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

Dose per monitor unit in photon fields generated by clinical linear accelerators can be affected by the backscattered radiation into the monitor chamber from collimator jaws. Thus, it is necessary to account for the backscattered radiation in computing monitor unit setting for a treatment field. In this work, we investigated effects of the backscatter from collimator jaws based on Monte Carlo simulations of a clinical linear accelerator. The backscattered radiation scored within the monitor chamber was identified as originating either from the upper jaws (Y jaws), or from the lower jaws (X jaws). From the results of Monte Carlo simulations, ratios of the monitor-chamber-scored dose caused by the backscatter to the dose caused by the forward radiation, R(x,y), were modeled as functions of the individual X and Y jaw positions. The amount of the backscattered radiation for any field setting was then computed as a compound contribution from both the X and Y jaws. The dose ratios of R(x,y) were then used to calculate the change in photon output caused by the backscatter, Scb(x,y). Results of these calculations were compared with available measured data based on counting the electron pulses or charge from the electron target of an accelerator. Data from this study showed that the backscattered radiation contributes approximately 3% to the monitor-chamber-scored dose. A majority of the backscattered radiation comes from the upper jaws, which are located closer to the monitor chamber. The amount of the backscatter decreases approximately in a linear fashion with the jaw opening. This results in about a 2% increase of photon output from a 10 cm x 10 cm field to a 40 cm x 40 cm field. The off-axis location of the jaw opening does not have a significant effect on the magnitude of the backscatter. The backscatter effect is significant for monitor chambers using kapton windows, particularly for treatment fields using moving jaws. Applying the backscatter correction improves the accuracy of monitor-unit calculation using a model-based dose calculation algorithm such as the convolution method.     

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.     

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.     

Monte Carlo simulation of MOSFET dosimeter for electron backscatter using the GEANT4 code

The aim of this study is to investigate the influence of the body of the metal-oxide-semiconductor field effect transistor (MOSFET) dosimeter in measuring the electron backscatter from lead. The electron backscatter factor (EBF), which is defined as the ratio of dose at the tissue-lead interface to the dose at the same point without the presence of backscatter, was calculated by the Monte Carlo simulation using the GEANT4 code. Electron beams with energies of 4, 6, 9, and 12 MeV were used in the simulation. It was found that in the presence of the MOSFET body, the EBFs were underestimated by about 2%-0.9% for electron beam energies of 4-12 MeV, respectively. The trend of the decrease of EBF with an increase of electron energy can be explained by the small MOSFET dosimeter, mainly made of epoxy and silicon, not only attenuated the electron fluence of the electron beam from upstream, but also the electron backscatter generated by the lead underneath the dosimeter. However, this variation of the EBF underestimation is within the same order of the statistical uncertainties as the Monte Carlo simulations, which ranged from 1.3% to 0.8% for the electron energies of 4-12 MeV, due to the small dosimetric volume. Such small EBF deviation is therefore insignificant when the uncertainty of the Monte Carlo simulation is taken into account. Corresponding measurements were carried out and uncertainties compared to Monte Carlo results were within +/- 2%. Spectra of energy deposited by the backscattered electrons in dosimetric volumes with and without the lead and MOSFET were determined by Monte Carlo simulations. It was found that in both cases, when the MOSFET body is either present or absent in the simulation, deviations of electron energy spectra with and without the lead decrease with an increase of the electron beam energy. Moreover, the softer spectrum of the backscattered electron when lead is present can result in a reduction of the MOSFET response due to stronger recombination in the SiO2 gate. It is concluded that the MOSFET dosimeter performed well for measuring the electron backscatter from lead using electron beams. The uncertainty of EBF determined by comparing the results of Monte Carlo simulations and measurements is well within the accuracy of the MOSFET dosimeter (< +/- 4.2%) provided by the manufacturer.     

