dimensional (2D) diode array (MapCHECK2) and gamma index analysis were used. The gamma passing rate (3%/3 mm) of an IMRT plan was found to be 98.5% for total beams. Also, comparison of the measured and Monte Carlo calculated doses at several points inside an inhomogeneous phantom for 6- and 18-MV photon beams showed a good agreement (within 1.5%). The accuracy and timing results of MCTPV showed that MCTPV could be used very efficiently for additional assessment of complicated plans such as IMRT plan.

A comprehensive procedure for characterizing arbitrary azimuthally symmetric photon beams

PURPOSE

A new Monte Carlo (MC) source model (SM) has been developed for azimuthally symmetric photon beams.

METHODS

The MC simulation tallied phase space file (PSF) is divided into two categories depending on the relationship of the particle track line to the beam central axis: multiple point source (MPS) and spatial mesh based surface source (SMBSS). To validate this SM, MCNPX2.6 was used to generate two PSFs for a 6 MV photon beam from a Varian 2100C/D linear accelerator.

RESULTS

PDDs and profiles were calculated using the SM and original PSF for different field sizes from 5 × 5 to 40 × 40 cm2. Agreement was within 2% of the maximum dose at 100 cm SSD and 2.5% of the maximum dose at 200 cm SSD for beam profiles at depths of 3.5 cm and 15 cm with respect to the original PSF. Differences between the source model and the PSF in the out-of-field regions were less than 0.5% of the profile maximum value at 100 cm SSD. Differences between measured and calculated points were less than 2% of the maximum dose or 2 mm distance to agreement (DTA) at 100 cm SSD.

CONCLUSIONS

This SM is unique in that it accounts for a higher level of energy dependence on the particle's direction and it is independent from accelerator components, unlike other published SMs. The model can be applied to any arbitrary azimuthally symmetric beam and has source biasing capabilities that significantly increase the simulation speed up to 3300 for certain field sizes.

Verification of monitor unit calculations for non-IMRT clinical radiotherapy: report of AAPM Task Group 114

The requirement of an independent verification of the monitor units (MU) or time calculated to deliver the prescribed dose to a patient has been a mainstay of radiation oncology quality assurance. The need for and value of such a verification was obvious when calculations were performed by hand using look-up tables, and the verification was achieved by a second person independently repeating the calculation. However, in a modern clinic using CT/MR/PET simulation, computerized 3D treatment planning, heterogeneity corrections, and complex calculation algorithms such as convolution/superposition and Monte Carlo, the purpose of and methodology for the MU verification have come into question. In addition, since the verification is often performed using a simpler geometrical model and calculation algorithm than the primary calculation, exact or almost exact agreement between the two can no longer be expected. Guidelines are needed to help the physicist set clinically reasonable action levels for agreement. This report addresses the following charges of the task group: (1) To re-evaluate the purpose and methods of the "independent second check" for monitor unit calculations for non-IMRT radiation treatment in light of the complexities of modern-day treatment planning. (2) To present recommendations on how to perform verification of monitor unit calculations in a modern clinic. (3) To provide recommendations on establishing action levels for agreement between primary calculations and verification, and to provide guidance in addressing discrepancies outside the action levels. These recommendations are to be used as guidelines only and shall not be interpreted as requirements.

Backscattered radiation into a transmission ionization chamber: measurement and Monte Carlo simulation

Backscattered radiation (BSR) from field-defining collimators can affect the response of a monitor chamber in X-radiation fields. This contribution must be considered since this kind of chamber is used to monitor the equipment response. In this work, the dependence of a transmission ionization chamber response on the aperture diameter of the collimators was studied experimentally and using a Monte Carlo (MC) technique. According to the results, the BSR increases the chamber response of over 4.0% in the case of a totally closed collimator and 50 kV energy beam, using both techniques. The results from Monte Carlo simulation confirm the validity of the simulated geometry.

Evaluation of calculation methods of collimator scatter factors in a linear accelerator equipped with MLC instead of lower collimators

In the monitor unit verification for high-energy radiation therapy, we evaluated methods of calculation of collimator scatter factors (S(c)) in a linear accelerator equipped with MLC instead of lower collimators. Routinely,S(c) is calculated from rectangular fields shaped by upper and lower jaws in the linear accelerator. However, this calculation method should not be used for the linear accelerator equipped with MLC instead of lower collimators. Consequently, we used a backprojected field at the flattening filter plane projected by calculation point's eye view on each MLC. We then attempted to deviseS(c) by using Clarkson's integration for these backprojected irregular fields. This method makes it possible to calculate collimator scatter factors in error of less than +/-0.3% in all of sixteen measured irregular fields.

