Monte Carlo calculation of nine megavoltage photon beam spectra using the BEAM code

Monte carlo model and output factors of elekta infinity™ 6 and 10 MV photon beam

Aim

This study aimed to commission the Elekta Infinity™ working in 6 and 10 MV photon beam installed in Concord International Hospital, Singapore, and compare the OFs between MC simulation and measurement using PTW semiflex and microDiamond detector for small field sizes.

Material and Methods

There are two main steps in this study: modelling of Linac 6 and 10 MV photon beam and analysis of the output factors for field size 2 × 2-10 × 10 cm². The EGSnrc/BEAMnrc-DOSXYZnrc code was used to model and characterize the Linac and to calculate the dose distributions in a water phantom. The dose distribution and OFs were compared to the measurement data in the same condition.

Results

The commissioning process was only conducted for a 10 × 10 cm² field size. The PDD obtained from MC simulation showed a good agreement with the measurement. The local dose difference of PDDs was less than 2% for 6 and 10 MV. The initial electron energy was 5.2 and 9.4 MeV for 6 and 10 MV photon beam, respectively. This Linac model can be used for dose calculation in other situations and different field sizes because this Linac has been commissioned and validated using Monte Carlo simulation. The 10 MV Linac produces higher electron contamination than that of 6 MV.

Conclusions

The Linac model in this study was acceptable. The most important result in this work comes from OFs resulted from MC calculation. This value was more significant than the OFs from measurement using semiflex and microDiamond for all beam energy and field sizes because of the CPE phenomenon.

Influence of a shape of gold nanoparticles on the dose enhancement in the wide range of gold mass concentration for high-energy X-ray beams from a medical linac

Aim

This work is focused on the Monte Carlo microdosimetric calculations taking into account the influence of the AuNPs' shape, size and mass concentration on the radiation dose enhancement for the high-energy 6 MV and 18 MV X-ray therapeutic beams from a medical linac.

Background

Due to a high atomic number and the photoelectric effect, gold nanoparticles can significantly enhance doses of ionizing radiation. However, this enhancement depends upon several parameters, such as, inter alia, nanoparticles' shape etc.

Method

The simulated system was composed of the therapeutic beam, a water phantom with the target volume (with and without AuNPs) located at the depth of the maximum dose, i.e. at 1.5 cm for the 6 MV beam and at 3.5 cm for the 18 MV one. In the study the GEANT4 code was used because it makes it possible to get a very short step of simulation which is required in case of simulating the radiation interactions with nanostructures.

Results

The dependence between the dose increase and the mass concentration of gold was determined and described by a simple mathematical formula for three different shapes of gold nanoparticles - two nanorods of different sizes and a flat 2D structure. The dose increase with the saturation occurring with the increasing mass concentration of gold was observed.

Conclusions

It was found that relatively large cylindrical gold nanoparticles can limit the increase of the dose absorbed in the target volume much more than the large 2D gold nanostructure.

Detector response in the buildup region of small MV fields

PURPOSE

The model used to calculate dose distributions in a radiotherapy treatment plan relies on the data entered during beam commissioning. The quality of these data heavily depends on the detector choice made, especially in small fields and in the buildup region. Therefore, it is necessary to identify suitable detectors for measurements in the buildup region of small fields. To aid the understanding of a detector's limitations, several factors that influence the detector signal are to be analyzed, for example, the volume effect due to the detector size, the response to electron contamination, the signal dependence on the polarity used, and the effective point of measurement chosen.

METHODS

We tested the suitability of different small field detectors for measurements of depth dose curves with a special focus on the surface-near area of dose buildup for fields sized between 10 × 10 and 0.6 × 0.6 cm² . Depth dose curves were measured with 14 different detectors including plane-parallel chambers, thimble chambers of different types and sizes, shielded and unshielded diodes as well as a diamond detector. Those curves were compared with depth dose curves acquired on Gafchromic film. Additionally, the magnitude of geometric volume corrections was estimated from film profiles in different depths. Furthermore, a lead foil was inserted into the beam to reduce contaminating electrons and to study the resulting changes of the detector response. The role of the effective point of measurement was investigated by quantifying the changes occurring when shifting depth dose curves. Last, measurements for the small ionization chambers taken at opposing biasing voltages were compared to study polarity effects.

RESULTS

Depth-dependent correction factors for relative depth dose curves with different detectors were derived. Film, the Farmer chamber FC23, a 0.13 cm³ scanning chamber CC13 and a plane-parallel chamber PPC05 agree very well in fields sized 4 × 4 and 10 × 10 cm² . For most detectors and in smaller fields, depth dose curves differ from the film. In general, shielded diodes require larger corrections than unshielded diodes. Neither the geometric volume effect nor the electron contamination can account for the detector differences. The biggest uncertainty arises from the positioning of a detector with respect to the water surface and from the choice of the detector's effective point of measurement. Depth dose curves acquired with small ionization chambers differ by over 15% in the buildup region depending on sign of the biasing voltage used.

CONCLUSIONS

A scanning chamber or a PPC40 chamber is suitable for fields larger than 4 × 4 cm² . Below that field size, the microDiamond or small ionization chambers perform best requiring the smallest corrections at depth as well as in the buildup region. Diode response changes considerably between the different types of detectors. The position of the effective point of measurement has a huge effect on the resulting curves, therefore detector specific rather than general shifts of half the inner radius of cylindrical ionization chambers for the effective point of measurement should be used. For small ionization chambers, averaging between both polarities is necessary for data obtained near the surface.

Different distributions of gold nanoparticles on the tumor and calculation of dose enhancement factor by Monte Carlo simulation

