Mass Collision Stopping-power Ratio Research Articles

Does Electron Spectrum Affect TLD-100 Dose Response in 6 MV Photon Beam Irradiation?

In this study, the electron spectrum effect on the TLD-100 dosimeter response to a 6 MV photon beam in different media like water, aluminum, polystyrene, iron, copper, and lead using Monte Carlo and Burlin cavity theory was evaluated. To calculate and compare the dose to medium to dose to cavity correction factors (f), the electronic equilibrium spectrum produced by the 6 MV photon beam and its maximum electron energy in different media were used. The electronic equilibrium spectra were obtained using Beamdp Monte Carlo Simulation. Using two different methods, the cavity theory was applied to obtain the response of the TLD-100 to 6 MV photon beam in the media considered. In the first method, the average mass collision stopping power ratios and the average mass effective attenuation coefficients were calculated using the electron spectrum of 6 MV. In the second method, these parameters were calculated based on the maximum energy value of 6 MV. The maximum difference between the f values obtained using the two methods was about 10 % for lead, while it was less than 2.5 % for other media. Consequently, the differences between f factors calculated using these two methods were insignificant except for lead.

Monte Carlo study of dosimetric impact of gadolinium contrast medium in transverse field MR-Linac system.

The aim of this study is to evaluate the dosimetric impact of gadolinium contrast medium (Gadovist) in a transverse MR-Linac system using Monte Carlo methods. The dose distributions were calculated using two heterogeneous multi-layer phantoms consisting of Gadovist, water, bone, and lung. The photon beam was irradiated with a filed size of 5×5cm2, and a transverse magnetic field of 0-3.0T was applied perpendicular to the incident photon beam. Next, dose distributions for brain, head and neck (H&N), and lung cancer patients were calculated using a patient voxel-based phantom with and without replacing the patient's GTV with Gadovist. The dose at the water-Gadovist interface increased by 8% without a magnetic field. A similar dose increment was observed at 0.35T. In contrast, the dose increment at the water-Gadovist interface was small at 1.5T and a dose decrement of 5% was observed at 3.0T. The dose variation at the lung-Gadovist interface was larger than that at the water-Gadovist interface. The mass collision stopping power ratio for Gadovist was 7% lower than that for water, whereas, the electron fluence spectra at the water-Gadovist interface increased by 17.5%. In a patient study, Gadovist increased the Dmean for brain, H&N, and lung cancer patients by 0.65-8.9%. The dose variation due to Gadovist grew large in the low-dose region in H&N and lung cancer. The GTV dose variation due to Gadovist in all treatment site was below 2% at 0-3T if the Gadovist concentration was lower than 0.2mmol/ml-1.

Ethylene-vinyl acetate foam as a new lung substitute in radiotherapy.

The purpose of this study was to evaluate ethylene-vinyl acetate (EVA) foam as a new lung substitute in radiotherapy and to study its physical and dosimetric characteristics. We calculated the ideal vinyl acetate (VA) content of EVA foam sheets to mimic the physical and dosimetric characteristics of the ICRU lung tissue. We also computed the water-to-medium mass collision stopping power ratios, mass attenuation coefficients, CT numbers, effective atomic numbers and electron densities for: ICRU lung tissue, the RANDO commercial phantom, scaled WATER and EVA foam sheets with varying VA contents in a range between the minimum and maximum values supplied by the manufacturer. For all these substitutes, we simulated percent depth-dose curves with EGSnrc Monte Carlo (MC PDDs) in a water-lung substitute-water slab phantom expressed as dose-to-medium and dose-to-water for 3×3- and 10×10-cm2 field sizes. PDD for the 10×10-cm2 field size was also calculated with the MultiGrid Superposition algorithm (MGS PDD) for a relative electron density to water ratio of 0.26. The latter was compared with the MC PDDs in dose-to-water for scaled WATER and EVA foam sheets with the VA content that was most similar to the calculated ideal content that is physically achievable in practice. We calculated an ideal VA content of 55%; however, the maximum physically achievable content with current manufacturing techniques is 40%. The physical characteristics of the EVA foam sheets with a VA content of 40% (EVA40) are very close to those of the ICRU lung reference. The physical densities of the EVA40 foam sheets ranged from 0.030 to 0.965g/cm3 , almost covering the entire physical density range of the inflated/deflated lung (0.260-1.050g/cm3 ). Its mass attenuation coefficient at the effective energy of a 6-MV photon beam agrees within 0.8% of the ICRU reference value, and its CT number agrees within 6HU. The effective atomic number for EVA40 varies by less than 0.42 of the ICRU value, and its effective electron density is within 0.9%. PDDs expressed in dose-to-medium and dose-to-water agree with the ICRU curve within 2% in all regions. PDDs calculated with both MC and MGS were within 1.5%. The EVA40 is an excellent cork-like lung substitute for radiotherapy applications. From a sole material used in footwear, it is possible to obtain a lung substitute that mimics the physical and dosimetric characteristics of ICRU lung tissue even better than the RANDO commercial phantom.