Clinical implementation of enhanced dynamic wedges into the Pinnacle treatment planning system: Monte Carlo validation and patient-specific QA

The goal of this work is to present a systematic Monte Carlo validation study on the clinical implementation of the enhanced dynamic wedges (EDWs) into the Pinnacle(3) (Philips Medical Systems, Fitchburg, WI) treatment planning system (TPS) and QA procedures for patient plan verification treated with EDWs. Modeling of EDW beams in the Pinnacle(3) TPS, which employs a collapsed-cone convolution superposition (CCCS) dose model, was based on a combination of measured open-beam data and the 'Golden Segmented Treatment Table' (GSTT) provided by Varian for each photon beam energy. To validate EDW models, dose profiles of 6 and 10 MV photon beams from a Clinac 2100 C/D were measured in virtual water at depths from near-surface to 30 cm for a wide range of field sizes and wedge angles using the Profiler 2 (Sun Nuclear Corporation, Melbourne, FL) diode array system. The EDW output factors (EDWOFs) for square fields from 4 to 20 cm wide were measured in virtual water using a small-volume Farmer-type ionization chamber placed at a depth of 10 cm on the central axis. Furthermore, the 6 and 10 MV photon beams emerging from the treatment head of Clinac 2100 C/D were fully modeled and the central-axis depth doses, the off-axis dose profiles and the output factors in water for open and dynamically wedged fields were calculated using the Monte Carlo (MC) package EGS4. Our results have shown that (1) both the central-axis depth doses and the off-axis dose profiles of various EDWs computed with the CCCS dose model and MC simulations showed good agreement with the measurements to within 2%/2 mm; (2) measured EDWOFs used for monitor-unit calculation in Pinnacle(3) TPS agreed well with the CCCS and MC predictions within 2%; (3) all the EDW fields satisfied our validation criteria of 1% relative dose difference and 2 mm distance-to-agreement (DTA) with 99-100% passing rate in routine patient treatment plan verification using MapCheck 2D diode array.

An efficient framework for photon Monte Carlo treatment planning

Currently photon Monte Carlo treatment planning (MCTP) for a patient stored in the patient database of a treatment planning system (TPS) can usually only be performed using a cumbersome multi-step procedure where many user interactions are needed. This means automation is needed for usage in clinical routine. In addition, because of the long computing time in MCTP, optimization of the MC calculations is essential. For these purposes a new graphical user interface (GUI)-based photon MC environment has been developed resulting in a very flexible framework. By this means appropriate MC transport methods are assigned to different geometric regions by still benefiting from the features included in the TPS. In order to provide a flexible MC environment, the MC particle transport has been divided into different parts: the source, beam modifiers and the patient. The source part includes the phase-space source, source models and full MC transport through the treatment head. The beam modifier part consists of one module for each beam modifier. To simulate the radiation transport through each individual beam modifier, one out of three full MC transport codes can be selected independently. Additionally, for each beam modifier a simple or an exact geometry can be chosen. Thereby, different complexity levels of radiation transport are applied during the simulation. For the patient dose calculation, two different MC codes are available. A special plug-in in Eclipse providing all necessary information by means of Dicom streams was used to start the developed MC GUI. The implementation of this framework separates the MC transport from the geometry and the modules pass the particles in memory; hence, no files are used as the interface. The implementation is realized for 6 and 15 MV beams of a Varian Clinac 2300 C/D. Several applications demonstrate the usefulness of the framework. Apart from applications dealing with the beam modifiers, two patient cases are shown. Thereby, comparisons are performed between MC calculated dose distributions and those calculated by a pencil beam or the AAA algorithm. Interfacing this flexible and efficient MC environment with Eclipse allows a widespread use for all kinds of investigations from timing and benchmarking studies to clinical patient studies. Additionally, it is possible to add modules keeping the system highly flexible and efficient.

An integrated Monte Carlo dosimetric verification system for radiotherapy treatment planning

An integrated Monte Carlo (MC) dose calculation system, MCRTV (Monte Carlo for radiotherapy treatment plan verification), has been developed for clinical treatment plan verification, especially for routine quality assurance (QA) of intensity-modulated radiotherapy (IMRT) plans. The MCRTV system consists of the EGS4/PRESTA MC codes originally written for particle transport through the accelerator, the multileaf collimator (MLC), and the patient/phantom, which run on a 28-CPU Linux cluster, and the associated software developed for the clinical implementation. MCRTV has an interface with a commercial treatment planning system (TPS) (Eclipse, Varian Medical Systems, Palo Alto, CA, USA) and reads the information needed for MC computation transferred in DICOM-RT format. The key features of MCRTV have been presented in detail in this paper. The phase-space data of our 15 MV photon beam from a Varian Clinac 2300C/D have been developed and several benchmarks have been performed under homogeneous and several inhomogeneous conditions (including water, aluminium, lung and bone media). The MC results agreed with the ionization chamber measurements to within 1% and 2% for homogeneous and inhomogeneous conditions, respectively. The MC calculation for a clinical prostate IMRT treatment plan validated the implementation of the beams and the patient/phantom configuration in MCRTV.