Gold nanoparticles can be used to increase the dose of the tumor due to its high atomic number as well as being free from apparent toxicity. The aim of this study is to evaluate the effect of distribution of gold nanoparticles models, as well as changes in nanoparticle sizes and spectrum of radiation energy along with the effects of nanoparticle penetration into surrounding tissues in dose enhancement factor DEF. Three mathematical models were considered for distribution of gold nanoparticles in the tumor, such as 1-uniform, 2- non-uniform distribution with no penetration margin and 3- non-uniform distribution with penetration margin of 2.7 mm of gold nanoparticles. For this purpose, a cube-shaped water phantom of 50 cm size in each side and a cube with 1 cm side placed at depth of 2 cm below the upper surface of the cubic phantom as the tumor was defined, and then 3 models of nanoparticle distribution were modeled. MCNPX code was used to simulate 3 distribution models. DEF was evaluated for sizes of 20, 25, 30, 50, 70, 90 and 100 nm of gold nanoparticles, and 50, 95, 250 keV and 4 MeV photon energies. In uniform distribution model the maximum DEF was observed at 100 nm and 50 keV being equal to 2.90, in non-uniform distribution with no penetration margin, the maximum DEF was measured at 100 nm and 50 keV being 1.69, and in non-uniform distribution with penetration margin of 2.7 mm, the maximum DEF was measured at 100 nm and 50 keV as 1.38, and the results have been showed that the dose was increased by injecting nanoparticles into the tumor. It is concluded that the highest DEF could be achieved in low energy photons and larger sizes of nanoparticles. Non-uniform distribution of gold nanoparticles can increase the dose and also decrease the DEF in comparison with the uniform distribution. The non-uniform distribution of nanoparticles with penetration margin showed a lower DEF than the non-uniform distribution without any margin and uniform distribution. Meanwhile, utilization of the real X-ray spectrum brought about a smaller DEF in comparison to mono-energetic X-ray photons.

Assessment of Radio-Induced Damage in Endothelial Cells Irradiated with 40 kVp, 220 kVp, and 4 MV X-rays by Means of Micro and Nanodosimetric Calculations

The objective of this work was to study the differences in terms of early biological effects that might exist between different X-rays energies by using a mechanistic approach. To this end, radiobiological experiments exposing cell monolayers to three X-ray energies were performed in order to assess the yields of early DNA damage, in particular of double-strand breaks (DSBs). The simulation of these irradiations was set in order to understand the differences in the obtained experimental results. Hence, simulated results in terms of microdosimetric spectra and early DSB induction were analyzed and compared to the experimental data. Human umbilical vein endothelial cells (HUVECs) were irradiated with 40, 220 kVp, and 4 MV X-rays. The Geant4 Monte Carlo simulation toolkit and its extension Geant4-DNA were used for the simulations. Microdosimetric calculations aiming to determine possible differences in the variability of the energy absorbed by the irradiated cell population for those photon spectra were performed on 10,000 endothelial cell nuclei representing a cell monolayer. Nanodosimetric simulations were also carried out using a computation chain that allowed the simulation of physical, physico-chemical, and chemical stages on a single realistic endothelial cell nucleus model including both heterochromatin and euchromatin. DNA damage was scored in terms of yields of prompt DSBs per Gray (Gy) and per giga (10⁹) base pair (Gbp) and DSB complexity was derived in order to be compared to experimental data expressed as numbers of histone variant H2AX (γ-H2AX) foci per cell. The calculated microdosimetric spread in the irradiated cell population was similar when comparing between 40 and 220 kVp X-rays and higher when comparing with 4 MV X-rays. Simulated yields of induced DSB/Gy/Gbp were found to be equivalent to those for 40 and 220 kVp but larger than those for 4 MV, resulting in a relative biological effectiveness (RBE) of 1.3. Additionally, DSB complexity was similar between the considered photon spectra. Simulated results were in good agreement with experimental data obtained by IRSN (Institut de radioprotection et de sûreté nucléaire) radiobiologists. Despite differences in photon energy, few differences were observed when comparing between 40 and 220 kVp X-rays in microdosimetric and nanodosimetric calculations. Nevertheless, variations were observed when comparing between 40/220 kVp and 4 MV X-rays. Thanks to the simulation results, these variations were able to be explained by the differences in the production of secondary electrons with energies below 10 keV.

Implementation of a simple clinical linear accelerator beam model in MCNP6 and comparison with measured beam characteristics

Monte Carlo N-Particle 6 (MCNP6) is the latest version of Los Alamos National Laboratory's powerful Monte Carlo software designed to compute general photon, neutron, and electron transport using stochastic algorithms. Here we provide a case study of modeling the photon beam of a Varian 600C Clinical Linear Accelerator (linac), which is used to deliver radiation therapy, along with a comparison to experimentally measured beam characteristics. The source definition parameters in MCNP6, including the energy spectrum and angular spectrum of the photons, secondary and tertiary collimators, and a water phantom that tallied dose delivered at different points along the phantom are included. The experimental data for comparison was in the form of a percent depth dose curve as well as crossline and inline beam profiles. Experimental depth dose curve and beam profiles were acquired using a standard 0.125 cc ion chamber within a water phantom. In the computational model, the simulated depth dose curve was computed by tallying the total energy deposited in a stack of vertical slices down the depth of the phantom for percent depth dose curves. The simulated beam profiles were computed in a similar fashion, by tallying the energy deposited in a horizontal row, both in the x- and y-directions of cubic cells located at various depths. For the percent depth dose curve, a mean absolute percentage difference of 1.02%, 1.07%, and 1.94% were calculated for field sizes of 5 × 5 cm², 10 × 10 cm² and 20 × 20 cm², respectively, between the model and experimental measurements were calculated. We present our model as an example to guide other MCNP6 users in the medical physics community to create similar beam models for biomedical dose estimation and research calculations for predicting dose to newly developed phantoms.

Effect of ICRU report 90 recommendations on Monte Carlo calculated kQ for ionization chambers listed in the Addendum to AAPM's TG-51 protocol

PURPOSE

The ICRU has published new recommendations for ionizing radiation dosimetry. In this work, the effect of recommendations on the water-to-air and graphite-to-air restricted mass electronic stopping power ratios (s_{w, air} and s_{g, air} ) and the individual perturbation correction factors P_i was calculated. The effect on the beam quality conversion factors k_Q for reference dosimetry of high-energy photon beams was estimated for all ionization chambers listed in the Addendum to AAPM's TG-51 protocol.

METHODS

The s_{w, air} , s_{g, air} , individual P_i, and k_Q were calculated using EGSnrc Monte Carlo code system and key data of both ICRU report 37 and ICRU report 90. First, the P_i and k_Q were calculated using precise models of eight ionization chambers: NE2571 (Nuclear Enterprise), 30013, 31010, 31021 (PTW), Exradin A12, A12S, A1SL (Standard imaging), and FC-65P (IBA). In this simulation, the radiation sources were one ⁶⁰ Co beam and ten photon beams with nominal energy between 4 MV and 25 MV. Then, the change in k_Q for ionization chambers listed in the Addendum to AAPM's TG-51 protocol was calculated by changing the specification of the simple-model of ionization chamber. The simple-models were made with only cylindrical component modules. In this simulation, the radiation sources of ⁶⁰ Co beam and 24 MV photon beam were used.

