Calculation of effective dose from measurements of secondary neutron spectra and scattered photon dose from dynamic MLC IMRT for 6 MV, 15 MV, and 18 MV beam energies

Effective doses were calculated from the delivery of 6 MV, 15 MV, and 18 MV conventional and intensity-modulated radiation therapy (IMRT) prostate treatment plans. ICRP-60 tissue weighting factors were used for the calculations. Photon doses were measured in phantom for all beam energies. Neutron spectra were measured for 15 MV and 18 MV and ICRP-74 quality conversion factors used to calculate ambient dose equivalents. The ambient dose equivalents were corrected for each tissue using neutron depth dose data from the literature. The depth corrected neutron doses were then used as a measure of the neutron component of the ICRP protection quantity, organ equivalent dose. IMRT resulted in an increased photon dose to many organs. However, the IMRT treatments resulted in an overall decrease in effective dose compared to conventional radiotherapy. This decrease correlates to the ability of an intensity-modulated field to minimize dose to critical normal structures in close proximity to the treatment volume. In a comparison of the three beam energies used for the IMRT treatments, 6 MV resulted in the lowest effective dose, while 18 MV resulted in the highest effective dose. This is attributed to the large neutron contribution for 18 MV compared to no neutron contribution for 6 MV.     

Assessing the effect of electron density in photon dose calculations

Photon dose calculation algorithms (such as the pencil beam and collapsed cone, CC) model the attenuation of a primary photon beam in media other than water, by using pathlength scaling based on the relative mass density of the media to water. In this study, we assess if differences in the electron density between the water and media, with different atomic composition, can influence the accuracy of conventional photon dose calculations algorithms. A comparison is performed between an electron-density scaling method and the standard mass-density scaling method for (i) tissues present in the human body (such as bone, muscle, etc.), and for (ii) water-equivalent plastics, used in radiotherapy dosimetry and quality assurance. We demonstrate that the important material property that should be taken into account by photon dose algorithms is the electron density, and not the mass density. The mass-density scaling method is shown to overestimate, relative to electrondensity predictions, the primary photon fluence for tissues in the human body and water-equivalent plastics, where 6%-7% and 10% differences were observed respectively for bone and air. However, in the case of patients, differences are expected to be smaller due to the large complexity of a treatment plan and of the patient anatomy and atomic composition and of the smaller thickness of bone/air that incident photon beams of a treatment plan may have to traverse. Differences have also been observed for conventional dose algorithms, such as CC, where an overestimate of the lung dose occurs, when irradiating lung tumors. The incorrect lung dose can be attributed to the.incorrect modeling of the photon beam attenuation through the rib cage (thickness of 2-3 cm in bone upstream of the lung tumor) and through the lung and the oversimplified modeling of electron transport in convolution algorithms. In the present study, the overestimation of the primary photon fluence, using the mass-density scaling method, was shown to be a consequence of the differences in the hydrogen content between the various media studied and water. On the other hand, the electron-density scaling method was shown to predict primary photon fluence in media other than water to within 1%-2% for all the materials studied and for energies up to 5 MeV. For energies above 5 MeV, the accuracy of the electron-density scaling method was shown to depend on the photon energy, where for materials with a high content of calcium (such as bone, cortical bone) or for primary photon energies above 10 MeV, the pair-production process could no longer be neglected. The electron-density scaling method was extended to account for pair-production attenuation of the primary photons. Therefore the scaling of the dose distributions in media other than water became dependent on the photon energy. The extended electron-scaling method was shown to estimate the photon range to within 1% for all materials studied and for energies from 100 keV to 20 MeV, allowing it to be used to scale dose distributions to media other than water and generated by clinical radiotherapy photon beams with accelerator energies from 4 to 20 MV.     

