Secondary electron fluence perturbation by high-Z interfaces in clinical proton beams: a Monte Carlo study.

Fluence perturbation of secondary electrons from clinical proton beams (50-250 MeV) by thin high-Z planar interfaces was studied with Monte Carlo simulations. Starting from monoenergetic proton pencil beams, proton depth doses and proton fluence spectra were calculated, both in homogeneous water and near thin high-Z interfaces by using the proton transport Monte Carlo code PTRAN. This code was modified extensively to enable modelling of proton transport in non-homogeneous geometries. From the proton fluence spectra in water and in the interface materials, electron generation spectra were calculated analytically and were then used as input for an electron transport calculation with the Monte Carlo code EGS4/PRESTAII to obtain electron doses and electron fluence spectra. The interface materials used in the study were graphite, Al, Ti, Cu, Sn and Au. We found significant electron fluence perturbations on both sides of the planar interfaces, resulting in an electron dose increase upstream and a decrease downstream from the interfaces, with the magnitude of the effect depending strongly on the atomic number of the interface. For the most extreme case studied, 250 MeV protons and a gold interface, we obtained an electron dose increase of 41% upstream of the interface and a decrease of 15% downstream with both perturbations having a spatial extent of about 700 microm. The total dose perturbation due to this effect amounts to a 5% increase upstream and a 2% decrease downstream. A detailed analysis of dose and fluence perturbation is presented for a wide range of materials and proton energies.     

Interface perturbation effects in high-energy electron beams

Near interfaces between two different media exposed to high-energy electron beams substantial dose and fluence perturbations due to backscatter can be observed. In this work, dose and fluence perturbations were studied for 4-19 MeV electron beams at backscatter interfaces of polystyrene, graphite, water, aluminium and lead. Measurements of relative dose using an NPL-designed thin-window plane-parallel ion chamber and a Markus ion chamber were performed to determine the effect of different interface materials and thicknesses. Results of Monte Carlo simulations with the EGSnrc code, including models of the ion chambers, were found to be in excellent agreement with the measurements. The well-documented increasing dose perturbation with increasing effective atomic number of the backscatter material and decreasing electron beam energy was confirmed. Simulations in a simplified slab geometry showed that, despite the decrease of average electron energy with depth in water, the dose perturbations decrease with increasing depth of the interface in water for all the materials in the study. This was ascribed to the change of the electron angular distribution with depth which has a different effect in water and in the presence of a high-Z interface. Electron fluence perturbations near a lead/water interface were found to cause small differences in unrestricted mass collision stopping power ratios, water to air. Effects of bremsstrahlung photons, characteristic photons and positrons from the backscattering material were found to be insignificant for electron interface dosimetry. When comparing simulations using EGSnrc and the older version of the same code, EGS4, underestimations of the dose perturbation effects of up to 7% were found when using the latter code to simulate 4 MeV electrons irradiating a lead/water interface. It is concluded that EGSnrc is a highly suitable tool for electron interface dosimetry studies.     

Evaluation of the EGSnrc Monte Carlo code for interface dosimetry near high-Z media exposed to kilovolt and 60Co photons

High atomic number (Z) heterogeneities in tissue exposed to photons with energies of up to about 1 MeV can cause significant dose perturbations in their immediate vicinity. The recently released Monte Carlo (MC) code EGSnrc (Kawrakow 2000a Med. Phys. 27 485-98) was used to investigate the dose perturbation of high-Z heterogeneities in tissue in kilovolt (kV) and 60Co photon beams. Simulations were performed of measurements with a dedicated thin-window parallel-plate ion chamber near a high-Z interface in a 60Co photon beam (Nilsson et al 1992 Med. Phys. 19 1413-21). Good agreement was obtained between simulations and measurements for a detailed set of experiments in which the thickness of the ion chamber window, the thickness of the air gap between ion chamber and heterogeneity, the depth of the ion chamber in polystyrene and the material of the interface was varied. The EGSnrc code offers several improvements in the electron and photon production and transport algorithms over the older EGS4/PRESTA code (Nelson et al 1985 Stanford Linear Accelerator Center Report SLAC-265. Bielajew and Rogers 1987 Nucl. Instrum. Methods Phys. Res. B 18 165-81). The influence of the new EGSnrc features was investigated for simulations of a planar slab of a high-Z medium embedded in water and exposed to kV or 60Co photons. It was found that using the new electron transport algorithm in EGSnrc, including relativistic spin effects in elastic scattering, significantly affects the calculation of dose distribution near high-Z interfaces. The simulations were found to be independent of the maximum fractional electron energy loss per step (ESTEPE), which was often a cause for concern in older EGS4 simulations. Concerning the new features of the photon transport algorithm sampling of the photoelectron angular distribution was found to have a significant effect, whereas the effect of binding energies in Compton scatter was found to be negligible. A slight dose artefact very close to high-Z interfaces exposed to kilovolt x-rays was discovered when atomic relaxation processes following excitation were omitted.     