RESULTS

The significant changes (p < 0.05) were observed for s_{w, air} , s_{g, air} , the wall correction factor P_wall , and the waterproofing sleeve correction factor P_sleeve . The decrease in s_{w, air} varied from -0.57% for a ⁶⁰ Co beam to -0.36% for the highest beam quality. The decrease in s_{g, air} varied from -0.72% to -1.12% in the same range. The changes in P_wall and P_sleeve were up to 0.41% and 0.14% and those maximum changes were observed for the ⁶⁰ Co beam. All changes in the central electrode correction factor P_cel , the stem correction factor P_stem , and the replacement correction factor P_repl were from -0.02% to 0.12%. Those changes were statistically insignificant (p = 0.07 or more) and were independent of photon energy. The change in k_Q was mainly characterized by the change in s_{w, air} , P_wall , and P_sleeve . The relationship between the change in k_Q and the beam quality index was linear approximately. The changes in k_Q of the simple-models were agreed with those of the precise-models within 0.08%.

CONCLUSION

The effects of ICRU-90 recommendations on k_Q for the ionization chambers listed in the Addendum to AAPM's TG-51 protocol were from -0.15% to 0.30%. To remove the known systematic effect on the clinical reference dosimetry, the k_Q based on ICRU-37 should be updated to the k_Q based on ICRU-90.

Monte Carlo Study of Unflattened Photon Beams Shaped by Multileaf Collimator

INTRODUCTION

This study investigates basic dosimetric properties of unflattened 6 MV photon beam shaped by multileaf collimator and compares them with those of flattened beams.

MATERIALS AND METHODS

Monte Carlo simulation model using BEAM code was developed for a 6MV photon beam based on Varian Clinic 600 unique performance linac operated with and without a flattening filter in beam line. Dosimetric features including lateral profiles, central axis depth dose, photon and electron spectra were calculated for flattened and unflattened cases, separately.

RESULTS

An increase in absolute depth dose with a factor of more than 2.4 was observed for unflattened beam which was dependent on depth. PDDs values were found to be lower for unflattened beam for all field sizes. Significant decrease in calculated mlc leakage was observed when the flattening filter was removed from the beam line. The total scatter factor, S_CP was found to show less variation with field sizes for unflattened beam indicating a decrease in head scatter. The beam profiles for unflattened case are found to have lower relative dose value in comparison with flattened beam near the field edge, and it falls off faster with distance.

CONCLUSION

Our study showed that increase in the dose rate and lower peripheral dose could be considered as realistic advantages for unflattened 6MV photon beams.

Part I: Out-of-field dose mapping for 6X and 6X-flattening-filter-free beams on the TrueBeam for extended distances

PURPOSE

With increasing cancer treatment success rates, many patients go on to live long, productive lives following recovery. Therefore, minimizing potential side effects due to dose outside the treated field is becoming a significant consideration in radiation therapy. With many potential treatment configurations available, it is important to quantify how out-of-field dose varies with common variables such as distance from isocenter, couch angle, jaw size, and flattening-filter setting. The accurate quantification of out-of-field dose at extended distances could also benefit researchers and detector developers. While data exist for out-of-field dose from older linear accelerator (Linac) models, the phenomenon has not been described for the latest generation of machines, such as the Varian TrueBeam. The purpose of this study was to comprehensively quantify out-of-field dose for the Varian TrueBeam Linac low energy photons in a wide range of positions and treatment geometries.

METHOD AND MATERIALS

Out-of-field doses were measured using two phantom setups: (a) A large volume ion chamber with a buildup sleeve to quantify head leakage and collimator scatter background dose; and (b) A farmer ion chamber in solid water to incorporate phantom scatter in addition to collimator scatter, and head leakage background dose. In both cases, the ion chamber was positioned with its length along the slowly varying transverse direction (perpendicular to the radial from isocenter). Doses were measured for four symmetric jaw settings (2 × 2 cm² , 4 × 4 cm² , 10 × 10 cm² , and 20 × 20 cm² ) for a range of distances from the isocenter (0-100 cm). The angular dependence of the out-of-field dose was measured using four different angles: 0°, 45°, 90°, and 135° with respect to the in-plane direction. All measurements were performed for both 6X and 6X-flattening-filter-free (FFF) beams.

RESULTS

The lowest out-of-field doses were observed at 60 cm away from isocenter in both in-plane and cross-plane directions for fields smaller than 10 × 10 cm² . Out-of-field dose decreased with decreasing jaw size (a factor of 4.7 for 6X-FFF and a factor of 3.1 for 6X going from 20 × 20 cm² to 2 × 2 cm² at 60 cm from isocenter in the in-plane direction). The 6X-FFF beam produced out-of-field doses as low as 64% of the 6X beam.

CONCLUSION

This study presents a comprehensive description of 6X and 6X-FFF out-of-field doses on a Varian TrueBeam Linac including measurements at a range of positions, angles, and jaw settings and with and without phantom scatter.

PENELOPE/PRIMO-calculated photon and electron spectra from clinical accelerators

BACKGROUND

The availability of photon and electron spectra in digital form from current accelerators and Monte Carlo (MC) systems is scarce, and one of the packages widely used refers to linacs with a reduced clinical use nowadays. Such spectra are mainly intended for the MC calculation of detector-related quantities in conventional broad beams, where the use of detailed phase-space files (PSFs) is less critical than for MC-based treatment planning applications, but unlike PSFs, spectra can easily be transferred to other computer systems and users.

METHODS

A set of spectra for a range of Varian linacs has been calculated using the PENELOPE/PRIMO MC system. They have been extracted from PSFs tallied for field sizes of 10 cm × 10 cm and 15 cm × 15 cm for photon and electron beams, respectively. The influence of the spectral bin width and of the beam central axis region used to extract the spectra have been analyzed.

RESULTS

Spectra have been compared to those by other authors showing good agreement with those obtained using the, now superseded, EGS4/BEAM MC code, but significant differences with the most widely used photon data set. Other spectra, particularly for electron beams, have not been published previously for the machines simulated in this work. The influence of the bin width on the spectrum mean energy for 6 and 10 MV beams has been found to be negligible. The size of the region used to extract the spectra yields differences of up to 40% for the mean energies in 10 MV beams, but the maximum difference for TPR _20,10 values derived from depth-dose distributions does not exceed 2% relative to those obtained using the PSFs. This corresponds to k_Q differences below 0.2% for a typical Farmer-type chamber, considered to be negligible for reference dosimetry. Different configurations for using electron spectra have been compared for 6 MeV beams, concluding that the geometry used for tallying the PSFs used to extract the spectra must be accounted for in subsequent calculations using the spectra as a source.