Monte Carlo simulation of backscatter from lead for clinical electron beams using EGSnrc

In electron radiotherapy of superficial lesions in the eyelid, lip, buccal mucosa, ear, and nose, backscattered electrons are produced from the lead shield used to protect the critical tissue underneath the tumor. In this study, the backscattered electrons, produced by clinical electron beams using a Varian 21 EX linear accelerator, were studied using Monte Carlo simulations. The electron backscatter factor (EBF), defined as the ratio of dose at the tissue-lead interface to the dose at the same point without the presence of backscatter, was calculated using the Monte Carlo EGSnrc-based code. The calculated EBFs were verified with measurements using metal-oxide-semiconductor field effect transistor detectors. The effect of the (1) initial electron beam energy, (2) thickness of bolus over the lead shield, (3) beam's angle of incidence, and (4) presence of an aluminum sheet used to absorb backscattered electrons, on the EBF, were studied. It is found that for lead shielding positioned at any fixed depth, the EBF decreases with an increase in initial electron beam energy (4-16 MeV). In addition, for depths within the electron practical range, Rp, and at a particular beam energy, the EBF increases with depth (or thickness of the treatment volume). When the electron beam angle increases from 0 degrees to 5 degrees, the EBF only decreases slightly (<4%) for all energies. The influence of the beam obliquity on the EBF is important when the treatment surface is not flat and perpendicular to the central beam axis. The use of an aluminum sheet to reduce backscattered electrons was also investigated. For a relatively low electron beam energy (4 MeV), a 2 mm aluminum sheet can reduce backscattering by 31%. While the electron beam energy increased, less backscattered electrons were produced and therefore removed by the same thickness of aluminum (only about 6% for 16 MeV). The Monte Carlo calculated EBFs from this study, characterized by the electron beam energy, depth of bolus above the lead shield, beam obliquity, and presence of an aluminum sheet, may provide important clinical information for radiation oncology staff when considering the effect of electron backscatter on radiotherapy using internal shielding.     

Energy spectra, angular spread, fluence profiles and dose distributions of 6 and 18 MV photon beams: results of monte carlo simulations for a varian 2100EX accelerator

The purpose of this study is to provide detailed characteristics of incident photon beams for different field sizes and beam energies. This information is critical to the future development of accurate treatment planning systems. It also enhances our knowledge of radiotherapy photon beams. The EGS4 Monte Carlo code, BEAM, has been used to simulate 6 and 18 MV photon beams from a Varian Clinac-2100EX accelerator. A simulated realistic beam is stored in a phase space data file, which contains details of each particle's complete history including where it has been and where it has interacted. The phase space files are analysed to obtain energy spectra, angular distribution, fluence profile and mean energy profiles at the phantom surface for particles separated according to their charge and history. The accuracy of a simulated beam is validated by the excellent agreement between the Monte Carlo calculated and measured dose distributions. Measured depth-dose curves are obtained from depth-ionization curves by accounting for newly introduced chamber fluence corrections and the stopping-power ratios for realistic beams. The study presents calculated depth-dose components from different particles as well as calculated surface dose and contribution from different particles to surface dose across the field. It is shown that the increase of surface dose with the increase of the field size is mainly due to the increase of incident contaminant charged particles. At 6 MV, the incident charged particles contribute 7% to 21% of maximum dose at the surface when the field size increases from 10 x 10 to 40 x 40 cm2. At 18 MV, their contributions are up to 11% and 29% of maximum dose at the surface for 10 x 10 cm2 and 40 x 40 cm2 fields respectively. However, the fluence of these incident charged particles is less than 1% of incident photon fluence in all cases.     

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.     

Monte Carlo study of backscatter in a flattening filter free clinical accelerator

In conventional linear accelerators, the flattening filter provides a uniform lateral dose profile. In intensity modulated radiation therapy applications, however, the flatness of the photon field and hence the presence of a flattening filter, is not necessary. Removing the filter may provide some advantages, such as faster treatments and smaller out-of-field doses to the patients. In clinical accelerators the backscattered radiation dose from the collimators must be taken into account when the dose to the target volume in the patient is being determined. In the case of a conventional machine, this backscatter is known to great precision. In a flattening filter free accelerator, however, the amount of backscatter may be different. In this study we determined the backscatter contribution to the monitor chamber signal in a flattening filter free clinical accelerator (Varian Clinac 21EX) with Monte Carlo simulations. We found that with the exception of very small fields in the 18-MV photon mode, the contribution of backscattered radiation to the monitor signal did not differ from that of conventional machines with a flattening filter. Hence, a flattening filter free clinical accelerator would not necessitate a different backscatter correction.     