A fast and sensitive TLD method for measurement of energy and homogeneity of electron beams using transmitted radiation through lead

A simple and fast, but sensitive TLD method for the measurement of energy and homogeneity of therapeutically used electron beams has been developed and tested. This method is based on the fact that when small thicknesses of high-Z absorbers such as lead are interposed in the high-energy electron beams, the transmitted radiation increases with the energy of the electron beams. Consequently, the ratio of readouts of TLDS held on the two sides of a lead plate varied sharply (by factor of 70) with a change in energy of the electron beam from 5 MeV to 18 MeV, offering a very sensitive method for the measurement of the energy of electron beams. By using the ratio of TL readouts of two types of TLD ribbon with widely different sensitivities, LiF TLD-700 ribbons on the upstream side and highly sensitive CaF2:Dy TLD-200 ribbons on the downstream side, an electron energy discrimination of better than +/- 0.1 MeV could be achieved. The homogeneity of the electron beam energy and the absorbed dose was measured by using a jig in which the TLDS were held in the desired array on both sides of a 4 mm thick lead plate. The method takes minimal beam time and makes it possible to carry out measurements for the audit of the quality of electron beams as well as for intercomparison of beams by mail.     

Dose perturbations at interfaces in photon beams: annihilation radiation

A model is presented for estimating the contribution of annihilation radiation to the dose perturbation at interfaces between high and low atomic number materials. The contribution is small, but not negligible relative to the total interface dose perturbation. The maximum contribution occurs for photon beams of about 8 MeV in energy. For an 8-MeV beam passing first through lead, then through polystyrene, the annihilation radiation contribution to the interface dose perturbation is about 8%, at a copper/polystyrene interface, the contribution is about 7%, and at an aluminum/polystyrene interface, the contribution is about 3%.     

Electron fluence correction factors for various materials in clinical electron beams

Relative to solid water, electron fluence correction factors at the depth of dose maximum in bone, lung, aluminum, and copper for nominal electron beam energies of 9 MeV and 15 MeV of the Clinac 18 accelerator have been determined experimentally and by Monte Carlo calculation. Thermoluminescent dosimeters were used to measure depth doses in these materials. The measured relative dose at dmax in the various materials versus that of solid water, when irradiated with the same number of monitor units, has been used to calculate the ratio of electron fluence for the various materials to that of solid water. The beams of the Clinac 18 were fully characterized using the EGS4/BEAM system. EGSnrc with the relativistic spin option turned on was used to optimize the primary electron energy at the exit window, and to calculate depth doses in the five phantom materials using the optimized phase-space data. Normalizing all depth doses to the dose maximum in solid water stopping power ratio corrected, measured depth doses and calculated depth doses differ by less than +/- 1% at the depth of dose maximum and by less than 4% elsewhere. Monte Carlo calculated ratios of doses in each material to dose in LiF were used to convert the TLD measurements at the dose maximum into dose at the center of the TLD in the phantom material. Fluence perturbation correction factors for a LiF TLD at the depth of dose maximum deduced from these calculations amount to less than 1% for 0.15 mm thick TLDs in low Z materials and are between 1% and 3% for TLDs in Al and Cu phantoms. Electron fluence ratios of the studied materials relative to solid water vary between 0.83+/-0.01 and 1.55+/-0.02 for materials varying in density from 0.27 g/cm3 (lung) to 8.96 g/cm3 (Cu). The difference in electron fluence ratios derived from measurements and calculations ranges from -1.6% to +0.2% at 9 MeV and from -1.9% to +0.2% at 15 MeV and is not significant at the 1sigma level. Excluding the data for Cu, electron fluence correction factors for open electron beams are approximately proportional to the electron density of the phantom material and only weakly dependent on electron beam energy.     