CONCLUSIONS

An up-to-date set of consistent spectra for Varian accelerators suitable for the calculation of detector-related quantities in conventional broad beams has been developed and made available in digital form.

Benchmarking of Monte Carlo model of Siemens Oncor® linear accelerator for 18MV photon beam: Determination of initial electron beam parameters

OBJECTIVE

This study aims to benchmark a Monte Carlo (MC) model of the 18 MV photon beam produced by the Siemens Oncor® linac using the BEAMnrc and DOSXYZnrc codes.

METHODS

By matching the percentage depth doses and beam profiles calculated by MC simulations with measurements, the initial electron beam parameters including electron energy, full width at half maximum (spatial FWHM), and mean angular spread were derived for the 10×10 cm2 and 20×20 cm2 field sizes. The MC model of the 18 MV photon beam was then validated against the measurements for different field sizes (5×5, 30×30 and 40×40 cm2) by gamma index analysis.

RESULTS

The optimum values for electron energy, spatial FWHM and mean angular spread were 14.2 MeV, 0.08 cm and 0.8 degree, respectively. The MC simulations yielded the comparable measurement results of these optimum parameters. The gamma passing rates (with acceptance criteria of 1% /1 mm) for percentage depth doses were found to be 100% for all field sizes. For cross-line profiles, the gamma passing rates were 100%, 97%, 95%, 96% and 95% for 5×5, 10×10, 20×20, 30×30 and 40×40 cm2 field sizes, respectively.

CONCLUSIONS

By validation of the MC model of Siemens Oncor® linac using various field sizes, it was found that both dose profiles of small and large field sizes were very sensitive to the changes in spatial FWHM and mean angular spread of the primary electron beam from the bending magnet. Hence, it is recommended that both small and large field sizes of the 18 MV photon beams should be considered in the Monte Carlo linac modeling.

Monte Carlo modeling of a 60Co MRI-guided radiotherapy system on Geant4 and experimental verification of dose calculation under a magnetic field of 0.35 T

Our purpose was to establish the commissioning procedure of Monte Carlo modeling on a magnetic resonance imaging-guided radiotherapy system (MRIdian, Viewray Inc.) under a magnetic field of 0.345 T through experimental measurements. To do this, we sought (i) to assess the depth-dose and lateral profiles generated by the Geant4 using either EBT3 film or the BJR-25 data; (ii) to assess the calculation accuracy under a magnetic field of 0.345 T. The radius of the electron trajectory caused by the electron return effect (ERE) in a vacuum was obtained both by the Geant4 and the theoretical methods. The surface dose on the phantom was calculated and compared with that obtained from the film measurements. The dose distribution in a phantom having two air gaps was calculated and measured with EBT 3 film. (i) The difference of depth-dose profile generated by the Geant4 from the BJR-25 data was 0.0 ± 0.8% and 0.3 ± 1.5% for field sizes of 4.5 and 27.3 cm2, respectively. Lateral dose profiles generated by Geant4 agreed well with those generated from the EBT3 film data. (ii) The radius of the electron trajectory generated by Geant4 agreed well with the theoretical values. A maximum of ~50% reduction of the surface dose under a magnetic field of 0.345 T was observed due to elimination of the electron contamination caused by the magnetic field, as determined by both the film measurements and the Geant4. Changes in the dose distributions in the air gaps caused by the ERE were observed on the Geant4 and in the film measurements. Gamma analysis (3%/3 mm) showed a pass rate of 95.1%. Commissioning procedures for the MRI-guided radiotherapy system on the Geant4 were established, and we concluded that the Geant4 had provided high calculation accuracy under a magnetic field of 0.345 T.

Towards photon radiotherapy treatment planning with high Z nanoparticle radiosensitisation agents: the Relative Biological Effective Dose (RBED) framework

A novel treatment planning framework, the Relative Biological Effective Dose (RBED), for high Z nanoparticle (NP)-enhanced photon radiotherapy is developed and tested in silico for the medical exemplar of neoadjuvant (preoperative) breast cancer MV photon radiotherapy. Two different treatment scenarios, conventional and high Z NP enhanced, were explored with a custom Geant4 application that was developed to emulate the administration of a single 2 Gy fraction as part of a 50 Gy radiotherapy treatment plan. It was illustrated that there was less than a 1% difference in the dose deposition throughout the standard and high Z NP-doped adult female phantom. Application of the RBED framework found that the extent of possible biological response with high Z NP doping was great than expected via the dose deposition alone. It is anticipated that this framework will assist the scientific community in future high Z NP-enhanced in-silico, pre-clinical and clinical trials.

Monte Carlo and analytic modeling of an Elekta Infinity linac with Agility MLC: Investigating the significance of accurate model parameters for small radiation fields

PURPOSE

To explain the deviation observed between measured and Monaco calculated dose profiles for a small field (i.e., alternating open-closed MLC pattern). A Monte Carlo (MC) model of an Elekta Infinity linac with Agility MLC was created and validated against measurements. In addition, an analytic model which predicts the fluence at the isocenter plane was used to study the impact of multiple beam parameters on the accuracy of dose calculations for small fields.

METHODS

A detailed MC model of a 6 MV Elekta Infinity linac with Agility MLC was created in EGSnrc/BEAMnrc and validated against measurements. An analytic model using primary and secondary virtual photon sources was created and benchmarked against the MC simulations and the impact of multiple beam parameters on the accuracy of the model for a small field was investigated. Both models were used to explain discrepancies observed between measured/EGSnrc simulated and Monaco calculated dose profiles for alternating open-closed MLC leaves.

RESULTS

MC-simulated dose profiles (PDDs, cross- and in-line profiles, etc.) were found to be in very good agreements with measurements. The best fit for the leaf bank rotation was found to be 9 mrad to model the defocusing of Agility MLC. Moreover, a very good agreement was observed between results from the analytic model and MC simulations for a small field. Modifying the radial size of the incident electron beam in the BEAMnrc model improved the agreement between Monaco and EGSnrc calculated dose profiles by approximately 16% and 30% in the position of maxima and minima, respectively.

CONCLUSION

Accurate modeling of the full-width-half-maximum (FWHM) of the primary photon source as well as the MLC leaf design (leaf bank rotation, etc.) is essential for accurate calculations of dose delivered by small radiation fields when using virtual source or MC models of the beam.