Ecliptic method for the determination of backscatter into the beam monitor chambers in photon beams of medical accelerators

A new method to measure the effect of the backscatter into the beam monitor chambers in linear accelerators is introduced from first principles. The technique, applicable to high-energy photon beams, is similar to the well-known telescopic method although here the heavy blocks are replaced by a very small, centred block on the shadow tray, thus the name 'ecliptic method'. This effect, caused mainly by backscattering from the secondary collimators, is known to be an output factor constituent and must be accounted for when detailed calculations involving the machine's head are required. Since its magnitude is generally small, experimental errors might obscure the behaviour of the phenomenon. Consequently, the procedure introduced goes along with an uncertainty assessment. Our theory was confirmed via measurements in cobalt-60 beams, where the studied effect does not contribute to the output factor. Measurements were also performed on our Saturne 41 linear accelerator and the results were qualitatively similar to those described elsewhere. The collimation systems were studied separately by varying one jaw setting while keeping the other at its maximum value. In the light of these results, we deduced an algorithm that can correlate the former data with the effect of backscattering to the beam monitor chambers for any rectangular field within 0.5%, which is of the order of the experimental uncertainty (0.6%). As we show, the experimental procedure is safe, simple, not invasive for the linac and requires only basic dosimetry equipment.

Determination of parameters for a multiple-source model of megavoltage photon beams using optimization methods

Accurate modelling of the radiation output of a medical linear accelerator is important for radiotherapy treatment planning. The major challenge is the adjustment of the model to a specific treatment unit. One approach is to use a multiple-source model containing a set of physical parameters. In this work, the parameters were derived from standard beam data measurements using optimization methods. The source model used includes sub-sources for bremsstrahlung radiation from the target, extra-focal photon radiation and electron contamination. The cost function includes a gamma error measure between measurements and current dose calculations. The procedure was applied to six beam data sets (6 MV to 23 MV) measured with accelerators from three vendors, but the results focus primarily on Varian accelerators. The obtained average gamma error (1%, 1 mm) between dose calculations and measurements used in optimization was smaller than 0.7 for each studied treatment beam and field size, and a minimum of 83% of measurement points passed the gamma < 1 criterion. For experiments made at different SSDs and for asymmetric fields, the average gamma errors were smaller than 1.1. For irregularly shaped MLC apertures, the differences in point doses were smaller than 1.0%. This work demonstrates that the source model parameters can be automatically derived from simple measurements using optimization methods. The developed procedure is applicable to a wide range of accelerators, and has an acceptable accuracy and processing time.

Monte Carlo calculations for absolute dosimetry to determine machine outputs for proton therapy fields

The prescribed dose in radiation therapy has to be converted into machine monitor units for patient treatment. This is done routinely for each spread-out Bragg peak (SOBP) field either by calibration measurements, by using analytical algorithms or by relying on empirical data. At the Northeast Proton Therapy Center, a monitor unit corresponds to a fixed amount of charge collected in a segmented transmission ionization chamber inside the treatment head. The goal of this work was to use a detailed Monte Carlo model of the treatment head to calculate the dose delivered to the patient as a function of ionization chamber reading, i.e. to yield absolute dose in patients in terms of machine monitor units. The results show excellent agreement with measurements. For 50 SOBP fields considered in this study, the mean absolute difference between the experimental and the calculated value is 1.5%, where approximately 50% of the fields agree within 1%. This is within the uncertainties of the data. The Monte Carlo method has advantages over analytical algorithms because it takes into account scattered and secondary radiation, does not rely on empirical parameters, and provides a tool to study the influence of parts of the treatment head on the ionization chamber reading. Compared to experimental methods the Monte Carlo method has the advantage of being able to verify the dose in the patient geometry.

A software tool for 2D/3D visualization and analysis of phase-space data generated by Monte Carlo modelling of medical linear accelerators

A computer program has been developed for novel 2D/3D visualization and analysis of the phase-space parameters of Monte Carlo simulations of medical accelerator radiation beams. The software is written in the IDL language and reads the phase-space data generated in the BEAMnrc/BEAM Monte Carlo code format. Contour and colour-wash plots of the fluence, mean energy, energy fluence, mean angle, spectra distribution, energy fluence distribution, angular distribution, and slices and projections of the 3D ZLAST distribution can be calculated and displayed. Based on our experience of using it at Massachusetts General Hospital, the software has proven to be a useful tool for analysis and verification of the Monte Carlo generated phase-space files. The software is in the public domain.

Absolute dose calculations for Monte Carlo simulations of radiotherapy beams

Monte Carlo (MC) simulations have traditionally been used for single field relative comparisons with experimental data or commercial treatment planning systems (TPS). However, clinical treatment plans commonly involve more than one field. Since the contribution of each field must be accurately quantified, multiple field MC simulations are only possible by employing absolute dosimetry. Therefore, we have developed a rigorous calibration method that allows the incorporation of monitor units (MU) in MC simulations. This absolute dosimetry formalism can be easily implemented by any BEAMnrc/DOSXYZnrc user, and applies to any configuration of open and blocked fields, including intensity-modulated radiation therapy (IMRT) plans. Our approach involves the relationship between the dose scored in the monitor ionization chamber of a radiotherapy linear accelerator (linac), the number of initial particles incident on the target, and the field size. We found that for a 10 x 10 cm2 field of a 6 MV photon beam, 1 MU corresponds, in our model, to 8.129 x 10(13) +/- 1.0% electrons incident on the target and a total dose of 20.87 cGy +/- 1.0% in the monitor chambers of the virtual linac. We present an extensive experimental verification of our MC results for open and intensity-modulated fields, including a dynamic 7-field IMRT plan simulated on the CT data sets of a cylindrical phantom and of a Rando anthropomorphic phantom, which were validated by measurements using ionization chambers and thermoluminescent dosimeters (TLD). Our simulation results are in excellent agreement with experiment, with percentage differences of less than 2%, in general, demonstrating the accuracy of our Monte Carlo absolute dose calculations.

