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.     

Comparison of characteristics of photon and electron beams generated by Philips/Elekta and Varian linear accelerators

Commissioning data of the common energy of 6 MV photon beams are analyzed for seven Philips/ Elekta linacs of five different models: SL 75/5, SL 15, SL 25, SLi Precise, and SLi. The observed beam quality %dd(10)x of 67.7% +/- 0.3 agrees with the manufacturer's specification of 67.5% +/- 2% but with a much smaller variation, suggesting the possibility of a more accurate beam quality guideline for acceptance commissioning of these linac models. Field size factors are less uniform, especially for the biggest field size of 40 x 40 cm2, but all agree within 1.1% between the five models. These 6 MV photon beams are also shown to be 0.4 MV harder than the Varian 2100C model. For commissioning electron beams generated by the Philips/Elekta models SL 15, SLi Precise and SLi, guidelines can be found from this work for the following parameters: actual field size at 100 SSD (up to +/- 0.6%) (different from manufacturer specified values at 95 SSD), and virtual source distance (up to +/- 0.7%). Significant differences are seen for R50 (up to 3.7 mm), sigma-theta-x, effective source distance, and cone size factor. However, except for R50 where Varian 2100C datasets are not available for comparison, the differences between the different Philips/Elekta models are comparable to those between the same Varian 2100C model linacs measured by us and Watts.     

A two-source model for electron beams: calculation of relative output factors

A two-source model for the calculation of relative output factors (ROF) for clinical applications of electron beams has been developed. The model consists of (1) an effective extended source above the final field-defining aperture (cutout) plane and (2) a source due to scattering from the aperture. Calculations are based on Fermi-Eyges theory and a pencil beam algorithm with parameters determined independently for each major scattering component. The model predicts a modified inverse square law for determining the dose rate for the electron beams. It also generalizes the "square-root method" and "one-dimensional method" that are often used clinically for ROF calculations. A computer program based on the model has been developed to calculate ROF for irregular fields. The predictions of ROF values have been compared with measurements on a Varian CLINAC 2100C/D accelerator for different cutout size, energies, applicators, and SSDs for square fields, rectangular fields, circular fields, and irregular fields. The agreement between prediction and measurement of the ROF for these wide range of conditions is generally within 1% for energies from 6 to 20 MeV. This two-source model can be used for clinical applications and it requires a minimal set of measured input data.     

Electron beam modeling and commissioning for Monte Carlo treatment planning

A hybrid approach for commissioning electron beam Monte Carlo treatment planning systems has been studied. The approach is based on the assumption that accelerators of the same type have very similar electron beam characteristics and the major difference comes from the on-site tuning of the electron incident energy at the exit window. For one type of accelerator, a reference machine can be selected and simulated with the Monte Carlo method. A multiple source model can be built on the full Monte Carlo simulation of the reference beam. When commissioning electron beams from other accelerators of the same type, the energy spectra in the source model are tuned to match the measured dose distributions. A Varian Clinac 2100C accelerator was chosen as the reference machine and a four-source beam model was established based on the Monte Carlo simulations. This simplified beam model can be used to generate Monte Carlo dose distributions accurately (within 2%/2 mm compared to those calculated with full phase space data) for electron beams from the reference machine with various nominal energies, applicator sizes, and SSDs. Three electron beams were commissioned by adjusting the energy spectra in the source model. The dose distributions calculated with the adjusted source model were compared with the dose distributions calculated using the phase space data for these beams. The agreement is within 1% in most of cases and 2% in all situations. This preliminary study has shown the capability of the commissioning approach for handling large variation in the electron incident energy. The possibility of making the approach more versatile is also discussed.     