Comparison of Measured and Monte Carlo Calculated Dose Distributions from Indigenously Developed 6 MV Flattening Filter Free Medical Linear Accelerator

Purpose:

Monte Carlo simulation was carried out for a 6 MV flattening filter-free (FFF) indigenously developed linear accelerator (linac) using the BEAMnrc user-code of the EGSnrc code system. The model was benchmarked against the measurements. A Gaussian distributed electron beam of kinetic energy 6.2 MeV with full-width half maximum of 1 mm was used in this study.

Methods:

The simulation of indigenously developed linac unit has been carried out by using the Monte Carlo-based BEAMnrc user-code of the EGSnrc code system. Using the simulated model, depth and lateral dose profiles were studied using the DOSXYZnrc user-code. The calculated dose data were compared against the measurements using an RFA dosimertic system made by PTW, Germany (water tank MP3-M and 0.125 cm³ ion chamber).

Results:

The BEAMDP code was used to analyze photon fluence spectra, mean energy distribution, and electron contamination fluence spectra. Percentage depth dose (PDD) and beam profiles (along both X and Y directions) were calculated for the field sizes 5 cm × 5 cm - 25 cm × 25 cm. The dose difference between the calculated and measured PDD and profile values were under 1%, except for the penumbra region where the maximum deviation was found to be around 3%.

Conclusions:

A Monte Carlo model of indigenous FFF linac (6 MV) has been developed and benchmarked against the measured data.

Characterization of the Cerenkov scatter function: a convolution kernel for Cerenkov light dosimetry

Cerenkov light is created in clinical applications involving high-energy radiation such as in radiation therapy. There is considerable interest in using Cerenkov light as a means to perform in vivo dosimetry during radiation therapy; however, a better understanding of the light-to-dose relationship is needed. One such method to solve this relationship is that of a deconvolution formulation, which relies on the Cerenkov scatter function (CSF). The CSF describes the creation of Cerenkov photons by a pencil beam of high-energy radiation, and the subsequent scattering that occurs before emission from the irradiated medium surface. This study investigated the dependence of the CSF on common radiation beam parameters (beam energy and incident angle) and the type of irradiated medium. An analytical equation with fitting coefficients of the CSF was obtained for common beam energies in a stratified skin model and optical phantom. Perturbation analysis was performed to investigate the dependence of the deconvolved Cerenkov images on the full-width at half-maximum and amplitude of the CSF. The irradiated material and beam angle had a large impact on the deconvolution process, whereas the beam energy had little effect.

Study of dosimetric properties of flattened and unflattened megavoltage x ray beam on high Z implant materials

PURPOSE

Addition of high Z implants in the treatment vicinity or beam path is unavoidable in certain clinical situation. In this work, we study the properties of radiation interaction parameters such as mass attenuation coefficient (MAC), x ray beam transmission factor (indirect beam attenuation), interface effects like backscatter dose perturbation factor (BSDF) and forward dose perturbation factor (FDPF) for flattened (FF) and unflattened (UF) x ray beams.

METHODS

MAC for stainless steel and titanium alloy was measured using CC13 chamber with appropriate buildup in narrow beam geometry. The x ray beam transmission factors were measured for stainless steel and titanium alloy for different field size, off-axis, and depths. Profile analysis was performed using a radiation field analyzer (RFA) as a function of field size and depth to study the influence of phantom scattering and spectral variation in the beam. In addition, interface effects such as BSDF and FDPF were measured with gafchromic films at maximum BSDF peak position calculated using Acuros XB algorithm and with PPC40 chamber measured at exit side of high Z material, respectively.

RESULTS

The MAC in both cases decreases with increase in energy for stainless steel (SS) and titanium (Ti) alloy. The MAC increases with the change in x ray beam type from flattened to UF beam because of relatively lower mean energy. The x ray beam transmission factor increases with the increase in energy, field size, and depth owing to increase in penetration power phantom scatter, respectively. The measured BSDF and FDPF were found to be in good agreement with AXB algorithm.

CONCLUSION

The dosimetric properties of x ray photon beam were studied comprehensively in the presence of high Z material like stainless steel and titanium alloy using both flattened and UF beams to understand and incorporate the findings of various parameters in clinical condition due to the variation in energy spectrum from FF to UF x ray beam.

Monte Carlo determination of k Q and k Qmsr values for the exradin A26 ionisation chamber for the Varian TrueBeam

We have calculated conversion factors, k _Q for the A26 micro ionisation chamber along with machine specific reference beam quality factors, k _Qmsr, for a number of field sizes and beam qualities for the Varian TrueBeam accelerator. The A12 ionisation chamber was simulated alongside the A26, so as to validate against known literature values. Both ionisation chambers were modelled from manufacturer data sheets and schematics. The egs_chamber Monte Carlo user code was used to simulate each absorbed dose relevant to the beam quality conversion factors k _Q and k _Qmsr. Tabulated spectra for beam energies of 4 through 25 MV were used in the k _Q calculations for both investigated chambers. Varian TrueBeam phase space files for 6 MV flattened as well as 6 and 10 MV unflattened beams were used in the simulations of the A26 chamber in field sizes from 2  ×  2 cm square to 20  ×  20 cm square in order to determine k _Qmsr values. The PDD(10)x  values of the tabulated spectra were found to be within variation between studies, with an average deviance of 0.4% from one prior study. The simulated A12 k _Q values matched the accepted literature values with an average variation of  <0.1%. The A26 k _Q values match the manufacturer provided values to within 0.5%. For all investigated field sizes the k _Qmsr values are within 0.006 of unity. There is no published data for this chamber for a direct comparison, but there is similarity between these results and results from other chambers regularly used in similar circumstances. Furthermore, the agreement of the simulated k _Q values to knowns, and the agreement of the PDD(10)x  factors would suggest the correctness and accuracy of the study.