Monte Carlo-based investigations on the impact of removing the flattening filter on beam quality specifiers for photon beam dosimetry.

The impact of removing the flattening filter in clinical electron accelerators on the relationship between dosimetric quantities such as beam quality specifiers and the mean photon and electron energies of the photon radiation field was investigated by Monte Carlo simulations. The purpose of this work was to determine the uncertainties when using the well-known beam quality specifiers or energy-based beam specifiers as predictors of dosimetric photon field properties when removing the flattening filter. Monte Carlo simulations applying eight different linear accelerator head models with and without flattening filter were performed in order to generate realistic radiation sources and calculate field properties such as restricted mass collision stopping power ratios (L¯/ρ)airwater, mean photon and secondary electron energies. To study the impact of removing the flattening filter on the beam quality correction factors kQ , this factor for detailed ionization chamber models was calculated by Monte Carlo simulations. Stopping power ratios (L¯/ρ)airwater and kQ values for different ionization chambers as a function of TPR1020 and %dd(10)x were calculated. Moreover, mean photon energies in air and at the point of measurement in water as well as mean secondary electron energies at the point of measurement were calculated. The results revealed that removing the flattening filter led to a change within 0.3% in the relationship between %dd(10)x and (L¯/ρ)airwater, whereby the relationship between TPR1020 and (L¯/ρ)airwater changed up to 0.8% for high energy photon beams. However, TPR1020 was a good predictor of (L¯/ρ)airwater for both types of linear accelerator with energies < 10MeV with a maximal deviation between both types of accelerators of 0.23%. According to the results, the mean photon energy below the linear accelerators head as well as at the point of measurement may not be suitable as a predictor of (L¯/ρ)airwater and kQ to merge the dosimetry of both linear accelerator types. It was possible to derive (L¯/ρ)airwater using the mean secondary electron energy at the point of measurement as a predictor with an accuracy of 0.17%. A bias between kQ for linear accelerators with and without flattening filter within 1.1% and 1.6% was observed for TPR1020 and %dd(10)x respectively. The results of this study have shown that removing the flattening filter led to a change in the relationship between the well-known beam quality specifiers and dosimetric quantities at the point of measurement, namely (L¯/ρ)airwater, mean photon and electron energy. Furthermore, the results show that a beam profile correction is important for dose measurements with large ionization chambers in flattening filter free beams.

A Proposal for the Absorbed Dose to Water Dosimetry for Flattening Filter-free Beams.