Photon spectral characteristics of dissimilar 6 MV linear accelerators

This work measures and compares the energy spectra of four dosimetrically matched 6 MV beams, generated from four physically different linear accelerators. The goal of this work is twofold. First, this study determines whether the spectra of dosimetrically matched beams are measurably different. This study also demonstrates that the spectra of clinical photon beams can be measured as a part of the beam data collection process for input to a three-dimensional (3D) treatment planning system. The spectra of 6 MV beams that are dosimetrically matched for clinical use were studied to determine if the beam spectra are similarly matched. Each of the four accelerators examined had a standing waveguide, but with different physical designs. The four accelerators were two Varian 2100C/Ds (one 6 MV/18 MV waveguide and one 6 MV/10 MV waveguide), one Varian 600 C with a vertically mounted waveguide and no bending magnet, and one Siemens MD 6740 with a 6 MV/10 MV waveguide. All four accelerators had percent depth dose curves for the 6 MV beam that were matched within 1.3%. Beam spectra were determined from narrow beam transmission measurements through successive thicknesses of pure aluminum along the central axis of the accelerator, made with a graphite Farmer ion chamber with a Lucite buildup cap. An iterative nonlinear fit using a Marquardt algorithm was used to find each spectrum. Reconstructed spectra show that all four beams have similar energy distributions with only subtle differences, despite the differences in accelerator design. The measured spectra of different 6 MV beams are similar regardless of accelerator design. The measured spectra show excellent agreement with those found by the auto-modeling algorithm in a commercial 3D treatment planning system that uses a convolution dose calculation algorithm. Thus, beam spectra can be acquired in a clinical setting at the time of commissioning as a part of the routine beam data collection.     

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.     

Response to high-energy photons of PTW31014 PinPoint ion chamber with a central aluminum electrode

Since its introduction the PinPoint (PTW-Freiburg) micro-ionization chamber has been proposed for relative dosimetry (output factors, depth dose curves, and beam profiles) as well as for determination of absolute dose of small high-energy photon beams. This paper investigates the dosimetric performance of a new design (type 31014) of the PinPoint ion chamber with a central aluminum electrode. The study included characterization of inherent and radiation-induced leakage, ion collection efficiency and polarity effect, relative response of the chamber, measurement of beam profiles, and depth dose curves. The 6 and 15 MV photon beams of a Varian 2100 C/D were considered. At the nominal operating voltage of 400 V the PinPoint type 31014 chamber was found to present a strong field size dependence of the polarity correction factor and an excess of the collected charge, which can lead to an underestimation of the collection efficiency if determined with the conventional "two-voltage" method. In comparison to the original PinPoint design (type 31006) the authors found for type 31014 chamber no overresponse to large-area fields if polarity correction is applied. If no correction is taken into consideration, the authors found the chamber's output to be inaccurate for large-area fields (0.5% accuracy limited up to the 12 x 12 and 20 x 20 cm2 field for the 6 and 15 MV beams, respectively), which is a direct consequence of the stem and polarity effects due to the chamber's very small sensitive volume (0.015 cc) and cable irradiation. Beam profiles and depth dose curves measured with type 31014 PinPoint chamber for small and medium size fields were compared to data measured with a 0.125 cc ion chamber and with high-resolution Kodak EDR2 films. Analysis of the penumbra (80%-20% distance) showed that the spatial resolution of type 31014 PinPoint ion chamber approaches (penumbra broadening < or = 0.6 mm) EDR2 film results.     

Comprehensive Monte Carlo calculation of the point spread function for a commercial a-Si EPID

Images produced by commercial amorphous silicon electronic portal imaging devices (a-Si EPIDs) are subject to multiple blurring processes. Implementation of these devices for fluence measurement requires that the blur be removed from the images. A standard deconvolution operation can be performed to accomplish this assuming the blur kernel is spatially invariant and accurately known. This study determines a comprehensive blur kernel for the Varian aS500 EPID. Monte Carlo techniques are used to derive a dose kernel and an optical kernel, which are then combined to yield an overall blur kernel for both 6 and 15 MV photon beams. Experimental measurement of the line spread function (LSF) is used to verify kernel shape. Kernel performance is gauged by comparing EPID image profiles with in-air dose profiles measured using a diamond detector (approximating fluence) both before and after the EPID images have been deconvolved. Quantitative comparisons are performed using the chi metric, an extension of the well-known y metric, using acceptance criteria of 0.0784 cm (1 pixel width) distance-to-agreement (deltad) and 2% of the relative central axis fluence (deltaD). Without incorporating any free parameters, acceptance was increased from 49.0% of pixels in a cross-plane profile for a 6 MV 10 x 10 cm2 open field to 92.0%. For a 10 x 10 cm2 physically wedged field, acceptance increased from 40.3% to 73.9%. The effect of the optical kernel was found to be negligible for these chi acceptance parameters, however for (deltaD= 1%, deltad = 0.0784 cm) we observed an improvement from 66.1% (without) to 78.6% (with) of chi scores <1 (from 20.6% before deconvolution). It is demonstrated that an empirical kernel having a triple exponential form or a semiempirical kernel based on a simplified model of the detector stack can match the performance of the comprehensive kernel.     