Relation between DNA double-strand breaks and energy spectra of secondary electrons produced by different X-ray energies

Purpose: In a radiological examination, low-energy X-radiation is used (<100 keV). For other radiological procedures, the energy used is several MeV. ICRP in publication 103 has currently considered that photons irrespective of their energy have the same radiation weighting factor. Nevertheless, there are topological differences at the nanoscale of X-ray energy deposition as a function of its energy spectrum, meaning that the different interactions with living matter could vary in biological efficacy. Materials and methods: To study these differences, we characterized our irradiation conditions in terms of initial photon energies, but especially in terms of energy spectra of secondary electrons at the cell nucleus level, using Monte Carlo simulations. We evaluated signaling of DNA damage by monitoring a large number of γH2A.X foci after exposure of G0/G1-phase synchronized human primary endothelial cells from 0.25 to 5 Gy at 40 kV, 220 kV and 4 MV X-rays. Number and spatial distribution of γH2A.X foci were explored. In parallel, we investigated cell behavior through cell death and ability of a mother cell to produce two daughter cells. We also studied the missegregation rate after cell division. Results: We report a higher number of DNA double-strand breaks signaled by γH2A.X for 40 kVp and/or 220 kVp compared to 4 MVp for the highest tested doses of 2 and 5 Gy. We observed no difference between the biological endpoint studies with 40 kVp and 220 kVp X-ray spectra. This lack of difference could be explained by the relative similarity of the calculated energy spectra of secondary electrons at the cell monolayer. Conclusion: The energy spectrum of secondary electrons seems to be more closely related to the level of DNA damage measured by γH2A.X than the initial spectrum of photon energy or voltage settings. Our results indicate that as the energy spectrum of secondary electrons increases, the DNA damage signaled by γH2A.X decreases and this effect is observable beyond 220 kVp.

Impact of new ICRU Report 90 recommendations on calculated correction factors for reference dosimetry

In 2016 the ICRU published a new report dealing with key data for ionizing radiation dosimetry (ICRU Report 90). New recommendations have been made for the mean excitation energies I for air, graphite and liquid water as well as for the graphite density to use when evaluating the density effect. In addition, the ICRU Report 90 discusses renormalized photoelectric cross sections, but refuses to give a recommendation on the use of renormalization factors. However, the Consultative Committee for Ionizing Radiation recommends to use renormalized photoeffect cross sections. Goal of the present work is to evaluate the impact of these new recommendations on clinical reference dosimetry for high energy photon and electron beams. The beam quality correction factor k _Q was calculated by Monte Carlo simulations for compact and parallel plate ionization chambers. In case of photons seven phase space files from clinical accelerators and twelve spectra taken from literature from 4 MV to 24 MV and additionally a ⁶⁰Co source were applied. As electron source thirteen electron spectra available in literature were used in the range of 4 MeV-21 MeV. The new ICRU recommendations have a small impact on Monte Carlo calculated k _Q values for the chosen ionization chambers in the range of 0.1%-0.35% only-the difference increases for higher photon energies. The impact of the ICRU Report 90 recommendations on Monte Carlo calculated stopping power ratios s _w,a , perturbation factors p and beam quality correction factors k _Q was investigated and confirmed a decrese of s _w,a by a fraction of a percent for photon and electron beams. This study indicates that the impact of the new ICRU recommendation is within 0.35%. The determined deviations should be taken into account, when widely published Monte Carlo calculated values are examined.

Technical Note: On maximizing Cherenkov emissions from medical linear accelerators

PURPOSE

Cherenkov light during MV radiotherapy has recently found imaging and therapeutic applications but is challenged by relatively low fluence. Our purpose is to investigate the feasibility of increasing Cherenkov light production during MV radiotherapy by increasing photon energy and applying specialized beam-hardening filtration.

METHODS

GAMOS 5.0.0, a GEANT4-based framework for Monte Carlo simulations, was used to model standard clinical linear accelerator primary photon beams. The photon source was incident upon a 17.8 cm³ cubic water phantom with a 94 cm source to surface distance. Dose and Cherenkov production was determined at depths of 3-9 cm. Filtration was simulated 15 cm below the photon beam source. Filter materials included aluminum, iron, and copper with thicknesses of 2-20 cm. Histories used depended on the level of attenuation from the filter, ranging from 100 million to 2 billion. Comparing average dose per history also allowed for evaluation of dose-rate reduction for different filters.

RESULTS

Overall, increasing photon beam energy is more effective at improving Cherenkov production per unit dose than is filtration, with a standard 18 MV beam yielding 3.3-4.0× more photons than 6 MV. Introducing an aluminum filter into an unfiltered 2400 cGy/min 10 MV beam increases the Cherenkov production by 1.6-1.7×, while maintaining a clinical dose rate of 300 cGy/min, compared to increases of ~1.5× for iron and copper. Aluminum was also more effective than the standard flattening filter, with the increase over the unfiltered beam being 1.4-1.5× (maintaining 600 cGy/min dose rate) vs 1.3-1.4× for the standard flattening filter. Applying a 10 cm aluminum filter to a standard 18 MV, photon beam increased the Cherenkov production per unit dose to 3.9-4.3× beyond that of 6 MV (vs 3.3-4.0× for 18 MV with no aluminum filter).

CONCLUSIONS

Through a combination of increasing photon energy and applying specialized beam-hardening filtration, the amount of Cherenkov photons per unit radiotherapy dose can be increased substantially.

A photon source model based on particle transport in a parameterized accelerator structure for Monte Carlo dose calculations

PURPOSE

An accurate source model of a medical linear accelerator is essential for Monte Carlo (MC) dose calculations. This study aims to propose an analytical photon source model based on particle transport in parameterized accelerator structures, focusing on a more realistic determination of linac photon spectra compared to existing approaches.

METHODS

We designed the primary and secondary photon sources based on the photons attenuated and scattered by a parameterized flattening filter. The primary photons were derived by attenuating bremsstrahlung photons based on the path length in the filter. Conversely, the secondary photons were derived from the decrement of the primary photons in the attenuation process. This design facilitates these sources to share the free parameters of the filter shape and be related to each other through the photon interaction in the filter. We introduced two other parameters of the primary photon source to describe the particle fluence in penumbral regions. All the parameters are optimized based on calculated dose curves in water using the pencil-beam-based algorithm. To verify the modeling accuracy, we compared the proposed model with the phase space data (PSD) of the Varian TrueBeam 6 and 15 MV accelerators in terms of the beam characteristics and the dose distributions. The EGS5 Monte Carlo code was used to calculate the dose distributions associated with the optimized model and reference PSD in a homogeneous water phantom and a heterogeneous lung phantom. We calculated the percentage of points passing 1D and 2D gamma analysis with 1%/1 mm criteria for the dose curves and lateral dose distributions, respectively.

RESULTS

The optimized model accurately reproduced the spectral curves of the reference PSD both on- and off-axis. The depth dose and lateral dose profiles of the optimized model also showed good agreement with those of the reference PSD. The passing rates of the 1D gamma analysis with 1%/1 mm criteria between the model and PSD were 100% for 4 × 4, 10 × 10, and 20 × 20 cm² fields at multiple depths. For the 2D dose distributions calculated in the heterogeneous lung phantom, the 2D gamma pass rate was 100% for 6 and 15 MV beams. The model optimization time was less than 4 min.