Commissioning stereotactic radiosurgery beams using both experimental and theoretical methods

The purpose of this investigation is to study the feasibility of using an alternative method to commission stereotactic radiosurgery beams shaped by micro multi-leaf collimators by using Monte Carlo simulations to obtain beam characteristics of small photon beams, such as incident beam particle fluence and energy distributions, scatter ratios, depth-dose curves and dose profiles where measurements are impossible or difficult. Ionization chambers and diode detectors with different sensitive volumes were used in the measurements in a water phantom and the Monte Carlo codes BEAMnrc/DOSXYZnrc were used in the simulation. The Monte Carlo calculated data were benchmarked against measured data for photon beams with energies of 6 MV and 10 MV produced from a Varian Trilogy accelerator. The measured scatter ratios and cross-beam dose profiles for very small fields are shown to be not only dependent on the size of the sensitive volume of the detector used but also on the type of detectors. It is known that the response of some detectors changes at small field sizes. Excellent agreement was seen between scatter ratios measured with a small ion chamber and those calculated from Monte Carlo simulations. The values of scatter ratios, for field sizes from 6 x 6 mm2 to 98 x 98 mm2, range from 0.67 to 1.0 and from 0.59 to 1.0 for 6 and 10 MV, respectively. The Monte Carlo calculations predicted that the incident beam particle fluence is strongly affected by the X-Y-jaw openings, especially for small fields due to the finite size of the radiation source. Our measurement confirmed this prediction. This study demonstrates that Monte Carlo calculations not only provide accurate dose distributions for small fields where measurements are difficult but also provide additional beam characteristics that cannot be obtained from experimental methods. Detailed beam characteristics such as incident photon fluence distribution, energy spectra, including composition of primary and scattered photons, can be independently used in dose calculation models and to improve the accuracy of measurements with detectors with an energy-dependent response. Furthermore, when there are discrepancies between results measured with different detectors, the Monte Carlo calculated values can indicate the most correct result. The data set presented in this study can be used as a reference in commissioning stereotactic radiosurgery beams shaped by a BrainLAB m3 on a Varian 2100EX or 600C accelerator.     

Evaluation of Al2O3:C optically stimulated luminescence (OSL) dosimeters for passive dosimetry of high-energy photon and electron beams in radiotherapy

This article investigates the performance of Al2O3: C optically stimulated luminescence dosimeters (OSLDs) for application in radiotherapy. Central-axis depth dose curves and optically stimulated luminescence (OSL) responses were obtained in a water phantom for 6 and 18 MV photons, and for 6, 9, 12, 16, and 20 MeV electron beams from a Varian 21EX linear accelerator. Single OSL measurements could be repeated with a precision of 0.7% (one standard deviation) and the differences between absorbed doses measured with OSLDs and an ionization chamber were within +/- 1% for photon beams. Similar results were obtained for electron beams in the low-gradient region after correction for a 1.9% photon-to-electron bias. The distance-to-agreement values were of the order of 0.5-1.0 mm for electrons in high dose gradient regions. Additional investigations also demonstrated that the OSL response dependence on dose rate, field size, and irradiation temperature is less than 1% in the conditions of the present study. Regarding the beam energy/quality dependence, the relative response of the OSLD for 18 MV was (0.51 +/- 0.48)% of the response for the 6 MV photon beam. The OSLD response for the electron beams relative to the 6 MV photon beam. The OSLD response for the electron beams relative to the 6 MV photon beam was in average 1.9% higher, but this result requires further confirmation. The relative response did not seem to vary with electron energy at dmax within the experimental uncertainties (0.5% in average) and, therefore, a fixed correction factor of 1.9% eliminated the energy dependence in our experimental conditions.     