Photon beam quality specification by narrow-beam transmission measurements

Radiation quality specifications in megavoltage photon beams are usually based on depth-dose measurements performed under reference conditions. Stopping-power ratios and various correction factors are then related to parameters such as TPR(10)20, which are extracted from depth-dose measurements. Stopping-power ratio determinations based on this concept were shown to be in error by more than 2% at high energies. Furthermore, electrons generated in the treatment head can, at high energies, contribute to the dose at a depth of 10 cm and thus significantly affect the TPR(10)20 ratio. This method was further shown to be inadequate when the dose in other parts of the field than the reference point was to be measured with ionization chamber dosimetry. A new standardized device for determining photon beam quality based on half value layer (HVL) measurements in water was developed and thoroughly investigated in both a low-energy, (4 MV) and a high-energy beam. A relation between HVL and stopping-power ratios water-to-air was determined by comparative measurements with air ionization chambers and liquid-filled ionization chambers together with Fricke dosimetry. Furthermore, different radiation quality gradients in the photon fields for different types of field-flattening systems, and field-compensating methods were discussed.     

Experimental determination of beam quality conversion factors k(Q) in clinical photon beams using ferrous sulphate (Fricke) dosimetry

The implementation of protocols based on absorbed dose to water standards requires beam quality conversion factors, k(Q). Calculated values of k(Q) are available for ionization chambers used for reference dosimetry. Ideally, k(Q) should be experimentally determined at the same beam qualities as that of the user. In this work we measure k(Q) factors in clinical photon beams and compare them with calculated and measured values. Beam quality conversion factors are determined for clinical photon beams of nominal energies 4 MV, 6 MV, 15 MV, and 25 MV, for commonly used cylindrical ionization chambers. Twelve chambers of eight different types are used. For three of them, no experimental data have previously been available. The experimental procedure is based on measurements with ionization chambers and Fricke dosimetry in the reference beam (60Co gamma radiation) and in clinical linear accelerator beams. The k(Q) values determined in this work generally agree within 0.5% with previously reported experimental values both when %dd(10)x and TPR2010 are used for beam quality specification. The agreement with calculated data is generally within 0.5%, except for the 15 MV beam. For this beam the measured values are usually between 0.5% and 1% lower than the data taken from the TG-51 protocol or the TRS-398 code of practice. For three NE2571 chambers and three NE2581 chambers, the maximum observed deviation of individual k(Q) values is 0.2% and 0.4%, respectively.     

有无均整器模式下臂架与准直器不同角度条件下的剂量学比较

目的:探讨臂架或准直器角度的改变对均整（FF）与非均整（FFF）两种模式的射线剂量的影响。方法:选用Versa HD直线加速器配备的6 MV/10 MV光子束FF/FFF模式4档能量在设定好九点位置的10 cm×10 cm标准射野内进行实验。首先,借助IMF等中心夹具将Mapcheck2固定于治疗机机头,并用Mapcheck2测量相同臂架与准直器角度条件下4种光子束输出的平面剂量值;其次,用Mapcheck2测量在相同臂架角度、不同准直器角度与相同准直器角度、不同臂架角度两种条件下4种光子束的中心轴剂量值;最后,固定准直器为0°,设立两组臂架对穿射野（0°与180°,90°与270°）。拆除Mapcheck2,采用固体水和FC65-G电离室建立一个测量模体来测量4种光子束在两组等中心对穿野的剂量。运用SPSS统计软件对该实验收集到的数据进行对比分析。结果:在相同臂架与准直器角度条件下,4种光子束辐照9个点的平面剂量之间均存在明显统计学差异（P6 MV FF =0.020, P6 MV FFF=0.017, P10 MV FF =0.030, P10 MV FFF=0.016）;而不同臂架角度或不同准直器角度条件下,4种能量光子束的中心轴点剂量值均无统计学差异。在0°与180°的对穿野,4种能量光子束的输出剂量存在统计学差异（P6 MV FF =0.001, P6 MV FFF=0.002, P10 MV FF =0.003, P10 MV FFF=0.001）,而在90°与270°的对穿野无统计学差异。结论:Versa HD直线加速器拥有优良的机械等中心性能。在实际工作时,臂架和准直器的旋转,均不影响光子束的中心轴剂量的准确输出。在FF模式下,射线能量越高,受治疗床影响越小;FFF模式射线由于射线质软,能量越高,更易受到治疗床的衰减作用,在实际中应引起重视。     