CONCLUSION

The proposed source model optimization process accurately produces photon fluence spectra from a linac using valid physical properties, without detailed knowledge of the geometry of the linac head, and with minimal optimization time.

On the impact of ICRU report 90 recommendations on kQ factors for high-energy photon beams

PURPOSE

To assess the impact of the ICRU report 90 recommendations on the beam-quality conversion factor, kQ , used for clinical reference dosimetry of megavoltage linac photon beams.

METHODS

The absorbed dose to water and the absorbed dose to the air in ionization chambers representative of those typically used for linac photon reference dosimetry are calculated at the reference depth in a water phantom using Monte Carlo simulations. Depth-dose calculations in water are also performed to investigate changes in beam quality specifiers. The calculations are performed in a cobalt-60 beam and MV photon beams with nominal energy between 6 MV and 25 MV using the EGSnrc simulation toolkit. Inputs to the calculations use stopping-power data for graphite and water from the original ICRU-37 report and the new proposed values from the recently published ICRU-90 report. Calculated kQ factors are compared using the two different recommendations for key dosimetry data and measured kQ factors.

RESULTS

Less than about 0.1% effects from ICRU-90 recommendations on the beam quality specifiers, the photon component of the percentage depth-dose at 10 cm, %dd(10)x , and the tissue-phantom ratio at 20 cm and 10 cm, TPR1020, are observed. Although using different recommendations for key dosimetric data impact water-to-air stopping-power ratios and ion chamber perturbation corrections by up to 0.54% and 0.40%, respectively, we observe little difference (≤0.14%) in calculated kQ factors. This is contradictory to the predictions in ICRU-90 that suggest differences up to 0.5% in high-energy photon beams. A slightly better agreement with experimental values is obtained when using ICRU-90 recommendations.

CONCLUSION

Users of the addendum to the TG-51 protocol for reference dosimetry of high-energy photon beams, which recommends Monte Carlo calculated kQ factors, can rest assured that the recommendations of ICRU report 90 on basic data have little impact on this central dosimetric parameter.

A mathematical deconvolution formulation for superficial dose distribution measurement by Cerenkov light dosimetry

PURPOSE

Cerenkov photons are created by high-energy radiation beams used for radiation therapy. In this study, we developed a Cerenkov light dosimetry technique to obtain a two-dimensional dose distribution in a superficial region of medium from the images of Cerenkov photons by using a deconvolution method.

METHODS

An integral equation was derived to represent the Cerenkov photon image acquired by a camera for a given incident high-energy photon beam by using convolution kernels. Subsequently, an equation relating the planar dose at a depth to a Cerenkov photon image using the well-known relationship between the incident beam fluence and the dose distribution in a medium was obtained. The final equation contained a convolution kernel called the Cerenkov dose scatter function (CDSF). The CDSF function was obtained by deconvolving the Cerenkov scatter function (CSF) with the dose scatter function (DSF). The GAMOS (Geant4-based Architecture for Medicine-Oriented Simulations) Monte Carlo particle simulation software was used to obtain the CSF and DSF. The dose distribution was calculated from the Cerenkov photon intensity data using an iterative deconvolution method with the CDSF. The theoretical formulation was experimentally evaluated by using an optical phantom irradiated by high-energy photon beams.

RESULTS

The intensity of the deconvolved Cerenkov photon image showed linear dependence on the dose rate and the photon beam energy. The relative intensity showed a field size dependence similar to the beam output factor. Deconvolved Cerenkov images showed improvement in dose profiles compared with the raw image data. In particular, the deconvolution significantly improved the agreement in the high dose gradient region, such as in the penumbra. Deconvolution with a single iteration was found to provide the most accurate solution of the dose. Two-dimensional dose distributions of the deconvolved Cerenkov images agreed well with the reference distributions for both square fields and a multileaf collimator (MLC) defined, irregularly shaped field.

CONCLUSIONS

The proposed technique improved the accuracy of the Cerenkov photon dosimetry in the penumbra region. The results of this study showed initial validation of the deconvolution method for beam profile measurements in a homogeneous media. The new formulation accounted for the physical processes of Cerenkov photon transport in the medium more accurately than previously published methods.

Effect of each component of a LINAC therapy head on neutron and photon spectra

Linear accelerators (LINACs) are widely applied in radiotherapy for their versatility and flexibility. Monte Carlo simulations were made to find the neutron and photon spectra at the isocenter (IC) of a LINAC operating at 10, 15, 18, and 24 MV by the MCNPX code. A detailed model of the LINAC head, consisting of flattening filter, secondary collimator, primary collimator, and multi-leaf collimator were used in the calculations. The effect of eliminating any of these components on contamination of a neutron spectrum and a photon spectrum was assessed. Photon and neutron ambient equivalent doses were found, and comparisons were made for the various structures. Lethargy neutron spectra at the IC were compared with spectra computed with the function reported by Tosi et al., which describes well neutron spectra for the energy region beyond 1 MeV, although tending to undervalue energy spectra below 1 MeV. The findings show that the photon and neutron fluences are enhanced when eliminating a LINAC component. The neutron and photon doses increased except when removing the primary collimator.

Image guidance doses delivered during radiotherapy: Quantification, management, and reduction: Report of the AAPM Therapy Physics Committee Task Group 180

BACKGROUND

With radiotherapy having entered the era of image guidance, or image-guided radiation therapy (IGRT), imaging procedures are routinely performed for patient positioning and target localization. The imaging dose delivered may result in excessive dose to sensitive organs and potentially increase the chance of secondary cancers and, therefore, needs to be managed.

AIMS

This task group was charged with: a) providing an overview on imaging dose, including megavoltage electronic portal imaging (MV EPI), kilovoltage digital radiography (kV DR), Tomotherapy MV-CT, megavoltage cone-beam CT (MV-CBCT) and kilovoltage cone-beam CT (kV-CBCT), and b) providing general guidelines for commissioning dose calculation methods and managing imaging dose to patients.

MATERIALS & METHODS

We briefly review the dose to radiotherapy (RT) patients resulting from different image guidance procedures and list typical organ doses resulting from MV and kV image acquisition procedures.

RESULTS

We provide recommendations for managing the imaging dose, including different methods for its calculation, and techniques for reducing it. The recommended threshold beyond which imaging dose should be considered in the treatment planning process is 5% of the therapeutic target dose.