The effect of collimator backscatter radiation on photon output of linear accelerators

The field-size dependence of the photon output of linear accelerators in air has been attributed in part to changes in the amount of radiation backscattered from the collimator jaws into the dose monitor chamber. This possible effect was investigated for a variety of accelerators with energies from 4 to 15 MV by measuring the monitor unit rate (MU/min) for different collimator openings. This measurement was made without dose rate feedback control, i.e., with constant electron beam current in the accelerator. The monitor unit rate was independent of collimator setting for all machines tested. Hence, it is concluded that backscattered radiation from the collimator jaws into the dose monitor chamber does not contribute to the variation of output with field size.     

Investigation of the optimal backscatter for an aSi electronic portal imaging device

The effects of backscattered radiation on the dosimetric response of the Varian aS500 amorphous silicon electronic portal imaging device (EPID) are studied. Measurements demonstrate that radiation backscattered from the EPID mechanical support structure causes 5% asymmetries in the detected signal. To minimize the effect of backscattered radiation from the support structure, this work proposes adding material downstream of the EPID phosphor which provides uniform backscattering material to the phosphor and attenuates backscatter from the support structure before it reaches the phosphor. Two material locations were studied: downstream of the existing image cassette and within the cassette, immediately downstream of the flat-panel imager glass panel. Monte Carlo simulations were used to determine the thicknesses of water, Pb and Cu backscattering materials required to saturate the backscattered signal response for 6 MV and 18 MV beams for material thicknesses up to 50 mm. Water was unable to saturate the backscattered signal for thicknesses up to 50 mm for both energies. For Pb, to obtain a signal within 1% of saturation, 3 mm was required at 6 MV, and 6.8 mm was required at 18 MV. For Cu, thicknesses of 20.6 mm and 22.6 mm were required for the 6 MV and 18 MV beams, respectively. For saturation thicknesses, at 6 MV, the Cu backscatter enhanced the signal more than for Pb (Cu 1.25, Pb 1.11), but at 18 MV the reverse was found (Cu 1.19, Pb 1.23). This is due to the fact that at 6 MV, the backscattered radiation signal is dominated by low-energy scattered photons, which are readily attenuated by the Pb, while at 18 MV, electron backscatter contributes substantially to the signal. Image blurring caused by backscatter spread was less for Pb than Cu. Placing Pb immediately downstream of the glass panel further reduced the signal spread and increased the backscatter enhancement to 1.20 and 1.39 for the 6 MV and 18 MV beams, respectively. Overall, it is determined that adding approximately 5 mm of Pb between the detector and the mechanical support structure will substantially reduce the nonuniformity in the backscattered signals for 6 MV and 18 MV photon beams.     

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.     

Characteristics of photon beams from Philips SL25 linear accelerators

The Philips SL25 accelerator is a multimodality machine offering asymmetric collimator jaws and a new type of beam bending and transport system. It produces photon beams, nominally at 6 and 25 MV, and a scattered electron beam with nine selectable energies between 4 and 22 MeV. Dosimetric characteristics for the 6- and 25-MV photon beams are presented with respect to field flatness, surface and depth dose characteristics, isodose distribution, field size factors for both open and wedged fields, and narrow beam transmission data in different materials.     

Derivation of electron and photon energy spectra from electron beam central axis depth dose curves

A method for deriving the electron and photon energy spectra from electron beam central axis percentage depth dose (PDD) curves has been investigated. The PDD curves of 6, 12 and 20 MeV electron beams obtained from the Monte Carlo full phase space simulations of the Varian linear accelerator treatment head have been used to test the method. We have employed a 'random creep' algorithm to determine the energy spectra of electrons and photons in a clinical electron beam. The fitted electron and photon energy spectra have been compared with the corresponding spectra obtained from the Monte Carlo full phase space simulations. Our fitted energy spectra are in good agreement with the Monte Carlo simulated spectra in terms of peak location, peak width, amplitude and smoothness of the spectrum. In addition, the derived depth dose curves of head-generated photons agree well in both shape and amplitude with those calculated using the full phase space data. The central axis depth dose curves and dose profiles at various depths have been compared using an automated electron beam commissioning procedure. The comparison has demonstrated that our method is capable of deriving the energy spectra for the Varian accelerator electron beams investigated. We have implemented this method in the electron beam commissioning procedure for Monte Carlo electron beam dose calculations.     