Monte Carlo dose calculations in homogeneous media and at interfaces: a comparison between GEPTS,EGSnrc, MCNP,and measurements

Three Monte Carlo photon/electron transport codes (GEPTS, EGSnrc, and MCNP) are bench-marked against dose measurements in homogeneous (both low- and high-Z) media as well as at interfaces. A brief overview on physical models used by each code for photon and electron (positron) transport is given. Absolute calorimetric dose measurements for 0.5 and 1 MeV electron beams incident on homogeneous and multilayer media are compared with the predictions of the three codes. Comparison with dose measurements in two-layer media exposed to a 60Co gamma source is also performed. In addition, comparisons between the codes (including the EGS4 code) are done for (a) 0.05 to 10 MeV electron beams and positron point sources in lead, (b) high-energy photons (10 and 20 MeV) irradiating a multilayer phantom (water/steel/air), and (c) simulation of a 90Sr/90Y brachytherapy source. A good agreement is observed between the calorimetric electron dose measurements and predictions of GEPTS and EGSnrc in both homogeneous and multilayer media. MCNP outputs are found to be dependent on the energy-indexing method (Default/ITS style). This dependence is significant in homogeneous media as well as at interfaces. MCNP(ITS) fits more closely the experimental data than MCNP(DEF), except for the case of Be. At low energy (0.05 and 0.1 MeV), MCNP(ITS) dose distributions in lead show higher maximums in comparison with GEPTS and EGSnrc. EGS4 produces too penetrating electron-dose distributions in high-Z media, especially at low energy (<0.1 MeV). For positrons, differences between GEPTS and EGSnrc are observed in lead because GEPTS distinguishes positrons from electrons for both elastic multiple scattering and bremsstrahlung emission models. For the 60Co source, a quite good agreement between calculations and measurements is observed with regards to the experimental uncertainty. For the other cases (10 and 20 MeV photon sources and the 90Sr/90Y beta source), a good agreement is found between the three codes. In conclusion, differences between GEPTS and EGSnrc results are found to be very small for almost all media and energies studied. MCNP results depend significantly on the electron energy-indexing method.     

Characteristics of the high-energy photon beam of a 25-MeV accelerator

The CGR Saturne 25 is an isocentrically mounted standing wave medical linear accelerator that produces dual-energy photon beams and a scanned electron beam with six selectable energies between 4 and 25 MeV. The highest energy photon beam is nominally referred to as 23 MV. For this beam the mean energy of the accelerated electron beam on the 1.3 radiation length (4 mm) tungsten x-ray target is found to be approximately 21 MeV, with the energy acceptance stated to be +/- 5%. The electron beam traverses a 270 degrees bending magnet upstream of the x-ray production target. The resulting bremsstrahlung beam passes through a combination steel and lead flattening filter, 4-cm maximum thickness. Dosimetric data for the 23-MV beam are presented with respect to rectangular field output factor, depth of maximum dose as a function of field size, surface and buildup dose, central axis percent depth dose, tissue-phantom ratios, beam profile, applicability of inverse square, and block transmission. Some data are also presented on the effect of different flattening filter designs on apparent beam energy.     

Accuracy of the photon and electron physics in GEANT4 for radiotherapy applications

This work involves a validation of the photon and electron transport of the GEANT4 particle simulation toolkit for radiotherapy physics applications. We examine the cross sections and sampling algorithms of the three electromagnetic physics models in version 4.6.1 of the toolkit: Standard, Low-energy, and Penelope. The depth dose distributions in water for incident monoenergetic and clinical beams are compared to the EGSNRC results. In photon beam simulations, all three models agree with EGSNRC to within 2%, except for the buildup region. Larger deviations are found for incident electron beams, and the differences are affected by user-imposed electron step limitations. Particle distributions through thin layers of clinical target materials, and perturbation effects near high-Z and low-Z interfaces are also investigated. The electron step size artifacts observed in our studies indicate potential problems with the condensed history algorithm. A careful selection of physics processes and transport parameters is needed for optimum efficiency and accuracy.     