Derivation of the distribution of extrafocal radiation for head scatter factor calculation

Head scatter factors for high energy photon beams from linear accelerators can be modeled using a two-source model consisting of focal and extrafocal radiation. The focal radiation can be approximated as a point source, and the distribution of the extrafocal radiation is a two-dimensional (2D) radial symmetric function. Various methods, including analytical, Monte Carlo, and empirical trial functions, have been used to determine the radial symmetric function of extrafocal radiation distribution. This article describes a method for directly determining the extrafocal radiation distribution without assuming any empirical trial function. The extrafocal radiation distribution is determined with measured head scatter factors for rectangular fields defined by the lower jaw (X) fixed at 40 cm and the upper jaw (Y) varying from 3 to 40 cm. The derivatives of the measured head scatter factors, with respect to the Y jaw position projected in the plane of extrafocal radiation, are proportional to the one-dimensional (1D) projection (also called the line spread function) of the extrafocal radiation distribution. Two methods are used to solve the radial function of extrafocal radiation from the 1D projection. The first method uses a 2D filtered backprojection algorithm, originally developed for parallel beam computed tomography reconstruction, to directly derive the radial dependence of the extrafocal radiation distribution. The method has been applied to 6 and 18 MV photon beams from a Siemens linear accelerator and has been tested by comparing measured and calculated head scatter factors for square and rectangular fields. The second method uses a Fourier transform followed by a Fourier-Bessel transform to solve the problem. The distributions of extrafocal radiation derived from these two methods are virtually identical.

Reference photon dosimetry data and reference phase space data for the 6MV photon beam from Varian Clinac 2100 series linear accelerators

The current study presents the reference photon dosimetry data (RPDD) and reference phase space data (RPSD) for the 6 MV photon beam from Varian 2100 series linear accelerators. The RPDD provide the basic photon dosimetry data, typically collected during the initial commissioning of a new linear accelerator, including output factors, depth dose data, and beam profile data in air and in water. The RPSD provide the full phase space information, such as position, direction, and energy for each particle generated inside the head of any particular linear accelerator in question. The dosimetric characteristics if the 6 MV photon beam from the majority of the aforementioned accelerators, which are unaltered from the manufacturer's original specifications, can be fully described with these two data sets within a clinically acceptable uncertainty (approximately +/-2 %). The current study also presents a detailed procedure to establish the RPDD and RPSD using measured data and Monte Carlo calculations. The RPDD were constructed by compiling our own measured data and the average data based on the analysis of more than 50 sets of measured data from the Radiological Physics Center (RPC) and 10 sets of clinical dosimetry data obtained from 10 different institutions participating in the RPC's quality assurance monitoring program. All the measured data from the RPC and the RPC-monitored institutions were found to be within a statistically tight range (i.e., 1sigma approximately 1% or less) for each dosimetric quantity. The manufacturer's standard data, except for in-air off-axis factors that are available only from the current study, were compared with the RPDD, showing that the manufacturer's standard data could also be used as the RPDD for the photon beam studied in this study. The RPSD were obtained from Monte Carlo calculations using the BEAMnrc/ DOSXYZnrc code system with 6.2 MeV (a spread of 3% full width at half maximum) and 1.0 mm full width at half maximum as the values of the energy and radial spread of a Gaussian electron pencil beam incident on the target, respectively. The RPSD were capable of generating Monte Carlo data that agreed with the RPDD within the acceptance criteria adopted in the current study (e.g., 1% or 1 mm for depth dose). A complete set of the RPDD and RPSD from the current study is available from the RPC website (http://rpc.mdanderson.org) or via mass storage media such as DVD or CD-ROM upon request.

An investigation of accelerator head scatter and output factor in air

Our purpose in this study was to investigate whether the Monte Carlo simulation can accurately predict output factors in air. Secondary goals were to study the head scatter components and investigate the collimator exchange effect. The Monte Carlo code, BEAMnrc, was used in the study. Photon beams of 6 and 18 MV were from a Varian Clinac 2100EX accelerator and the measurements were performed using an ionization chamber in a mini-phantom. The Monte Carlo calculated in air output factors was within 1% of measured values. The simulation provided information of the origin and the magnitude of the collimator exchange effect. It was shown that the collimator backscatter to the beam monitor chamber played a significant role in the beam output factors. However the magnitude of the scattered dose contributions from the collimator at the isocenter is negligible. The maximum scattered dose contribution from the collimators was about 0.15% and 0.4% of the total dose at the isocenter for a 6 and 18 MV beam, respectively. The scattered dose contributions from the flattening filter at the isocenter were about 0.9-3% and 0.2-6% of the total dose for field sizes of 4x4 cm2-40x40 cm2 for the 6 and 18 MV beam, respectively. The study suggests that measurements of head scatter factors be done at large depth well beyond the depth of electron contamination. The insight information may have some implications for developing generalized empirical models to calculate the head scatter.