Photonuclear dose calculations for high-energy photon beams from Siemens and Varian linacs

The dose from photon-induced nuclear particles (neutrons, protons, and alpha particles) generated by high-energy photon beams from medical linacs is investigated. Monte Carlo calculations using the MCNPX code are performed for three different photon beams from two different machines: Siemens 18 MV, Varian 15 MV, and Varian 18 MV. The linac head components are simulated in detail. The dose distributions from photons, neutrons, protons, and alpha particles are calculated in a tissue-equivalent phantom. Neutrons are generated in both the linac head and the phantom. This study includes (a) field size effects, (b) off-axis dose profiles, (c) neutron contribution from the linac head, (d) dose contribution from capture gamma rays, (e) phantom heterogeneity effects, and (f) effects of primary electron energy shift. Results are presented in terms of absolute dose distributions and also in terms of DER (dose equivalent ratio). The DER is the maximum dose from the particle (neutron, proton, or alpha) divided by the maximum photon dose, multiplied by the particle quality factor and the modulation scaling factor. The total DER including neutrons, protons, and alphas is about 0.66 cSv/Gy for the Siemens 18 MV beam (10 cm x 10 cm). The neutron DER decreases with decreasing field size while the proton (or alpha) DER does not vary significantly except for the 1 cm x 1 cm field. Both Varian beams (15 and 18 MV) produce more neutrons, protons, and alphas particles than the Siemens 18 MV beam. This is mainly due to their higher primary electron energies: 15 and 18.3 MeV, respectively, vs 14 MeV for the Siemens 18 MV beam. For all beams, neutrons contribute more than 75% of the total DER, except for the 1 cm x 1 cm field (approximately 50%). The total DER is 1.52 and 2.86 cSv/Gy for the 15 and 18 MV Varian beams (10 cm x 10 cm), respectively. Media with relatively high-Z elements like bone may increase the dose from heavy charged particles by a factor 4. The total DER is sensitive to primary electron energy shift. A Siemens 18 MV beam with 15 MeV (instead of 14 MeV) primary electrons would increase by 40% the neutron DER and by 210% the proton + alpha DER. Comparisons with measurements (neutron yields from different materials and neutron dose equivalent) are also presented. Using the NCRP risk assessment method, we found that the dose equivalent from leakage neutrons (at 50-cm off-axis distance) represent 1.1, 1.1, and 2.0% likelihood of fatal secondary cancer for a 70 Gy treatment delivered by the Siemens 18 MV, Varian 15 MV, and Varian 18 MV beams, respectively.     

Measurements of backscattered radiation from Therac-20 collimator and trimmer jaws into beam monitor chamber

The field size dependent photon output is known to be influenced by the existence of backscattered radiation (BSR) generated in the collimator or trimmer jaws of a linear accelerator. This paper describes the results of measurements made to study the existence of such backscatter by simulating the geometry of the treatment head of a Therac-20 linear accelerator. The machine's monitor chamber, flattening filter, and collimator jaws were simulated by another real monitor chamber, a 1-cm thick lead sheet and 2.5-cm thick low-melting-point alloy divergent blocks, respectively. BSR from the simulated collimator jaws (SCJ) was measured with the simulated monitor chamber (SMC) as a function of the openings of the SCJ and as a function of distance between SMC and SCJ. The present results demonstrate the presence of BSR in an 18-MV photon beam from a Therac-20 linear accelerator.     

Clinical implementation of a Monte Carlo treatment planning system.

The purpose of this study was to implement the Monte Carlo method for clinical radiotherapy dose calculations. We used the EGS4/BEAM code to obtain the phase-space data for 6-20 MeV electron beams and 4, 6, and 15 MV photon beams for Varian Clinac 1800, 2100C, and 2300CD accelerators. A multiple-source model was used to reconstruct the phase-space data for both electron and photon beams, which retained the accuracy of the Monte Carlo beam data. The multiple-source model reduced the phase-space data storage requirement by a factor of 1000 and the accelerator simulation time by a factor of 10 or more. Agreement within 2% was achieved between the Monte Carlo calculations and measurements of the dose distributions in homogeneous and heterogeneous phantoms for various field sizes, source-surface distances, and beam modulations. The Monte Carlo calculated electron output factors were within 2% of the measured values for various treatment fields while the heterogeneity correction factors for various lung and bone phantoms were within 1% for photon beams and within 2% for electron beams. The EGS4/DOSXYZ Monte Carlo code was used for phantom and patient dose calculations. The results were compared to the dose distributions produced by a conventional treatment planning system and an intensity-modulated radiotherapy inverse-planning system. Significant differences (>5% in dose and >5 mm shift in isodose lines) were found between Monte Carlo calculations and the analytical calculations implemented in the commercial systems. Treatment sites showing the largest dose differences were for head and neck, lung, and breast cases.     