Technical note: on cerrobend shielding for 18-22 MeV electron beams

The purpose of this study is to investigate (1) the depth at which the measurement of the block transmission factor should be made, and (2) the level of the transmission of 18 and 22 MeV electron beams through conventional Cerrobend. We measured the block transmission in water phantom as ionization profiles across the beam and as ionization distributions along the central axis of the beam for 18 and 22 MeV electron beams, for cone sizes ranging from 6 x 10 cm2 to 25 x 25 cm2. In our analysis, we separated the bremsstrahlung component produced in the Cerrobend block from the component originating in the head in the transmitted dose under the standard Cerrobend block. The block transmission for both beam energies and cone sizes was maximum on the central axis of the beam at depths between 0.4 and 0.7 cm. For the 18 MeV beam, the maximum transmission was 6.2% for the 6 x 10 cm2 cone, and 7.4% for the 25 x 25 cm2 cone. For the 22 MeV beam, it was 9.5% for the 6 x 10 cm2 cone, and 11.3% for the 25 x 25 cm2 cone. For the 22 MeV beam and 15 x 15 cm2 cone, it takes 2.95 and 1.4 cm of Cerrobend to reduce the maximum block transmission to 5% and 10%, respectively. The maximum dose under a blocked electron beam occurs on the central axis closer to the surface than it does for the open beam, and the block transmission factor should be defined at this shallower depth. To decrease the block transmission factor to the level of 5% on the central axis, electron beams with energy 18 MeV and greater require additional shielding.     

Bone and mucosal dosimetry in skin radiation therapy: a Monte Carlo study using kilovoltage photon and megavoltage electron beams

This study examines variations of bone and mucosal doses with variable soft tissue and bone thicknesses, mimicking the oral or nasal cavity in skin radiation therapy. Monte Carlo simulations (EGSnrc-based codes) using the clinical kilovoltage (kVp) photon and megavoltage (MeV) electron beams, and the pencil-beam algorithm (Pinnacle(3)?treatment planning system) using the MeV electron beams were performed in dose calculations. Phase-space files for the 105 and 220 kVp beams (Gulmay D3225 x-ray machine), and the 4 and 6?MeV electron beams (Varian 21 EX linear accelerator) with a field size of 5?cm diameter were generated using the BEAMnrc code, and verified using measurements. Inhomogeneous phantoms containing uniform water, bone and air layers were irradiated by the kVp photon and MeV electron beams. Relative depth, bone and mucosal doses were calculated for the uniform water and bone layers which were varied in thickness in the ranges of 0.5-2?cm and 0.2-1?cm. A uniform water layer of bolus with thickness equal to the depth of maximum dose (d(max)) of the electron beams (0.7?cm for 4 MeV and 1.5?cm for 6 MeV) was added on top of the phantom to ensure that the maximum dose was at the phantom surface. From our Monte Carlo results, the 4 and 6 MeV electron beams were found to produce insignificant bone and mucosal dose (<1%), when the uniform water layer at the phantom surface was thicker than 1.5?cm. When considering the 0.5?cm thin uniform water and bone layers, the 4 MeV electron beam deposited less bone and mucosal dose than the 6 MeV beam. Moreover, it was found that the 105 kVp beam produced more than twice the dose to bone than the 220 kVp beam when the uniform water thickness at the phantom surface was small (0.5?cm). However, the difference in bone dose enhancement between the 105 and 220 kVp beams became smaller when the thicknesses of the uniform water and bone layers in the phantom increased. Dose in the second bone layer interfacing with air was found to be higher for the 220 kVp beam than that of the 105 kVp beam, when the bone thickness was 1?cm. In this study, dose deviations of bone and mucosal layers of 18% and 17% were found between our results from Monte Carlo simulation and the pencil-beam algorithm, which overestimated the doses. Relative depth, bone and mucosal doses were studied by varying the beam nature, beam energy and thicknesses of the bone and uniform water using an inhomogeneous phantom to model the oral or nasal cavity. While the dose distribution in the pharynx region is unavailable due to the lack of a commercial treatment planning system commissioned for kVp beam planning in skin radiation therapy, our study provided an essential insight into the radiation staff to justify and estimate bone and mucosal dose.     