Monte Carlo computation of dosimetric amorphous silicon electronic portal images

This study develops and tests a method to compute dosimetric images for an amorphous silicon (a-Si) flat-panel detector so that an accurate quantitative comparison between measured and computed portal images may be made. An EGS4-based Monte Carlo (MC) algorithm is developed to efficiently tally the energy deposition through the use of a virtual detector dose-scoring methodology. The complete geometry of the a-Si imager is utilized in the MC calculation up to the imager rear housing, which is replaced with a uniform thickness material slab. The detector-mounting hardware is modeled as a uniform backscattering material. The amount of backscatter material required to reproduce the measured backscatter is 0.98 g/cm2 of water. A flood-field irradiation, performed in the measurement imaging session, is used to cross-calibrate the computed images with the measured images. Calibrated MC-computed images reproduce measured field-size dependencies of the electronic portal imaging device (EPID) response to within <1%, without the need for optical glare or other empirical corrections. A 10% dose difference between measured and computed images was observed outside the field edge for a 10 x 10 cm2 field that was entirely blocked by the multileaf collimator (MLC). However, this error corresponded with less than 0.15% of the open-field dose. For 10 x 10 cm2 fields produced by 5 and 20 mm dynamically sweeping MLC gaps, more than 98% of the points were found to have a gamma less than one with a 2%, 2 mm criteria. For an intensity modulated radiation therapy (IMRT) patient test field, over 99% of the points were found to have a gamma less than one with a 2%, 2 mm criteria. This study demonstrates that MC can be an effective tool for predicting measured a-Si portal images and may be useful for IMRT EPID-based dosimetry.

Using Monte Carlo simulations to commission photon beam output factors—a feasibility study

This study investigates the feasibility of using Monte Carlo methods to assist the commissioning of photon beam output factors from a medical accelerator. The Monte Carlo code, BEAMnrc, was used to model 6 MV and 18 MV photon beams from a Varian linear accelerator. When excellent agreements were obtained between the Monte Carlo simulated and measured dose distributions in a water phantom, the entire geometry including the accelerator head and the water phantom was simulated to calculate the relative output factors. Simulated output factors were compared with measured data, which consist of a typical commission dataset for the output factors. The measurements were done using an ionization chamber in a water phantom at a depth of 10 cm with a source-detector distance of 100 cm. Square fields and rectangular fields with widths and lengths ranging from 4 cm to 40 cm were studied. The result shows a very good agreement (< 1.5%) between the Monte Carlo calculated and the measured relative output factors for a typical commissioning dataset. The Monte Carlo calculated backscatter factors to the beam monitor chamber agree well with measured data in the literature. Monte Carlo simulations have also been shown to be able to accurately predict the collimator exchange effect and its component for rectangular fields. The information obtained is also useful to develop an algorithm for accurate beam modelling. This investigation indicates that Monte Carlo methods can be used to assist commissioning of output factors for photon beams.

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.

Head scatter off-axis for megavoltage x rays

Our objective in this study has been to investigate how head scatter varies with the off-axis position in a 6 MV x-ray beam. We define the head-scatter off-axis ratio, HOA, as the ratio of the kerma due to head-scatter photons at the off-axis position x to the kerma from direct primary photons on the central axis. "Direct primary" are those photons that come from the source without interactions in the intervening structures. We determined HOA from measurements with an ionization chamber in a miniphantom. Head-scatter and direct primary photons contribute to a measurement of the ionization per mu Q(x) at the off-axis position x in the open field cx x cy. The ionization per mu QP(x), measured in the same position but with the field collimated to the smallest possible opening (cx x 3 cm), is intended to include only direct primary photons. Head-scatter photons cannot be completely eliminated, and the errors due to remaining head scatter and radiation back-scattered by the movable collimators into the monitor were estimated. For normalization of the final results, ionization due to direct primary photons was also measured on the central axis, QP(0). HOA was derived from these three measurements as HOA(cx,cy,x)=(Q(cx,cy,x) - QP(cx,cy,x))/QP(cx,cy,0). On the central axis (x=y=0), HOA represents the "scatter-to-primary ratio" between head scatter and the direct primary dose. Monte Carlo simulations were made to help with the interpretation and evaluation of the results. HOA could be fitted to a Gaussian model with two components corresponding to sources of widths 1.8 and 14 cm, projected on a plane 5 cm below the x-ray source. The narrow Gaussian component is interpreted as the source of photons scattered in the flattening filter and the primary collimator. The broad component is attributed to photons scattered in the secondary (variable) collimators. Conventional head-scatter models (e.g., a single Gaussian source model) do not fit the measured HOA data for large collimator settings (c>20 cm) or outside beam collimation. The full width at half-maximum (FWHM) of HOA(x) across the field increased with the field width (cx) in the direction of the measurements in a manner consistent with the field of view of the two sources. It was not sensitive to the field measure in the orthogonal direction (cy). Head scatter outside the field also increased with field size, reflecting an increased contribution of photons scattered at large angles. It exceeds the leakage through the collimator 2 cm outside the edge for square fields c>10 cm. Monte Carlo calculations showed considerably less head scatter outside the field than measurements.