Flattening filter-free (FFF) beams generated by linear accelerators have been widely adopted in many hospitals recently for radiation therapy. FFF technology can provide higher dose rates so that shortening of the treatment time and less intra-fraction motion error are expected.In Japan, the current way of determining absorbed dose to water for FFF beams is to follow the Standard Dosimetry 12 protocol which was developed for flattened beams. Since it has been reported that the flattened beams and FFF beams have different beam properties, it is necessary to evaluate the usefulness of Standard Dosimetry 12 protocol for FFF beam dosimetry.This report reviews physical and dosimetric properties of FFF beams especially in terms of the effect on absorbed dose to water dosimetry using an ionization chamber. From the review, it became evident that the absorbed dose to water is underestimated by volume averaging effect of the ionization chamber. On the other hand, the absorbed dose to water is overestimated by using the beam-quality specifier TPR20,10 to predict the restricted mass collision stopping power ratio for FFF beams. Therefore, an alternative method was proposed for absorbed dose to water dosimetry of FFF beams based on Standard Dosimetry 12.

Experimental validation of the dual parameter beam quality specifier for reference dosimetry in flattening-filter-free (FFF) photon beams

Removal of the flattening filter alters the energy spectrum of the photon beam such that current beam quality specifiers may not correctly account for this change when predicting the Spencer–Attix restricted water-to-air mass collision stopping-power ratio, . Johnsson et al ( Phys. Med. Biol. 45 2733–45) proposed a beam quality specifier, known as the dual parameter beam quality specifier, which was calculated via Monte Carlo (MC) simulations using transmission data of primary kerma through two differing thicknesses of water material. Ceberg et al ( Med. Phys. 37 1164–8) extended this MC study to include relevant flattening filter free (FFF) beam data. Experimental investigations of this dual parameter beam quality specifier have not previously been published, therefore the purpose of this work was to validate that the dual parameter beam quality specifier could be measured experimentally for clinical beams (both with a flattening filter (WFF) and without (FFF)).Transmission measurements of primary kerma were performed by employing the setup outlined in Johnsson et al ( Phys. Med. Biol. 44 2445–50). Varying absorber thicknesses, in 5 cm increments from 5 to 40 cm, were placed at isocentre with the chamber positioned at an extended source to chamber distance of 300 cm. Experimental setup for TPR20,10 and %dd(10)x followed the methodology outlined in IAEA TRS398 (2004) and TG-51 (1999) with AAPM Addendum to TG-51 (2014) respectively.The maximum difference of determined using the different beam quality specifiers was found to be 0.35%. Analysis of the absorber thickness combination found that small thicknesses (<10 cm) for the first absorber and absorbers similar in thickness (<10 cm) should be avoided.Stopping-power ratios of the beams investigated were determined using three different beam quality specifiers. The results demonstrated successful experimental determination of the dual parameter beam quality specifier, indicating its potential as an alternate beam quality specifier for FFF beams.

Combining tissue-phantom ratios to provide a beam-quality specifier for flattening filter free photon beams.