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.     

Monte Carlo study of si diode response in electron beams

Silicon semiconductor diodes measure almost the same depth-dose distributions in both photon and electron beams as those measured by ion chambers. A recent study in ion chamber dosimetry has suggested that the wall correction factor for a parallel-plate ion chamber in electron beams changes with depth by as much as 6%. To investigate diode detector response with respect to depth, a silicon diode model is constructed and the water/silicon dose ratio at various depths in electron beams is calculated using EGSnrc. The results indicate that, for this particular diode model, the diode response per unit water dose (or water/diode dose ratio) in both 6 and 18 MeV electron beams is flat within 2% versus depth, from near the phantom surface to the depth of R50 (with calculation uncertainty <0.3%). This suggests that there must be some other correction factors for ion chambers that counter-balance the large wall correction factor at depth in electron beams. In addition, the beam quality and field-size dependence of the diode model are also calculated. The results show that the water/diode dose ratio remains constant within 2% over the electron energy range from 6 to 18 MeV. The water/diode dose ratio does not depend on field size as long as the incident electron beam is broad and the electron energy is high. However, for a very small beam size (1 X 1 cm(2)) and low electron energy (6 MeV), the water/diode dose ratio may decrease by more than 2% compared to that of a broad beam.     

Monte Carlo feasibility study of orthogonal bremsstrahlung beams for improved radiation therapy imaging

The basic characteristics of orthogonal bremsstrahlung beams are studied and the feasibility of improved contrast imaging with such a beam is evaluated. In the context of this work, orthogonal bremsstrahlung beams represent the component of the bremsstrahlung distribution perpendicular to the electron beam impinging on an accelerator target. The BEAMnrc Monte Carlo code was used to study target characteristics, energy spectra and relative fluences of orthogonal beams to optimize target design. The reliability of the simulations was verified by comparing our results with benchmark experiments. Using the results of the Monte Carlo optimization, the targets with various materials and a collimator were designed and built. The primary pencil electron beam from the research port of a Varian Clinac-18 accelerator striking on Al, Pb and C targets was used to create orthogonal beams. For these beams, diagnostic image contrast was tested by placing simple Lucite objects in the path of the beams and comparing image contrast obtained in the orthogonal direction to the one obtained in the forward direction. The simulations for various target materials and various primary electron energies showed that a width of 80% of the continuous-slowing-down approximation range (RCSDA) is sufficient to remove electron contamination in the orthogonal direction. The photon fluence of the orthogonal beam for high Z targets is larger compared to low Z targets, i.e. by a factor of 20 for W compared to Be. For a 6 MeV electron beam, the mean energy for low Z targets is calculated to be 320 keV for Al and 150 keV for Be, and for a high Z target like Pb to be 980 keV. For irradiation times of 1.2 s in an electron mode of the linac, the contrast of diagnostic images created with orthogonal beams from the Al target is superior to that in the forward direction. The image contrast and the beam profile of the bremsstrahlung beams were also studied. Both the Monte Carlo study and experiment showed an improvement of the contrast for lower Z target materials. This study confirms the feasibility, both in terms of intensity and image contrast, of orthogonal bremsstrahlung beams for radiation therapy imaging.     

Generation and modelling of megavoltage photon beams for contrast-enhanced radiation therapy

Contrast-enhanced radiation therapy (CERT) is a treatment approach involving the irradiation of tumours containing high atomic number (Z) contrast media, using low-quality x-ray beams. This work describes the experimental generation of x-ray beams using a linear accelerator with low-Z target materials (beryllium and aluminium), in order to produce photon energy spectra appropriate for CERT. Measurements were made to compare the experimental beams to conventional linear accelerator photon beams in terms of per cent depth dose. Monte Carlo simulation was used to model the generation of each beam, and models were validated against experimental measurement. Validated models were used to demonstrate changes in photon spectra as well as to quantify the variation of tumour dose enhancement with iodinated contrast medium concentration in a simulated tumour volume. Finally, the ratio of the linear attenuation coefficient for iodinated contrast medium relative to water was determined experimentally as a function of iodine concentration. Beams created with low-Z targets show significant changes in energy spectra compared to conventional beams. For the 4 MeV/Be beam, for example, 33% of photons have energies below 60 keV. Measurements and calculation show that both the linear attenuation coefficient ratio and dose enhancement factor (DEF) increase most rapidly at concentrations below 46 mg I ml(-1). There is a significant dependence of DEF on electron energy and a lesser dependence on target material. The 4 MeV/Be beam is the most promising in terms of magnitude of DEF - for example, DEF values of 1.16 and 1.29 are obtained for concentrations of 20 mg I ml(-1) and 50 mg I ml(-1), respectively. DEF will increase or decrease, respectively, for shallower or deeper tumours at a rate of approximately 1.1% cm(-1). In summary, we show that significant dose enhancement is possible by altering the linear accelerator target and filtration, but the magnitude is highly dependent on contrast medium concentration.     