Determining the incident electron fluence for Monte Carlo-based photon treatment planning using a standard measured data set

An accurate dose calculation in phantom and patient geometries requires an accurate description of the radiation source. Errors in the radiation source description are propagated through the dose calculation. With the emergence of linear accelerators whose dosimetric characteristics are similar to within measurement uncertainty, the same radiation source description can be used as the input to dose calculation for treatment planning at many institutions with the same linear accelerator model. Our goal in the current research was to determine the initial electron fluence above the linear accelerator target for such an accelerator to allow a dose calculation in water to within 1% or 1 mm of the measured data supplied by the manufacturer. The method used for both the radiation source description and the patient transport was Monte Carlo. The linac geometry was input into the Monte Carlo code using the accelerator's manufacturer's specifications. Assumptions about the initial electron source above the target were made based on previous studies. The free parameters derived for the calculations were the mean energy and radial Gaussian width of the initial electron fluence and the target density. A combination of the free parameters yielded an initial electron fluence that, when transported through the linear accelerator and into the phantom, allowed a dose-calculation agreement to the experimental ion chamber data to within the specified criteria at both 6 and 18 MV nominal beam energies, except near the surface, particularly for the 18 MV beam. To save time during Monte Carlo treatment planning, the initial electron fluence was transported through part of the treatment head to a plane between the monitor chambers and the jaws and saved as phase-space files. These files are used for clinical Monte Carlo-based treatment planning and are freely available from the authors.

A DICOM-RT-based toolbox for the evaluation and verification of radiotherapy plans

The verification of radiotherapy plans is an essential step in the treatment planning process. This is especially important for highly conformal and IMRT plans which produce non-intuitive fluence maps and complex 3D dose distributions. In this work we present a DICOM (Digital Imaging and Communication in Medicine) based toolbox, developed for the evaluation and the verification of radiotherapy treatment plans. The toolbox offers the possibility of importing treatment plans generated with different calculation algorithms and/or different optimization engines and evaluating dose distributions on an independent platform. Furthermore the radiotherapy set-up can be exported to the BEAM Monte Carlo code system for dose verification. This can be done by simulating the irradiation of the patient CT dataset or the irradiation of a software-generated water phantom. We show the application of some of the functions implemented in this toolbox for the evaluation and verification of an IMRT treatment of the head and neck region.

A three-source model for the calculation of head scatter factors

Accurate determination of the head scatter factor Sc is an important issue, especially for intensity modulated radiation therapy, where the segmented fields are often very irregular and much less than the collimator jaw settings. In this work, we report an Sc calculation algorithm for symmetric, asymmetric, and irregular open fields shaped by the tertiary collimator (a multileaf collimator or blocks) at different source-to-chamber distance. The algorithm was based on a three-source model, in which the photon radiation to the point of calculation was treated as if it originated from three effective sources: one source for the primary photons from the target and two extra-focal photon sources for the scattered photons from the primary collimator and the flattening filter, respectively. The field mapping method proposed by Kim et al. [Phys. Med. Biol. 43, 1593-1604 (1998)] was extended to two extra-focal source planes and the scatter contributions were integrated over the projected areas (determined by the detector's eye view) in the three source planes considering the source intensity distributions. The algorithm was implemented using Microsoft Visual C/C++ in the MS Windows environment. The only input data required were head scatter factors for symmetric square fields, which are normally acquired during machine commissioning. A large number of different fields were used to evaluate the algorithm and the results were compared with measurements. We found that most of the calculated Sc's agreed with the measured values to within 0.4%. The algorithm can also be easily applied to deal with irregular fields shaped by a multileaf collimator that replaces the upper or lower collimator jaws.

Dosimetric characteristics of dynamic wedged fields: a Monte Carlo study

We have developed a Monte Carlo (MC) technique using the EGS4/BEAM system to calculate dosimetric characteristics of dynamic wedges (DW) for photon beam radiotherapy. The simulation of DW was accomplished by weighting the history numbers of the electrons, which are incident on the target in accordance with the segmented treatment table. Calculations were performed for DW with wedge angles ranging from 15 degrees to 60 degrees as well as for open fields with different field sizes for both degrees 6 and 18 MV beams. The MC-calculated percentage depth dose (PDD) and beam profiles agreed with the measurements within +/- 2% (of the dose maximum along the beam axis) or +/- 2 mm in high dose gradient region. The DW slightly affects energy spectra of photons and contaminating electrons. These slight changes have no significant effects on PDD as compared to the open field. The MC-calculated dynamic wedge factors agree with the measurements within +/- 2%. The MC method enables us to provide more detailed beam characteristics for DW fields than a measurement method. This beam characteristic includes photon energy spectra, mean energy, spectra of contaminating electrons and effects of moving jaw on off-axis beam quality. These data are potentially important for treatment planning involving dynamic wedges.