There are currently several commercially available radiotherapy treatment units without a flattening filter in the beam line. Unflattened photon beams have an energy and lateral fluence distribution that is different from conventional beams and, thus, their attenuation properties differ. As a consequence, for flattening filter free (FFF) beams, the relationship between the beam-quality specifier TPR20,10 and the Spencer-Attix restricted water-to-air mass collision stopping-power ratios, L̄/ρair (water), may have to be refined in order to be used with equivalent accuracy as for beams with a flattening filter. The purpose of this work was twofold. First, to study the relationship between TPR20,10 and L̄/ρair (water) for FFF beams, where the flattening filter has been replaced by a metal plate as in most clinical FFF beams. Second, to investigate the potential of increasing the accuracy in determining L̄/ρair (water) by adding another beam-quality metric, TPR10,5. The relationship between L̄/ρair (water) and %dd(10)x for beams with and without a flattening filter was also included in this study. A total of 24 realistic photon beams (10 with and 14 without a flattening filter) from three different treatment units have been used to calculate L̄/ρair (water), TPR20,10, and TPR10,5 using the EGSnrc Monte Carlo package. The relationship between L̄/ρair (water) and the dual beam-quality specifier TPR20,10 and TPR10,5 was described by a simple bilinear equation. The relationship between the photon beam-quality specifier %dd(10)x used in the AAPM's TG-51 dosimetry protocol and L̄/ρair (water) was also investigated for the beams used in this study, by calculating the photon component of the percentage depth dose at 10 cm depth with SSD 100 cm. The calculated L̄/ρair (water) for beams without a flattening filter was 0.3% lower, on average, than for beams with a flattening filter and comparable TPR20,10. Using the relationship in IAEA, TRS-398 resulted in a root mean square deviation (RMSD) of 0.0028 with a maximum deviation of 0.0043 (0.39%) from Monte Carlo calculated values. For all beams in this study, the RMSD between the proposed model and the Monte Carlo calculated values was 0.0006 with a maximum deviation of 0.0013 (0.1%). Using an earlier proposed relationship [Xiong and Rogers, Med. Phys. 35, 2104-2109 (2008)] between %dd(10)x and L̄/ρair (water) gave a RMSD of 0.0018 with a maximum deviation of 0.0029 (0.26%) for all beams in this study (compared to RMSD 0.0015 and a maximum deviation of 0.0048 (0.47%) for the relationship used in AAPM TG-51 published by Almond et al. [Med. Phys. 26, 1847-1870 (1999)]). Using TPR20,10 as a beam-quality specifier, for the flattening filter free beams used in this study, gave a maximum difference of 0.39% between L̄/ρair (water) predicted using IAEA TRS-398 and Monte Carlo calculations. An additional parameter for determining L̄/ρair (water) has been presented. This parameter is easy to measure; it requires only an additional dose measurement at 5 cm depth with SSD 95 cm, and provides information for accurate determination of the L̄/ρair (water) ratio for beams both with and without a flattening filter at the investigated energies.

Conversion from dose-to-graphite to dose-to-water in an 80 MeV/A carbon ion beam

Based on experiments and numerical simulations, a study is carried out pertaining to the conversion of dose-to-graphite to dose-to-water in a carbon ion beam. This conversion is needed to establish graphite calorimeters as primary standards of absorbed dose in these beams. It is governed by the water-to-graphite mass collision stopping power ratio and fluence correction factors, which depend on the particle fluence distributions in each of the two media. The paper focuses on the experimental and numerical determination of this fluence correction factor for an 80 MeV/A carbon ion beam. Measurements have been performed in the nuclear physics laboratory INFN-LNS in Catania (Sicily, Italy). The numerical simulations have been made with a Geant4 Monte Carlo code through the GATE simulation platform. The experimental data are in good agreement with the simulated results for the fluence correction factors and are found to be close to unity. The experimental values increase with depth reaching 1.010 before the Bragg peak region. They have been determined with an uncertainty of 0.25%. Different numerical results are obtained depending on the level of approximation made in calculating the fluence correction factors. When considering carbon ions only, the difference between measured and calculated values is maximal just before the Bragg peak, but its value is less than 1.005. The numerical value is close to unity at the surface and increases to 1.005 near the Bragg peak. When the fluence of all charged particles is considered, the fluence correction factors are lower than unity at the surface and increase with depth up to 1.025 before the Bragg peak. Besides carbon ions, secondary particles created due to nuclear interactions have to be included in the analysis: boron ions (10B and 11B), beryllium ions (7Be), alpha particles and protons. At the conclusion of this work, we have the conversion of dose-to-graphite to dose-to-water to apply to the response of a graphite calorimeter in an 80 MeV/A carbon ion beam. This conversion consists of the product of two contributions: the water-to-graphite electronic mass collision stopping power ratio, which is equal to 1.115, and the fluence correction factor which varies linearly with depth, as kfl, all = 0.9995 + 0.0048 zw-eq. The latter has been determined on the basis of experiments and numerical simulations.