Perturbation correction factors for the NACP-02 plane-parallel ionization chamber in water in high-energy electron beams

Recent dosimetry protocols for clinical high-energy electron beams recommend measurements of absorbed dose-to-water with a plane-parallel or cylindrical ionization chamber. For well-guarded plane-parallel ionization chambers, the ionization chamber perturbation factor in water, p(Q), has a recommended value of unity in all protocols. This assumption was investigated in detail in this study for one of the recommended ionization chambers in the protocols: the Scanditronix NACP-02 plane-parallel ionization chamber. Monte Carlo (MC) simulations of the NACP-02 ionization chamber with the EGSnrc code were validated against backscatter experiments. MC simulations were then used to calculate p(wall), p(cav) and p(Q) perturbation factors and water-to-air Spencer-Attix stopping powers in 4-19 MeV electron beams of a calibration laboratory (NPL), and in 6-22 MeV clinical electron beams from a Varian CL2300 accelerator. Differences between calculated and the currently recommended (Burns et al 1996 Med. Phys. 23 383-8) stopping powers, water-to-air, were found to be limited to 0.9% at depths between the reference depth z(ref) and the depth where the dose has decreased to 50% of the maximum dose, R50. p(wall) was found to exceed unity by 2.3% in the 4 MeV NPL calibration beam at z(ref). For higher energy electron beams p(wall) decreased to a value of about 1%. Combined with a p(cav) about 1% below unity for all energies at z(ref), this was found to cause p(Q) to exceed unity significantly for all energies. In clinical electron beams all three perturbation factors were found to increase with depth. Our findings indicate that the perturbation factors have to be taken into account in calibration procedures and for clinical depth dose measurements with the NACP-02 ionization chamber.     

Dose properties of a laser accelerated electron beam and prospects for clinical application

Laser wakefield acceleration (LWFA) technology has evolved to where it should be evaluated for its potential as a future competitor to existing technology that produces electron and x-ray beams. The purpose of the present work is to investigate the dosimetric properties of an electron beam that should be achievable using existing LWFA technology, and to document the necessary improvements to make radiotherapy application for LWFA viable. This paper first qualitatively reviews the fundamental principles of LWFA and describes a potential design for a 30 cm accelerator chamber containing a gas target. Electron beam energy spectra, upon which our dose calculations are based, were obtained from a uniform energy distribution and from two-dimensional particle-in-cell (2D PIC) simulations. The 2D PIC simulation parameters are consistent with those reported by a previous LWFA experiment. According to the 2D PIC simulations, only approximately 0.3% of the LWFA electrons are emitted with an energy greater than 1 MeV. We studied only the high-energy electrons to determine their potential for clinical electron beams of central energy from 9 to 21 MeV. Each electron beam was broadened and flattened by designing a dual scattering foil system to produce a uniform beam (103%>off-axis ratio>95%) over a 25 x 25 cm2 field. An energy window (deltaE) ranging from 0.5 to 6.5 MeV was selected to study central-axis depth dose, beam flatness, and dose rate. Dose was calculated in water at a 100 cm source-to-surface distance using the EGS/BEAM Monte Carlo algorithm. Calculations showed that the beam flatness was fairly insensitive to deltaE. However, since the falloff of the depth-dose curve (R10-R90) and the dose rate both increase with deltaE, a tradeoff between minimizing (R10-R90) and maximizing dose rate is implied. If deltaE is constrained so that R10-R90 is within 0.5 cm of its value for a monoenergetic beam, the maximum practical dose rate based on 2D PIC is approximately 0.1 Gy min(-1) for a 9 MeV beam and 0.03 Gy min(-1) for a 15 MeV beam. It was concluded that current LWFA technology should allow a table-top terawatt (T3) laser to produce therapeutic electron beams that have acceptable flatness, penetration, and falloff of depth dose; however, the dose rate is still 1%-3% of that which would be acceptable, especially for higher-energy electron beams. Further progress in laser technology, e.g., increasing the pulse repetition rate or number of high energy electrons generated per pulse, is necessary to give dose rates acceptable for electron beams. Future measurements confirming dosimetric calculations are required to substantiate our results. In addition to achieving adequate dose rate, significant engineering developments are needed for this technology to compete with current electron acceleration technology. Also, the functional benefits of LWFA electron beams require further study and evaluation.     