Monte Carlo simulation and dosimetric verification of radiotherapy beam modifiers

Monte Carlo simulation of beam modifiers such as physical wedges and compensating filters has been performed with a rectilinear voxel geometry module. A modified version of the EGS4/DOSXYZ code has been developed for this purpose. The new implementations have been validated against the BEAM Monte Carlo code using its standard component modules (CMs) in several geometrical conditions. No significant disagreements were found within the statistical errors of 0.5% for photons and 2% for electrons. The clinical applicability and flexibility of the new version of the code has been assessed through an extensive verification versus dosimetric data. Both Varian multi-leaf collimator (MLC) wedges and standard wedges have been simulated and compared against experiments for 6MV photon beams and different field sizes. Good agreement was found between calculated and measured depth doses and lateral dose profiles along both wedged and unwedged directions for different depths and focus-to-surface distances. Furthermore, Monte Carlo-generated output factors for both open and wedged fields agreed with linac commissioning beam data within statistical uncertainties of the calculations (<3% at largest depths). Compensating filters of both low-density and high-density materials have also been successfully simulated. As a demonstration, a wax compensating filter with a complex three-dimensional concave and convex geometry has been modelled through a CT scan import. Calculated depth doses and lateral dose profiles for different field sizes agreed well with experiments. The code was used to investigate the performance of a commercial treatment planning system in designing compensators. Dose distributions in a heterogeneous water phantom emulating the head and neck region were calculated with the convolution-superposition method (pencil beam and collapsed cone implementations) and compared against those from the MC code developed herein. The new technique presented in this work is versatile, DICOM-RT compliant and accurate in the simulation of beam modulators. This paper addresses the need to reduce the sources of error in the modelling of beam modifiers since they remain a viable alternative to the MLC technique in the delivery of IMRT beams.

A method of simulating dynamic multileaf collimators using Monte Carlo techniques for intensity-modulated radiation therapy

A method of modelling the dynamic motion of multileaf collimators (MLCs) for intensity-modulated radiation therapy (IMRT) was developed and implemented into the Monte Carlo simulation. The simulation of the dynamic MLCs (DMLCs) was based on randomizing leaf positions during a simulation so that the number of particle histories being simulated for each possible leaf position was proportional to the monitor units delivered to that position. This approach was incorporated into an EGS4 Monte Carlo program, and was evaluated in simulating the DMLCs for Varian accelerators (Varian Medical Systems, Palo Alto. CA, USA). The MU index of each segment, which was specified in the DMLC-control data, was used to compute the cumulative probability distribution function (CPDF) for the leaf positions. This CPDF was then used to sample the leaf positions during a real-time simulation, which allowed for either the step-shoot or sweeping-leaf motion in the beam delivery. Dose intensity maps for IMRT fields were computed using the above Monte Carlo method, with its accuracy verified by film measurements. The DMLC simulation improved the operational efficiency by eliminating the need to simulate multiple segments individually. More importantly, the dynamic motion of the leaves could be simulated more faithfully by using the above leaf-position sampling technique in the Monte Carlo simulation.

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.

Modeling the output ratio in air for megavoltage photon beams

The output ratio in air, OR, for a high-energy x-ray beam describes how the incident central axis photon fluence varies with collimator setting. For field sizes larger than 3 x 3 cm2, its variation is caused by the scatter of photons in structures in the accelerator head (primarily the flattening filter and the wedge, if one is used) and by the backscatter of radiation into the monitor ionization chamber. The objective of this study was to evaluate the use of an analytical function to parametrize OR for square collimator setting c: OR = (1 + a1.c).[1 + a2.erf(c/lambda)2].H0. For open beams, these parameters can be attributed to explicit physical meanings within the systematical uncertainty of the model: a1 accounts for backscatter into the monitor, a2 is the maximum scatter-to-primary ratio for head-scattered photons, and lambda represents the effective width of the "source" of head-scatter photons. H0 is a constant that sets OR = 1 for c = 10 cm. This formula also fits OR for wedge beams and a Co-60 unit, although the fitting parameters lose their physical interpretations. To calculate the output ratio for a rectangular field, cx x cy, an equivalent square can be used: c = (1 + k).cy x cx/(k.cx + cy), where k is a constant. The study included a number of different accelerators and a cobalt-60 unit. The fits for square fields agreed with measurements with a standard deviation (SD) of less than 0.5%. Using k = lx.(f - ly)/ly.(f - lx), where lx and ly are the source-to-collimator distances and f is the source-to-detector distance, measurements and calculations agreed within a SD of 0.7% for rectangular fields. Sufficient data for the three parameters are presented to suggest constraints that can be used for quality assurance of the measured output ratio in air.