Testing of the analytical anisotropic algorithm for photon dose calculation

The analytical anisotropic algorithm (AAA) was implemented in the Eclipse (Varian Medical Systems) treatment planning system to replace the single pencil beam (SPB) algorithm for the calculation of dose distributions for photon beams. AAA was developed to improve the dose calculation accuracy, especially in heterogeneous media. The total dose deposition is calculated as the superposition of the dose deposited by two photon sources (primary and secondary) and by an electron contamination source. The photon dose is calculated as a three-dimensional convolution of Monte-Carlo precalculated scatter kernels, scaled according to the electron density matrix. For the configuration of AAA, an optimization algorithm determines the parameters characterizing the multiple source model by optimizing the agreement between the calculated and measured depth dose curves and profiles for the basic beam data. We have combined the acceptance tests obtained in three different departments for 6, 15, and 18 MV photon beams. The accuracy of AAA was tested for different field sizes (symmetric and asymmetric) for open fields, wedged fields, and static and dynamic multileaf collimation fields. Depth dose behavior at different source-to-phantom distances was investigated. Measurements were performed on homogeneous, water equivalent phantoms, on simple phantoms containing cork inhomogeneities, and on the thorax of an anthropomorphic phantom. Comparisons were made among measurements, AAA, and SPB calculations. The optimization procedure for the configuration of the algorithm was successful in reproducing the basic beam data with an overall accuracy of 3%, 1 mm in the build-up region, and 1%, 1 mm elsewhere. Testing of the algorithm in more clinical setups showed comparable results for depth dose curves, profiles, and monitor units of symmetric open and wedged beams below dmax. The electron contamination model was found to be suboptimal to model the dose around dmax, especially for physical wedges at smaller source to phantom distances. For the asymmetric field verification, absolute dose difference of up to 4% were observed for the most extreme asymmetries. Compared to the SPB, the penumbra modeling is considerably improved (1%, 1 mm). At the interface between solid water and cork, profiles show a better agreement with AAA. Depth dose curves in the cork are substantially better with AAA than with SPB. Improvements are more pronounced for 18 MV than for 6 MV. Point dose measurements in the thoracic phantom are mostly within 5%. In general, we can conclude that, compared to SPB, AAA improves the accuracy of dose calculations. Particular progress was made with respect to the penumbra and low dose regions. In heterogeneous materials, improvements are substantial and more pronounced for high (18 MV) than for low (6 MV) energies.     

Peripheral dose outside applicators in electron beams

The peripheral dose outside the applicators in electron beams was studied using a Varian 21 EX linear accelerator. To measure the peripheral dose profiles and point doses for the applicator, a solid water phantom was used with calibrated Kodak TL films. Peak dose spot was observed in the 4 MeV beam outside the applicator. The peripheral dose peak was very small in the 6 MeV beam and was ignorable at higher energies. Using the 10 x 10 cm(2) cutout and applicator, the dose peak for the 4 MeV beam was about 12 cm away from the field central beam axis (CAX) and the peripheral dose profiles did not change with depths measured at 0.2, 0.5 and 1 cm. The peripheral doses and profiles were further measured by varying the angle of obliquity, cutout and applicator size for the 4 MeV beam. The local peak dose was increased with about 3% per degree angle of obliquity, and was about 1% of the prescribed dose (angle of obliquity equals zero) at 1 cm depth in the phantom using the 10 x 10 cm(2) cutout and applicator. The peak dose position was also shifted 7 mm towards the CAX when the angle of obliquity was increased from 0 to 15 degrees.     