Re-evaluation of the product of (W/e)air and the graphite to air stopping-power ratio for 60Co air kerma standards

Experiments which determine the product of (W/e)air, the average energy deposited per coulomb of charge of one sign released by an electron coming to rest in dry air, and , the Spencer–Attix mean restricted mass collision stopping-power ratio for graphite to air, in a 60Co or 137Cs beam are reanalysed. Correction factors, e.g., to account for gaps about a calorimeter core or perturbations due to a cavity's presence, are calculated using the EGSnrc Monte Carlo code system and these generally decrease the value of (W/e)air for each experiment. Stopping-power ratios are calculated for different choices of density correction and average excitation energy (I-value) for graphite. To calculate an average value (W/e)air for the BIPM air kerma standard, each experimental result is multiplied by the ratio /. While individual values of are sensitive to the I-values and density corrections assumed, this ratio varies by less than 0.1% for different choices. Hence, the product (W/e)air is relatively insensitive to these choices. The weighted mean of the updated data is (W/e)air J C−1 ± 0.2%, suggesting that the accepted value of 33.97 J C−1 ± 0.1% is 0.8% too high. This has implications for primary 60Co air kerma standards worldwide and potentially for the choice of graphite I-value and density correction for the calculation of the graphite stopping power, as well as the value of (W/e)air.

SU-GG-T-616: The Perturbation Correction Factors for Plane-Parallel Ion Chamber Dose Measurements in the Build-Up Region of Megavoltage Photon Beams

Purpose: To calculate perturbation factors for plane-parallel ion chamber dose measurements in the build-up region of megavoltage photon beams. Method and Materials: We simulated NACP-02 and ROOS plane-parallel chambers by using the EGSnrc/cavity user code and calculated dose to water (Dair)pp in build-up region for 6 and 18 MV photon beam. Also, the dose to air in the chamber with no wall Dair is calculated. The perturbation correction factor for presence of wall and cavity, Pwall and Prepl, were calculated from Prepl =(Dw / Dair) / (L / ρ)airw and Pwall=Dair/(Dair)pp where Dw represents dose to water calculated with cylindrical slab of 0.02 cm thickness and 1.0 cm radius, (L / ρ)airw is the average restricted mass collision stopping-power ratio of water to air. The overall perturbation factors Pq (the product of Prepl and Pwall) were calculated from [Dw / (Dair)pp] / (L / ρ)airw. The field sizes were 10×10 cm2 and 30×30 cm2 at a source-surface distance of 100 cm. Results: Pwall at depths of 0.1 cm to 2 cm for ROOS ranged from 1.016 to 1.022 for 6 MV and for NACP-02 varied from 0.919 to 1.028. For 18 MV, Pwall of both chambers was a similar tendency. Prepl for ROOS and NACP-02 in 6 MV was 0921 and 0.951, respectively, at 0.1 cm and nearly unity at 1 cm. In 18 MV beam, Prepl for ROOS and NACP-02 was 0.931 and 0.911, respectively, at 0.1cm. PQ for ROOS in 6 MV varied from 0.972 to 1.025 at depths of 0.1 cm to 2 cm and PQ for NACP-02 varied from 0.847 to 1.026. Similarly, PQ for ROOS in 18 MV was 0.946 to 1.015 and for NACP-02 was 0.803 to 1.023 at depths of 0.1 cm to 3.5 cm. Conclusions: It was found that ROOS is more suitable than NACP-02 for dose measurements in the build-up region of megavoltage photon beams.