Absorbed dose beam quality correction factors kappaQ for the NE2571 chamber in a 5 MV and a 10 MV photon beam

Dose to water (Dw) determination in clinical high-energy photon beams with ionization chambers calibrated in terms of absorbed dose to water has been proposed as an alternative to ionization chamber dosimetry based on air kerma calibrations. Dw in the clinical beam is derived using a kappaQ factor that scales the absorbed dose calibration factor in the reference beam to the absorbed dose calibration factor in the user beam. In the present study kappaQ values were determined for the NE2571 chamber in a 5 MV and a 10 MV high-energy photon beam generated at the 15 MeV high-intensity electron linac of the University of Gent. A set of three NE2571 chambers was calibrated relative to the Gent sealed water calorimeter both in 60Co and in the linac beam at a depth of 5 cm and a source to detector distance of 100 cm. Two high-purity chemical water systems were used in the detection vessel of the calorimeter, H2-saturated and Ar-saturated pure water, which are both supposed to give a zero heat defect. TPR20(10) and %dd(10) have been evaluated as beam quality specifiers. Simulations using the BEAM/DOSXYZ Monte Carlo system were performed to evaluate potential corrections on the measured beam qualities. The average kappaQ values measured for the three NE2571 chambers in the 5 MV and 10 MV photon beams are 0.995 +/- 0.005 and 0.979 +/- 0.005 respectively. For the three chambers used, the maximum deviation of individual kappaQ values is 0.2%. The measured beam quality specifiers %dd(10) and TPR20(10) are 67.0 and 0.705 for the 5 MV beam and 75.0 and 0.759 for the 10 MV beam. Although our beam design is very different from those used by other investigators for the measurement of kappaQ values, the agreement with their results is satisfactory showing a slightly better agreement when %dd(10) is used as the beam quality specifier.     

Validity of transition-zone dosimetry at high atomic number interfaces in megavoltage photon beams

Measurement of dose or dose perturbation factors at high atomic number interfaces are usually performed with a thin-window parallel-plate ion chamber. In a transition region, under nonequilibrium conditions, accuracy of ion chamber readings for the dose measurements has often been questioned. This paper critically analyzes the factors (stopping power ratio and charge collection) for the dose measurements at interfaces. Monte Carlo simulations were performed to investigate the secondary electron spectrum produced by photon beams and to calculate the stopping power ratios at the point of measurement. The validity of dose measurements was studied for the photon beams in the range of Co-60 gamma rays to 24-MV x rays at bone and lead interfaces with polystyrene, using thermoluminescent dosimeters, extrapolation chamber and several types of commercially available parallel-plate ion chambers. It is observed that for energies greater than 10 MV most parallel-plate chambers can be used to measure dose accurately. At lower energies, however significant differences between measured doses with different detectors were noticed. It is suggested that at high-Z interfaces and lower energies, the dose measurements should be performed with ultrathin-window parallel-plate ion chambers or extrapolation chambers.     

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。     

Large efficiency improvements in BEAMnrc using directional bremsstrahlung splitting

The introduction into the BEAMnrc code of a new variance reduction technique, called directional bremsstrahlung splitting (DBS), is described. DBS uses a combination of interaction splitting for bremsstrahlung, annihilation, Compton scattering, pair production and photoabsorption, and Russian Roulette to achieve a much better efficiency of photon beam treatment head simulations compared to the splitting techniques already available in BEAMnrc (selective bremsstrahlung splitting, SBS, and uniform bremsstrahlung splitting, UBS). In a simulated 6 MV photon beam (10 x 10 cm2 field) photon fluence efficiency in the beam using DBS is over 8 times higher than with optimized SBS and over 20 times higher than with UBS, with a similar improvement in electron fluence efficiency in the beam. Total dose efficiency in a central-axis depth-dose curve improves by a factor of 6.4 over SBS at all depths in the phantom. The performance of DBS depends on the details of the accelerator being simulated. At higher energies, the relative improvement in efficiency due to DBS decreases somewhat, but is still a factor of 3.5 improvement over SBS for total dose efficiency using DBS in a simulated 18 MV photon beam. Increasing the field size of the simulated 6 MV beam to 40 x 40 cm2 (broad beam) causes the relative efficiency improvement of DBS to decrease by a factor of approximately 1.7 but is still up to 7 times more efficient than with SBS.     