Surface dosimetry for oblique tangential photon beams: a Monte Carlo simulation study

The effect of beam obliquity on the surface relative dose profiles for the tangential photon beams was studied. The 6 and 15 MV photon beams with 4 x 4 and 10 x 10 cm2 field sizes produced by a Varian 21 EX linear accelerator were used. Phase-space models of the photon beams were created using Monte Carlo simulations based on the EGSnrc code, and were verified using film measurements. The relative dose profiles in the phantom skin, at 2 mm depth from the surface of the half-phantom geometry, or HPG, were calculated for increasing gantry angles from 270 to 280 deg clockwise. Relative dose profiles of a full phantom enclosing the whole tangential beam (full phantom geometry, or FPG) were also calculated using Monte Carlo simulation as a control for comparison. The results showed that, although the relative dose profiles in the phantom skin did not change significantly with an oblique beam using a FPG, the surface relative depth dose was increased for the HPG. In the HPG, with 6 MV photon beams and field size = 10 x 10 cm2, when the beam angle, starting from 270 deg, was increased from 1 to 3 deg, the relative depth doses in the phantom skin were increased from 68% to 79% at 10 cm depth. This increase in dose was slightly larger than the dose from 15 MV photon beams with the same field size and beam angles, where the relative depth doses in phantom skin were increased from 81% to 87% at 10 cm depth. A parameter called the percent depth dose (PDD) ratio, defined as the relative depth dose from the HPG to the relative depth dose from the FPG at a given depth along the phantom skin, was used to evaluate the effect of the phantom-air interface. It is found that the PDD ratio increased significantly when the beam angle was changed from zero to 1-3 degrees. Moreover, the PDD ratio, for a given field size, experienced a greater increase for 6 MV than for 15 MV. For the same photon beam energy, the PDD ratio increased more with a 4 x 4 cm2 field compared to 10 x 10 cm2. The results in this study will be useful for physicists and dosimetrists to predict the surface relative dose variations when using clinical tangential-like photon beams in radiation therapy.     

Dose discrepancies between Monte Carlo calculations and measurements in the buildup region for a high-energy photon beam

This study investigates a possible cause of reported significant dose discrepancies between Monte Carlo calculations and measurements in the buildup region for high-energy photon beams in large fields. A proposed hypothesis was that the discrepancy was caused by a source of electrons in the accelerator head that was not fully accounted for in the treatment head simulation. In this investigation, a lead foil is added just below the accelerator head in order to study this hypothesis. The lead foil effectively removes charged particles generated inside the accelerator head. The charged particles generated by the lead foil can be accounted for fully because the simple geometry can be simulated accurately. An 18 MV photon beam from a Varian Clinac-2100EX is measured using a WELLHOFER WP700 beam scanner with an IC-10 ionization chamber (cavity radius=3 mm). The BEAM Monte Carlo code is used in the incident beam simulations. Both EGS4/DOSXYZ and EGSnrc/DOSRZnrc are used in the dose calculations in a water phantom. The Monte Carlo calculated depth-dose curve is scaled so that it has the same values at 10 cm depth as the measured curve. It is found that the discrepancies between Monte Carlo calculations and measurements remain significant in the buildup region even after applying necessary corrections to the measured data. The discrepancies have only been modestly decreased with the lead foil in place compared to the 40 x 40 cm2 open field. At a depth of 1 cm, discrepancies of about 5% are still observed in the buildup region for the field with the lead foil. Therefore a new explanation for the unresolved discrepancy remains to be found.