DISCUSSION

Although the imaging dose resulting from current kV acquisition procedures is generally below this threshold, the ALARA principle should always be applied in practice. Medical physicists should make radiation oncologists aware of the imaging doses delivered to patients under their care.

CONCLUSION

Balancing ALARA with the requirement for effective target localization requires that imaging dose be managed based on the consideration of weighing risks and benefits to the patient.

Neutron and photon scattering properties of high density concretes used in radiation therapy facilities: A Monte Carlo study

In the current study the neutron and photon scattering properties of some newly developed high density concretes (HDCs) were calculated by using MCNPX Monte Carlo code. Five high-density concretes including Steel-Magnetite, Barite, Datolite-Galena, Ilmenite-ilmenite, Magnetite-Lead with the densities ranging from 5.11 g/cm3 and ordinary concrete with density of 2.3 g/cm3 were studied in our simulations. The photon beam spectra of 4 and 18 MV from Varian linac and neutron spectra of clinical 18 MeV photon beam was used for calculations. The fluence of scattered photon and neutron from all studied concretes was calculated in different angles. Overall, the ordinary concrete showed higher scattered photons and Datolite-Galena concrete (4.42 g/cm3) had the lowest scattered photons among all studied concretes. For neutron scattering, fluence at the angle of 180 was higher relative to other angles while for photons scattering fluence was maximum at 90 degree. The scattering fluence for photons and neutrons was dependent on the angle and composition of concrete. The results showed that the fluence of scattered photons and neutrons changes with the composition of high density concrete. Also, for high density concretes, the variation of scattered fluence with angle was very pronounced for neutrons but it changed slightly for photons. The results can be used for design of radiation therapy bunkers.

Radiation dosimetry in magnetic fields with Farmer-type ionization chambers: determination of magnetic field correction factors for different magnetic field strengths and field orientations

The aim of this work was to determine magnetic field correction factors that are needed for dosimetry in hybrid devices for MR-guided radiotherapy for Farmer-type ionization chambers for different magnetic field strengths and field orientations. The response of six custom-built Farmer-type chambers irradiated at a 6 MV linac was measured in a water tank positioned in a magnet with magnetic field strengths between 0.0 T and 1.1 T. Chamber axis, beam and magnetic field were perpendicular to each other and both magnetic field directions were investigated. EGSnrc Monte Carlo simulations were compared to the measurements and simulations with different field orientations were performed. For all geometries, magnetic field correction factors, [Formula: see text], and perturbation factors were calculated. A maximum increase of 8.8% in chamber response was measured for the magnetic field perpendicular to chamber and beam axis. The measured chamber response could be reproduced by adjusting the dead volume layer near the chamber stem in the Monte Carlo simulations. For the magnetic field parallel to the chamber axis or parallel to the beam, the simulated response increased by 1.1% at maximum for field strengths up to 1.1 T. A complex dependence of the response was found on chamber radius, magnetic field strength and orientation of beam, chamber axis and magnetic field direction. Especially for magnetic fields perpendicular to beam and chamber axis, the exact sensitive volume has to be considered in the simulations. To minimize magnetic field correction factors and the influence of dead volumes on the response of Farmer chambers, a measurement set-up with the magnetic field parallel to the chamber axis or parallel to the beam is recommended for dosimetry.

Effect of VERO pan-tilt motion on the dose distribution

Tumor tracking is an option for intra-fractional motion management in radiotherapy. The VERO gimbal tracking system creates a unique beam geometry and understanding the effect of the gimbal motion in terms of dose distribution is important to assess the dose deviation from the reference conditions. Beam profiles, output factors (OF) and percentage depth doses (PDD) were measured and evaluated to investigate this effect. In order to find regions affected by the pan-tilt motion, synthesized 2D dose distributions were generated. An evaluation of the 2D dose distribution with the reference position was done using dose difference criteria 1%-4%. The OF and point dose at central axis were measured and compared with the reference position. Furthermore, the PDDs were measured using a special monitoring approach to filtering inaccurate points during the acquisition. Beam profiles evaluation showed that the effect of pan-tilt at inline direction was stronger than at the crossline direction. The maximum average deviation of the full width half maximum (FWHM), flatness, symmetry, penumbra left and right were 0.39 ± 0.25 mm, 0.62 ± 0.50%, 0.76 ± 0.59%, 0.22 ± 0.16 mm, and 0.19 ± 0.15 mm respectively. The ÔF and the measured dose average deviation were <0.5%. The mechanical accuracies during the PDD measurements were 0.28 ± 0.09 mm and 0.21 ± 0.09 mm for pan and tilt and pan or tilt position. The PDD average deviations were 0.58 ± 0.26 % and 0.54 ± 0.25 % for pan-or-tilt and pan-and-tilt position respectively. All the results showed that the deviation at pan and tilt position are higher than pan or tilt. The most influences were observed for the penumbra region and the shift of radiation beam path.

Development of a Monte Carlo multiple source model for inclusion in a dose calculation auditing tool

PURPOSE

The Imaging and Radiation Oncology Core Houston (IROC-H) (formerly the Radiological Physics Center) has reported varying levels of agreement in their anthropomorphic phantom audits. There is reason to believe one source of error in this observed disagreement is the accuracy of the dose calculation algorithms and heterogeneity corrections used. To audit this component of the radiotherapy treatment process, an independent dose calculation tool is needed.

METHODS

Monte Carlo multiple source models for Elekta 6 MV and 10 MV therapeutic x-ray beams were commissioned based on measurement of central axis depth dose data for a 10 × 10 cm² field size and dose profiles for a 40 × 40 cm² field size. The models were validated against open field measurements consisting of depth dose data and dose profiles for field sizes ranging from 3 × 3 cm² to 30 × 30 cm² . The models were then benchmarked against measurements in IROC-H's anthropomorphic head and neck and lung phantoms.

RESULTS

Validation results showed 97.9% and 96.8% of depth dose data passed a ±2% Van Dyk criterion for 6 MV and 10 MV models respectively. Dose profile comparisons showed an average agreement using a ±2%/2 mm criterion of 98.0% and 99.0% for 6 MV and 10 MV models respectively. Phantom plan comparisons were evaluated using ±3%/2 mm gamma criterion, and averaged passing rates between Monte Carlo and measurements were 87.4% and 89.9% for 6 MV and 10 MV models respectively.

CONCLUSIONS

Accurate multiple source models for Elekta 6 MV and 10 MV x-ray beams have been developed for inclusion in an independent dose calculation tool for use in clinical trial audits.