The replacement correction factor for the BIPM flat cavity ion chamber and the value of

A graphite flat cavity ionization chamber is used at the BIPM in France to determine the absorbed dose to graphite in a 60Co photon beam and thereby used to determine the product of the value of W/e, the average energy required to produce an ion pair in dry air, and the value of (LDelta/p)a(C), the mean restricted mass collision stopping-power ratio for graphite to air in a 60Co beam. The accuracy of the (W/e) (LDelta/p)a(C) value thus determined depends upon the accuracy of the perturbation correction factors adopted for this chamber. The perturbation effect of this chamber was accounted for by the replacement correction factor whose value was calculated by an analytical method and confirmed by an EGS4 Monte Carlo calculation. The purpose of this study is to investigate the validity of the analytical and the EGS4 calculations by using recently established methods and the EGSnrc Monte Carlo code, a much improved version of EGS4, to calculate the replacement correction factors for the graphite chamber. It is found that the replacement correction factors used for the BIPM chamber are not correct: the values used are smaller than they should be by about 1%. This leads to a 1% overestimation of the (W/e) (LDelta/p)a(C) value determined by using this chamber. This implies that 60Co air kerma standards that are directly proportional to this product need to be reduced by 1%. Based on the values of the replacement correction factors calculated in this study, and on the value of (LDelta/p)a(C) evaluated from ICRU Report No. 37 stopping power for graphite, the value of W/e determined by using the BIPM chamber should be 33.61 +/- 0.08 J/C. If a more recent value of mean excitation energy for graphite (86.8 eV) and grain density are used to evaluate the graphite stopping power, then the value obtained for W/e is 34.15 +/- 0.08 J/C.

Relationship between and stopping‐power ratios for flattening filter free accelerators: A Monte Carlo study

The relationship between the photon beam quality specifier %dd(10)x and the Spencer-Attix water water to air restricted mass collision stopping-power ratio, (L/rho))air(water), is studied using Monte Carlo simulation with realistic beams in contrast to the previously used realistic but uniform spectra from an isotropic point source. The differences between accelerators with and without flattening filters are investigated since flattening filter free accelerators appear to be useful for IMRT. Our results show that the standard relationship between %dd(10)x and (L/rho)air(water), which is used in the TG-51 protocol to calculate the quality conversion factor kQ, is acceptable for beams with or without a flattening filter with a maximum error of 0.4%, although a fit to the new data would reduce the maximum error to 0.2%. Reasons for differences between the individual values of %dd(10)x and (L/ rho)air(water) with and without a flattening filter are studied. Specifically the differences due to the softening of the beam, the change in shape of the profile, and the inclusion of radial variations in the photon energy spectra, are investigated. It is shown that if TPR10(20) is used as a beam quality specifier, there are two different relationships between TPR10(20) and (L/rho)air(water) which differ by 0.4%-1%. When using TPR10(20) as a beam quality specifier in a beam without a flattening filter, one should subtract 0.5% from the value of kQ for a given value of TPR10(20).

Physical parameters and correction factors for ionization chambers for absolute measurement of air kerma in γ-ray fields

Values of physical parameters and correction factors essential for the absolute measurement of air kerma in 137Cs and 60Co γ-ray fields were obtained using an EGS5 program for spherical, cylindrical and pancake ionization chambers. The mean mass collision stopping power ratio for graphite and air, was found to vary depending on the cutoff energy of electrons employed in calculation. The ratio between the energies deposited in cavity air due to Compton electrons emitted from the air and those from the graphite wall increases as the chamber size is increased. It also increases as the γ-ray energy is reduced and is equal to 0.09 for 137Cs γ-rays in a spherical ionization chamber of cavity diameter 12 cm. Correction factors for γ-ray attenuation in chamber walls and those for the contribution of scattered γ-rays to chamber responses were obtained separately. The wall correction factor, which is equal to the product of these two factors, is close to unity for pancake chambers.