An investigation of the photon energy dependence of the EPR alanine dosimetry system

The electron paramagnetic resonance (EPR) alanine dosimetry system is based on EPR measurements of radicals formed in alanine by ionizing radiation. The system has been studied to determine its energy dependence for photons in the 10-30 MV region relative to those of 60Co and to find out if the system would be suitable for dosimetry comparisons. The irradiations were carried out at the National Research Council, Ottawa, Canada and the doses ranged from 8 to 54 Gy. The EPR measurements were performed at the University of Oslo, Norway. The ratio of the slope of the alanine reading versus dose-to-water curve for a certain linac photon beam quality and the corresponding slope for a reference 60Co gamma-radiation gives an experimental measure of the relative dose-to-water response of the EPR alanine dosimetry system. For calculating the linear regression coefficients of these alanine reading versus dose curves, the method of weighted least squares was used. This method is assumed to produce more accurate regression coefficients when applied to EPR dosimetry than the common method of standard least squares. The overall uncertainty on the ratio of slopes was between 0.5 and 0.6% for all three linac energies. The relative response for all the linac beams compared to cobalt was less than unity: by about 0.5% for the 20 and 30 MV points but by more than 1% for the 10 MV point. The given standard uncertainties negate concluding that there is any significant internal variation in the measured response as a function of beam quality between the three linac energies. Thus, we calculated the average dose response for all three energies and found that the alanine response is 0.8% (+/-0.5%) lower for high energy x-rays than for 60Co gamma-rays. This result indicates a small energy dependence in the alanine response for the high-energy photons relative to 60Co which may be significant. This result is specific to our dosimetry system (alanine with 20% polyethylene binder pressed into a particular shape) including its waterproofing sleeve of PMMA (2 mm thick); however, we expect that this result may apply to other similar detectors.     

Comparing two methods for calculating phantom scatter

Two methods are compared for calculating the field-size dependence of the phantom scatter component of dose for x-ray beams. One model sums three Gaussian distributions; the other model is a two-parameter function. With a measurement of the beam quality as input to determine parameters, both models accurately reproduce the relative phantom scatter. However, there are important differences between the models. For all beam energies, the two-parameter model characterizes the absolute phantom scatter as a function of depth and field size, while, also for all beam energies, the six-parameter Gaussian model characterizes the relative phantom scatter at a single depth of 10 cm. For small field sizes, the phantom scatter calculated from the two-parameter model agrees with Monte Carlo calculations better than the Gaussian model. In the Gaussian model, the parameters can be obtained for beam energies between 60Co and 25 MV by linear interpolation based on the measured beam quality. In the two-parameter model, and for energies above 4 MV, the parameters can be obtained using linear functions of the dose-weighted average linear attenuation coefficient, which is related to beam quality.     

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.     

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.     

Comparison of dosimetric standards of Canada and France for photons at 60Co and higher energies

We report the results of a comparison of the dosimetric standards of Canada and France for photon beams at 60Co and a few higher energies. The present primary standard of absorbed dose to water for NRC, Canada is based on measurements made with a sealed water calorimeter. The corresponding standard of the LNHB, France is based on measurements made with a graphite calorimeter at 60Co energy and transferred to absorbed dose to water for 60Co and higher-energy photon beams using both ion chambers and Fricke dosemeters as transfer instruments. To make this comparison, we used three graphite-walled NE2571 Farmer chambers. The absorbed dose to water determined by the LNHB was greater than that determined by NRC by 0.20% at 60Co energy. This difference is not significant given the uncertainties on the standards. In order to do the comparison for higher-energy photons, we interpolated the NRC data set at the beam qualities used at the LNHB. When %dd(10)x is used as the method of specifying beam quality, the determination of absorbed dose to water by the LNHB is about 0.2% greater than that determined by NRC and consistent with the results at 60Co. However, when using TPR20,10 as the beam quality specifier, the LNHB determination is greater than the NRC's determination by 0.8% and 1.2% at 12 and 20 MV respectively. This discrepancy, which systematically increases with increasing energy, eventually exceeds the uncertainties in the ratio of the standards, estimated to be 0.7%. This underscores the importance of selecting the method of specifying beam quality, either %dd(10)x or TPR20,10, at least for the 'soft' beams used by NRC in this comparison. In the case of the air kerma standards, which were also compared at 60Co energy, the LNHB determination was greater than NRC's by 0.14%, which is not significant given the uncertainties on the standards.     