Energy correction factors of LiF powder TLDs irradiated in high-energy electron beams and applied to mailed dosimetry for quality assurance networks

Absorbed dose determination with thermoluminescent dosimeters (TLDs) generally relies on calibration in 60Co gamma-ray reference beams. The energy correction factor fCo(E) for electron beams takes into account the difference between the response of the TLD in the beam of energy E and in the 60Co gamma-ray beam. In this work, fCo(E) was evaluated for an LiF powder irradiated in electron beams of 6 to 20 MeV (Varian 2300C/D) and 10 to 50 MeV (Racetrack MM50), and its variation with electron energy, TLD size and nature of the surrounding medium was also studied for LiF powder. The results have been applied to the ESTRO-EQUAL mailed dosimetry quality assurance network. Monte Carlo calculations (EGS4, PENELOPE) and experiments have been performed for the LiF powder (rho = 1.4 g cm3) (DTL937, Philitech, France), read on a home made reader and a PCL3 automatic reader (Fimel, France). The TLDs were calibrated using Fricke dosimetry and compared with three ionization chambers (NE2571, NACP02, ROOS). The combined uncertainties in the experimental fCo(E) factors determined in this work are less than about 0.4% (1 SD), which is appreciably smaller than the uncertainties up to 1.4% (1 SD) reported for other calculated values in the literature. Concerning the Varian 2300C/D beams, the measured fCo(E) values decrease from 1.065 to 1.049 +/- 0.004 (1 SD) when the energy at depth in water increases from 2.6 to 14.1 MeV; the agreement with Monte Carlo calculations is better than 0.5%. For the Racetrack MM50 pulsed-scanned beams, the average experimental value of fCo(E) is 1.071 +/- 0.005 (1 SD) for a mean electron energy at depth Ez ranging from 4.3 to 36.3 MeV: fCo(E) is up to 2% higher for the MM50 beams than for the 2300C/D beams in the range of the tested energies. The energy correction factor for LiF powder (3 mm diameter and 15 mm length) varies with beam quality and type (pulsed or pulsed-scanning), cavity size and nature of the surrounding medium. The fCo(E) values obtained for the LiF powder (3 mm diameter and 15 mm length) irradiated in water, have been applied to the EQUAL external audit network, leading to a good agreement between stated and measured doses, with a mean value of 1.002 +/- 0.022 (1 SD), for 170 beam outputs checked (36 electron beam energies) in 13 'reference' radiotherapy centres in Europe. Such fCo(E) data improve the accuracy of the absorbed dose TLD determination in electron beams, justifying their use for quality control in radiotherapy.     

Small-field electron dosimetry for the Philips SL25 linear accelerator

Electron-beam characteristics of a Philips SL25 linear accelerator have been studied. Central-axis percentage depth doses, cross-beam profiles and beam output factors of 6-, 10-, and 20-MeV beams, selected from the available energy range of 4 to 22 MeV, are reported in this paper. The main thrust of this work is to determine the systematic variation of beam characteristics, especially the output factor, with standard cone sizes and cerrobend beam-shaping cutouts down to a field size of 2 X 2 cm Output factors for the standard cones (open field) are energy dependent in a complex manner, increasing with the cone size for the 6-MeV beam whereas decreasing for 10- and 20-MeV beams. The output factor falls below unity at lower energies (6 and 10 MeV) for fields with at least one side smaller than 6 cm, and stays nearly constant for the 20-MeV beam. Measured output factors of small fields are least squares fitted by a second-order polynomial function. Output factors for small rectangular fields have been derived from the one-dimensional and square-root formulas, and the equivalent-square method. Only the one-dimensional formula predicts the measured output factors of highly elongated fields to within +/- 1% experimental uncertainties. Different cones with the same size electron cutout show a varied dose response, primarily due to variation in scattered electron contamination from the cones.     