Measurements of output factors with different detector types and Monte Carlo calculations of stopping-power ratios for degraded electron beams

The aim of the present study was to investigate three different detector types (a parallel-plate ionization chamber, a p-type silicon diode and a diamond detector) with regard to output factor measurements in degraded electron beams, such as those encountered in small-electron-field radiotherapy and intraoperative radiation therapy (IORT). The Monte Carlo method was used to calculate mass collision stopping-power ratios between water and the different detector materials for these complex electron beams (nominal energies of 6, 12 and 20 MeV). The diamond detector was shown to exhibit excellent properties for output factor measurements in degraded beams and was therefore used as a reference. The diode detector was found to be well suited for practical measurements of output factors, although the water-to-silicon stopping-power ratio was shown to vary slightly with treatment set-up and irradiation depth (especially for lower electron energies). Application of ionization-chamber-based dosimetry, according to international dosimetry protocols, will introduce uncertainties smaller than 0.3% into the output factor determination for conventional IORT beams if the variation of the water-to-air stopping-power ratio is not taken into account. The IORT system at our department includes a 0.3 cm thin plastic scatterer inside the therapeutic beam, which furthermore increases the energy degradation of the electrons. By ignoring the change in the water-to-air stopping-power ratio due to this scatterer, the output factor could be underestimated by up to 1.3%. This was verified by the measurements. In small-electron-beam dosimetry, the water-to-air stopping-power ratio variation with field size could mostly be ignored. For fields with flat lateral dose profiles (>3 × 3 cm2), output factors determined with the ionization chamber were found to be in close agreement with the results of the diamond detector. For smaller field sizes the lateral extension of the ionization chamber hampers its use. We therefore recommend that the readily available silicon diode detector should be used for output factor measurements in complex electron fields.

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.

Preliminary results on a dedicated silicon diode detector for proton dosimetry.

The present work reports preliminary measurements on the behaviour of a new p-type stereotactic silicon diode. Hi-pSi, produced by Scanditronix and dedicated to proton dosimetry. Diode response was investigated in low-energy proton beams (26.7 MeV and 12 MeV nominal energy), mainly with attention to stability, linearity, dose rate and energy dependence of the detector response. Three different Hi-pSi diodes of the same type were investigated. The diode response was linear with dose and the standard deviation of repeated readings was less than 2.5%. A marked dependence on dose rate was observed for one of the diodes (a response increase of 47% in the 0.7-11 Gy x min(-1) range). After the dose rate and water to silicon mass collision stopping power ratio correction of the diode response in the depth dose measurements, the difference, at the Bragg peak, with respect to the reference chamber was about 4%, ascribed to poor knowledge of the materials in front of the sensitive volume. The diode response was also nearly independent of linear energy transfer (LET) in the 9.6-21.5 MeV effective energy range.

Stopping-power ratios for clinical electron beams from a scatter-foil linear accelerator

Restricted mass collision stopping-power ratios for electron beams from a scatter-foil medical linear accelerator (Varian Clinac 2100C) were calculated for various combinations of beams, phantoms and detector materials using the Monte Carlo method. The beams were of nominal energy 6, 12 or 20 MeV, with square dimensions 1 × 1 cm2 to 10 × 10 cm2. They were incident at nominal SSDs of 100 or 120 cm and inclined at 90° or 30° to the surface of homogeneous water phantoms or water phantoms interspersed with layered lung or bone-like materials. The broad beam water-to-air stopping-power ratios were within 1.3% of the AAPM TG21 protocol values and consistent with the results of Ding et al to within 0.2%. On the central axis the stopping-power ratio variations for narrow beams compared with normally incident broad beams were 0.1% or less for water-to-LiF-100, graphite, ferrous sulfate dosimeter solution, polystyrene and PMMA, 0.5% for water-to-silicon and 1% for water-to-air and water-to-photographic-film materials. The transverse variations of the stopping-power ratios were up to 4% for water-to-silicon, 7% for water-to-photographic-film materials and 10% for water-to-air in the penumbral regions (where the dose was 10% of the global dose maximum) at shallow depths compared with the values at the same depths on the central axis. In the inhomogeneous phantoms studied, the stopping-power ratio correction factors varied more significantly for air, followed by photographic materials and silicon, at various depths on the central axis in the heterogeneous regions. For the simple layered phantoms studied, the estimation of the stopping-power ratio correction factors based on the relative electron-density derived effective depth approach yielded results that were within 0.5% of the Monte Carlo derived values for all the detector materials studied.