Monte Carlo calculations of scatter to primary ratios for normalisation of primary and scatter dose

The separation of total absorbed dose into primary and scatter components is a commonly used technique in photon dose calculations. The primary dose component can be characterised by a measured narrow beam attenuation coefficient and a single normalisation value which establishes the relative proportion of the primary to the total dose at some reference depth and field size. The determination of this normalisation value from measured data requires an extrapolation of measured values for finite field sizes to obtain a zero field size value. We have used Monte Carlo simulations to score primary and scatter dose for photon beams of 4, 6, 10, 15 and 24 MV and report values of the scatter to primary ratio at the depth of dose maximum for the circular equivalent of a 10 cm x 10 cm field. These values have an uncertainty of less than 1% and can be used in lieu of extrapolation of measured data to establish the relative magnitude of the primary dose for a wide range of photon beam energies.     

A Monte Carlo comparison of the response of the PTW-diamond and the TL-diamond detectors in megavoltage photon beams

A detailed Monte Carlo study of the PTW-diamond solid state detector response in megavoltage photon beams (60Co gamma rays to 25 MV x rays) has been performed with the EGS4 Monte Carlo Code. The sensitive volume of the diamond detector is a disk of diameter 4.4 mm and thickness 0.40 mm. The phantom material was water and the irradiation depth was usually 3 cm but additional simulations were performed at six other depths for the 10 and 25 MV x rays. Results show that the PTW-diamond detector response per unit of absorbed dose is constant within 1% for photon beam energies ranging from 60Co gamma rays to 25 MV x rays. Accurate depth dose curves for 10 and 25 MV x-ray beams may be measured with the diamond detector since the response per unit of absorbed dose at different depths in a water phantom is also constant to within 1% for depths ranging from 3 to 25 cm and field sizes ranging from 2.5 cm by 2.5 cm to 10 cm by 10 cm. An examination of the difference between the PTW-diamond detector and the wall-less form of the detector (e.g., TLDs) revealed that there is no significant difference in their response in megavoltage photon beams. This implies that the encapsulation of the diamond dosimeter causes less than a 1.3% change in its response for these megavoltage photon beams. Analysis of the total dose deposited in the sensitive volume of the detector shows that the PTW-diamond detector behaves as an intermediate-sized cavity, not a simple Bragg-Gray cavity, since the dose contribution from photon interactions within the cavity (alpha(c)) to the total cavity dose is 8% for 25 MV x rays and increases to 42% for 60Co gamma rays.     

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.     

Dose enhancement close to platinum implants for the 4, 6, and 10 MV stereotactic radiosurgery

Three photon interaction processes, namely, the photoelectric effect, Compton effect, and pair production, can occur when materials with high atomic numbers are irradiated by the high- and low-energy bremsstrahlung photons from a linear accelerator. A dose enhancement, due to the photoelectric effect and pair production, near targets with platinum implants (with a high atomic number) in radiosurgery cannot be predicted by the XKnife radiosurgery treatment planning system. In the present work, Monte Carlo simulations using PRESTA EGS4 were employed to investigate the resulting dose enhancements from 4, 6, and 10 MV energies commonly used in the stereotactic radiosurgery system. Dose enhancements from 32% to 68% were observed close to the platinum implant for the above energies when using a 12.5 mm collimator. Comparatively higher dose enhancements were observed when using smaller collimators. It was found that this dose enhancement increased with beam energy but decreased as beam size increased.