On the dosimetric behaviour of photon dose calculation algorithms in the presence of simple geometric heterogeneities: comparison with Monte Carlo calculations

A comparative study was performed to reveal differences and relative figures of merit of seven different calculation algorithms for photon beams when applied to inhomogeneous media. The following algorithms were investigated: Varian Eclipse: the anisotropic analytical algorithm, and the pencil beam with modified Batho correction; Nucletron Helax-TMS: the collapsed cone and the pencil beam with equivalent path length correction; CMS XiO: the multigrid superposition and the fast Fourier transform convolution; Philips Pinnacle: the collapsed cone. Monte Carlo simulations (MC) performed with the EGSnrc codes BEAMnrc and DOSxyznrc from NRCC in Ottawa were used as a benchmark. The study was carried out in simple geometrical water phantoms (rho = 1.00 g cm(-3)) with inserts of different densities simulating light lung tissue (rho = 0.035 g cm(-3)), normal lung (rho = 0.20 g cm(-3)) and cortical bone tissue (rho = 1.80 g cm(-3)). Experiments were performed for low- and high-energy photon beams (6 and 15 MV) and for square (13 x 13 cm2) and elongated rectangular (2.8 x 13 cm2) fields. Analysis was carried out on the basis of depth dose curves and transverse profiles at several depths. Assuming the MC data as reference, gamma index analysis was carried out distinguishing between regions inside the non-water inserts or inside the uniform water. For this study, a distance to agreement was set to 3 mm while the dose difference varied from 2% to 10%. In general all algorithms based on pencil-beam convolutions showed a systematic deficiency in managing the presence of heterogeneous media. In contrast, complicated patterns were observed for the advanced algorithms with significant discrepancies observed between algorithms in the lighter materials (rho = 0.035 g cm(-3)), enhanced for the most energetic beam. For denser, and more clinical, densities a better agreement among the sophisticated algorithms with respect to MC was observed.     

基于EPID采用参数化梯度法测定光子束射野边界

【摘 要】 目的:探究参数化梯度方法（PGM）测量电子射野影像系统（EPID）光子束射野大小的可行性。 方法:PGM通过一个修改的双曲正切函数拟合Profile半影区。瓦里安EDGE机载aS1200采集6 MV和10 MV FF及FFF射束EPID数据,TrueBeam机载aS1000采集6 MV FF射束EPID数据。γ分析1 mm/1%标准量化PGM拟合Profile半影区与EPID测量半影区一致性。比较半高宽方法与PGM测量的FF射束射野大小,比较最大斜率方法与PGM测量的FFF射束射野大小;比较PGM在不同射束能量、不同EPID探测器类型和引入铅门位置误差后测量射野边界的稳定性和扩展性。 结果:半影区PGM拟合与EPID实测数据Pearson相关系数大于0.999,γ值小于0.2。FF射束,半高宽方法测定射野均大于PGM,且随着射野增大而增大,Profile本影去除后,两种方法测量差值显著减小;FFF射束,最大斜率方法与PGM测定射野大小差值在0.1 mm以内。PGM能够稳定测量不同能量、不同模态、不同EPID探测器类型射野边界,能够准确识别铅门1 mm位置变动。 结论:PGM可作为一种鲁棒通用的方法适用于EPID光子束射野质量保障。     

The use of an extra-focal electron source to model collimator-scattered electrons using the pencil-beam redefinition algorithm