Problems and solutions in achieving uniform dose distribution in superficial total body electron therapy.

In treating mycosis fungoides (MF) and Sezary syndrome patients with electron beam, the entire thickness and the area of the skin from crown to sole should be irradiated uniformly. To achieve irradiation of the entire thickness of the skin, electron beams of 3 - 4 MeV energy with 80 percent depth dose at 6 mm is sufficient. This unique property of limited penetration of electron beam does not cause any systemic toxicity during or after total body electron therapy. However, this property of limited penetration of electrons poses the problem of self-shielding in the curvaceous human body. The optic lens, which is within the range of penetrability of electron beam energy used for total body electron therapy, is to be shielded artificially.     

Dose enhancement by a thin foil of high-Z material: a Monte Carlo study.

The purpose of this work is to study the dose enhancement by a thin foil (thickness of 0.2-4 mm) of high-Z material in a water phantom, irradiated by high-energy photon beams. EGS4 Monte Carlo technique was used. Perturbations on the beam spectra due to the presence of the foils, and dose enhancement dependence of photon-beam quality, beam incident angle, atomic number (Z), the thickness and size of the foil, and the depth of the foil situated in the phantom were studied. Analysis of photon and secondary-electron spectra indicates that the dose enhancement near an inhomogeneity interface is primarily due to secondary electrons. A calculation for 1-mm-thick planar lead foil in a water phantom shows that the dose enhancements at 0.25, 1, 2 and 3 mm away from the foil in the backward region were 58%, 37%, 24% and 17%, respectively, for a 15 MV beam. Calculations for a variety of planar foils and photon beams show that dose enhancement: (a) increases with Z; (b) decreases with decreasing foil thickness when the foils are thinner than a certain value (1 mm for lead foil for 15 MV); (c) decreases with decreasing incident photon-beam energies; (d) changes slightly for beam incident angles less than 45 degrees and more prominently for larger angles; (e) increases with size of foil; and (f) is almost independent of the depth at which the foil is situated when the foil is placed beyond the range of secondary electrons. The dose enhancement calculation is also performed for a cylindrically shaped lead foil irradiated by a four-field-box. The dose enhancement of 34%/13% was obtained at 0.25/2 mm away from the cylindrical outer interface for a 15 MV four-field-box.     

The characterization of the Advanced Markus ionization chamber for use in reference electron dosimetry in the UK

The IPEM Code of Practice (IPEM 2003) for electron dosimetry for radiotherapy beams recommends design requirements for parallel-plate ionization chambers used to determine absorbed dose to water in an electron beam. The Classic Markus design has been found not to meet these requirements. The Advanced Markus ionization chamber has been designed to rectify the problems associated with the Classic Markus ionization chamber. The response of three Advanced Markus ionization chambers was investigated and compared to the designated chamber types. Absorbed dose to water calibration factors were derived at the National Physical Laboratory (NPL) for each ionization chamber at seven electron energies in the range nominally 4-19 MeV. Investigations were carried out into chamber settling, polarity effects, ion recombination and the chamber perturbation. The response of the ionization chambers in a clinical beam was also investigated. In general all three Advanced Markus ionization chambers showed the same energy response. The magnitude of the polarity effect was typically 5% at a nominal energy of 4 MeV. There was discrepancy between the polarity measurements made at the NPL and in the clinic. The recommendation of this study is that this chamber type is not suitable for reference dosimetry in electron beams.