Calculation of stopping-power ratios using realistic clinical electron beams.

The Spencer-Attix water/air restricted mass collision stopping-power ratio is calculated in realistic electron beams in the energy range from 5-50 MeV for a variety of clinical accelerators including the Varian Clinac 2100C, the Philips SL75-20, the Siemens KD2, the AECL Therac 20, and the Scanditronix Medical Microtron 50. The realistic clinical beams are obtained from full Monte Carlo simulations of the clinical linear accelerators using the code BEAM. The stopping-power ratios calculated using clinical beams are compared with those determined according to the AAPM and the IAEA protocols which were calculated by using monoenergetic parallel beams. Using the energy-range relationship of Rogers and Bielajew [Med. Phys. 13, 687-694 (1986)] leads to the most consistent picture in which the stopping-power ratios at dmax derived from mono-energetic calculations underestimate the stopping-power ratios calculated with the realistic beam by 0.3% at 5 MeV and up to 1.4% at 20 MeV. The stopping-power ratios at dmax determined according to the AAPM TG-21 protocol (1983) are shown to overestimate the realistic stopping-power ratios by up to 0.6% for a 5-MeV beam and underestimate them by up to 1.2% for a 20-MeV beam. Those determined according to the IAEA (1987) protocol overestimate the realistic stopping-power ratios by up to 0.3% for a 5-MeV beam and underestimate them by up to a 1.1% for a 20-MeV beam at reference depth. The causes of the differences in the stopping-power ratios between the realistic clinical mono-energetic beams are analyzed quantitatively. The changes in the stopping-power ratios at dmax are mainly due to the energy spread of the electron beam and the contaminant photons in the clinical beams. The effect of the angular spread of electrons is rather small except at the surface. Data are presented which give the corrected stopping-power ratios at dmax or reference depth starting from those determined according to protocols for any energy of clinical electron beams with scattering foils. For scanned clinical electron beams the correction to stopping-power ratios determined according to protocols is found to be less than 0.5% at dmax or reference depth for all beam energies studied. We quantify the differences in the stopping-power ratios determined using the depth of 50% ionization level and the depth of 50% dose level. The differences are very small except for very-high-energy beams (50 MeV) where they can be up to 0.8%.

Dosimetry of 106Ru eye applicators with a p-type silicon detector

Beta emitting 106Ru-applicators are used to treat choroidal melanoma. In order to improve the accuracy and simplify the dosimetry of these applicators, the suitability of using a p-type silicon semiconductor detector has been investigated. The detector is calibrated in a low energy electron beam. An important property of the detector for this application is that the linear scattering power of the epoxy resin that surrounds the silicon crystal is within 1% of that for water, in the energy interval of interest. Furthermore the variation in the mass collision stopping power ratio of water to silicon is within 4% in this energy interval. The accuracy of the dose-rate measured with the detector is about 5% for this application.

Comparison of ferrous sulfate (Fricke) and ionization dosimetry for high‐energy photon and electron beams

Dose measurements using Fricke and ionization methods were compared for 60Co gamma rays, 4-25-MV photons, and 10-25-MeV electrons. Fricke derived doses based on a constant yield (epsilon mG) were in good agreement with ionization derived doses based on the American Association of Physicists in Medicine Task Group 21 protocol and the National Research Council of Canada (NRC) ND calibration or the NRC proposed Nx. These measurements also confirmed the validity of the double-voltage technique in the collection efficiency correction, even for swept electron beams. Assuming the correctness of the ionization derived doses, the radiation yield appeared to be 1% higher and to increase with photon energy when irradiation vessels were made of Pyrex but not with polystyrene cells. These glass wall effects could be due to the scattering perturbation of electrons between inhomogeneous materials and, in particular for photon beams, due to the mismatch in mass energy absorption ratios and mass collision stopping power ratios between the Fricke dosimeter and the wall materials.