Currently, the pencil-beam redefinition algorithm (PBRA) utilizes a single electron source to model clinical electron beams. In the single-source model, the electrons appear to originate from a virtual source located near the scattering foils. Although this approach may be acceptable for most treatment machines, previous studies have shown dose differences as high as 8% relative to the given dose for small fields for some machines such as the Varian Clinac 1800. In such machines collimation-scattered electrons originating from the photon jaws and the applicator give rise to extra-focal electron sources. In this study, we examined the impact of modeling an additional electron source to better account for the collimator-scattered electrons. The desired dose calculation accuracy in water throughout the dose distribution is 3% or better relative to the given dose. We present here a methodology for determining the electron-source parameters for the dual-source model using a minimal set of data, that is, two central-axis depth-dose curves and two off-axis profiles. A Varian Clinac 1800 accelerator was modeled for beam energies of 20 and 9 MeV and applicator sizes of 15 x 15 and 6 x 6 cm2. The improvement in the accuracy of PBRA-calculated dose, evaluated using measured two-dimensional dose distributions in water, was characterized using the figure of merit, FA3%, which represents the fractional area containing dose differences greater than 3%. For the 15 x 15 cm2 field the evaluation was restricted to the penumbral region, and for the 6 x 6 cm2 field the central region of the beam was included as it was impacted by the penumbra. The greatest improvement in dose accuracy was for the 6 x 6 cm2 applicator. At 9 MeV, FA3% decreased from 15% to 0% at 100 cm SSD and from 34% to 4% at 110 cm SSD. At 20 MeV, FA3% decreased from 17% to 2% at 100 cm SSD and from 41% to 10% at 110 cm SSD. In the penumbra of the 15 x 15 cm2 applicator, the improvement was less, but still significant. At 9 MeV, FA3% changed from 11% to 1% at 100 cm SSD and from 10% to 12% at 110 cm SSD. At 20 MeV, FA3% decreased from 12% to 8% at 100 cm SSD and from 14% to 5% at 110 cm SSD. Results demonstrate that use of a dual-source beam model can provide significantly improved accuracy in the PBRA-calculated dose distribution that was not achievable with a single-source beam model when modeling the Varian Clinac 1800 electron beams. Time of PBRA dose calculation was approximately doubled; however, dual-source beam modeling of newer accelerators (e.g., the Varian Clinac 2100) may not be necessary because of less impact of collimator-scattered electrons on dosimetry.     

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.     

Influence of focal spot on characteristics of very small diameter radiosurgical beams

Percentage depth dose (PDD) distributions and beam profiles of very small diameter (1.5-5 mm) megavoltage radiosurgical beams calculated with Monte Carlo (MC) technique critically depend on the diameter of the circular focal spot used in the simulation: The smaller is the field diameter, the larger is the effect. Thus, in simulations of radiosurgical fields that have diameters of the order of the focal spot size, an accurate focal spot geometry should be used. We used a simplified moving slit technique in conjunction with a diode detector for evaluation of the focal spot size and shape of a megavoltage 6 MV linac as well as for determination of the equivalent focal spot diameter of the linac for use in MC simulations. The measured total diode signal contains three components: A direct focal spot signal, a background signal, and an extra-focal radiation signal. A single profile scan of the focal spot signal is Gaussian like in shape, and its full width at half maximum is used to define the focal spot dimension for this scan. The focal spot of our 6 MV linac is approximated with a Gaussian circle, and when the geometry of the effective focal spot circle is used in MC simulations, the agreement between MC-calculated and measured PDD distributions as well as beam profiles is good even for radiosurgical fields as small as 1.5 mm in diameter. Our results also confirm that matching the penumbral areas of accurately measured large-field beam profiles to the same areas of the MC-calculated beam profiles reliably leads to a realistic effective focal spot size for use in MC simulations of very small diameter beams.     

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.     

VarianClinaciX15MeV光子束能谱重建

目的：精确重建VarianClinaciX15MeV光子束能谱。方法：利用先验模型和遗传算法,以光子束中轴百分深度剂量（PDD）为基础数据实现医用直线加速器光子能谱重建。1．EGS模拟仿真VarianClinacix治疗头和标准水模体,获得15MeV光子束的模拟能谱以及单能光子中轴PDD数据;2．根据测量得到的中轴PDD数据以及模拟得到的单能光子中轴PDD数据,运用遗传算法优化求解先验模型的参数：3．将优化后的先验模型所计算的结果作为初始化种群．再用遗传算法二次优化重建光子能谱。结果：重建能谱与蒙特卡洛模拟得到的能谱具有良好的一致性,相关系数为0．9970;重建能谱的平均能量与由相空间文件分析所得平均能量的相对误差为1．16％;根据重建能谱计算得到的中轴PDD数据与实际测量的中轴PDD数据之间的相关系数为O．9999。结论：利用先验模型和遗传算法进行光子束能谱重建可靠有效．具有实用价值。