A multiple source model for 6 MV photon beam dose calculations using Monte Carlo

A multiple source model (MSM) for the 6 MV beam of a Varian Clinac 2300 C/D was developed by simulating radiation transport through the accelerator head for a set of square fields using the GEANT Monte Carlo (MC) code. The corresponding phase space (PS) data enabled the characterization of 12 sources representing the main components of the beam defining system. By parametrizing the source characteristics and by evaluating the dependence of the parameters on field size, it was possible to extend the validity of the model to arbitrary rectangular fields which include the central 3 x 3 cm2 field without additional precalculated PS data. Finally, a sampling procedure was developed in order to reproduce the PS data. To validate the MSM, the fluence, energy fluence and mean energy distributions determined from the original and the reproduced PS data were compared and showed very good agreement. In addition, the MC calculated primary energy spectrum was verified by an energy spectrum derived from transmission measurements. Comparisons of MC calculated depth dose curves and profiles, using original and PS data reproduced by the MSM, agree within 1% and 1 mm. Deviations from measured dose distributions are within 1.5% and 1 mm. However, the real beam leads to some larger deviations outside the geometrical beam area for large fields. Calculated output factors in 10 cm water depth agree within 1.5% with experimentally determined data. In conclusion, the MSM produces accurate PS data for MC photon dose calculations for the rectangular fields specified.

Monte Carlo dose calculations for dynamic IMRT treatments

Dose calculations for intensity modulated radiation therapy (IMRT) face new challenges due to the complex leaf geometry and time dependent nature of the delivery. A fast method of particle transport through a dynamic multileaf collimator (MLC) geometry that accounts for photon attenuation and first-scattered Compton photon production has been incorporated into an existing Monte Carlo code used for patient dose calculations. Dosimetric agreement between calculation and measurement for two photon energies and MLC types is within experimental error for the sliding window tests. For a patient IMRT field, the Monte Carlo calculations are closer to measured dose than similar superposition or pencil beam calculations.

Incorporating dynamic collimator motion in Monte Carlo simulations: an application in modelling a dynamic wedge

In radiation therapy, new treatment modalities employing dynamic collimation and intensity modulation increase the complexity of dose calculation because a new dimension, time, has to be incorporated into the traditional three-dimensional problem. In this work, we investigated two classes of sampling technique to incorporate dynamic collimator motion in Monte Carlo simulation. The methods were initially evaluated for modelling enhanced dynamic wedges (EDWs) from Varian accelerators (Varian Medical Systems, Palo Alto, USA). In the position-probability-sampling or PPS method, a cumulative probability distribution function (CPDF) was computed for the collimator position, which could then be sampled during simulations. In the static-component-simulation or SCS method, a dynamic field is approximated by multiple static fields in a step-shoot fashion. The weights of the particles or the number of particles simulated for each component field are computed from the probability distribution function (PDF) of the collimator position. The CPDF and PDF were computed from the segmented treatment tables (STTs) for the EDWs. An output correction factor had to be applied in this calculation to account for the backscattered radiation affecting monitor chamber readings. Comparison of the phase-space data from the PPS method (with the step-shoot motion) with those from the SCS method showed excellent agreement. The accuracy of the PPS method was further verified from the agreement between the measured and calculated dose distributions. Compared to the SCS method, the PPS method is more automated and efficient from an operational point of view. The principle of the PPS method can be extended to simulate other dynamic motions, and in particular, intensity-modulated beams using multileaf collimators.

Modeling the extrafocal radiation and monitor chamber backscatter for photon beam dose calculation

A simple analytical approach has been developed to model extrafocal radiation and monitor chamber backscatter for clinical photon beams. Model parameters for both the extrafocal source and monitor chamber backscatter are determined simultaneously using conventional measured data, i.e., in-air output factors for square and rectangular fields defined by the photon jaws. The model has been applied to 6 MV and 15 MV photon beams produced by a Varian Clinac 2300C/D accelerator. Contributions to the in-air output factor from the extrafocal radiation and monitor chamber backscatter, as predicted by the model, are in good agreement with the measurements. The model can be used to calculate the in-air output factors analytically, with an accuracy of 0.2% for symmetric or asymmetric rectangular fields defined by jaws when the calculation point is at the isocenter and 0.5% when the calculation point is at an extended SSD. For MLC-defined fields, with the jaws at the recommended positions, calculated in-air output factors agree with the measured data to within 0.3% at the isocenter and 0.7% at off-axis positions. The model has been incorporated into a Monte Carlo dose algorithm to calculate the absolute dose distributions in patients or phantoms. For three MLC-defined irregular fields (triangle shape, C-shape, and L-shape), the calculations agree with the measurements to about 1% even for points at off-axis positions. The model will be particularly useful for IMRT dose calculations because it accurately predicts beam output and penumbra dose.