光子射野剂量计算参数研究

目的：介绍医用加速器常规光子射线的机器数据测量方法及剂量计算模型中基本参数的计算过程。以百分深度剂量与散射因子为基础数据,根据原散射线模型通过测量数据推导出原射线组织最大剂量比、散射最大剂量比、原射线在水中线性衰减系数、能量注量等,为进一步还原射野在水模体中的剂量分布提供方法与理论。方法：用Blue Phantom三维水箱在医科达Synergy加速器上测量6MV光子线的百分深度剂量、离轴比剂量、总散射因子、准直器散射因子,先从测量的百分深度剂量曲线中按照原散射模型剥离出原射线百分深度剂量,然后在Matlab软件中拟合处理测量的散射因子数据,外推出零野的模体散射因子,从而按照给定公式计算出组织最大剂量比、散射最大剂量比。按照离轴比剂量,利用平方反比规律推出最大开野在模体表面的能量注量。结果：计算出准直器散射因子、总散射因子的拟合公式,外推零野模体散射因子（s。）、根据原射线的百分深度剂量曲线计算出原射线在水中线性衰减系数,组织最大剂量比（TMR）、散射最大剂量比（SMR）、以及射野能量注量分布（Fluence Matrix）。结论：这些基本参数是剂量计算建模的关键,也是进一步研究各种剂量计算模型的基础。     

CT值对剂量计算结果的影响

目的：通过CT值对剂量计算结果影响的研究。分析不做组织均匀性校正剂量计算在人体不同部位带来的剂量误差。材料和方法：在Varianeclipse治疗计划系统（版本7．3．10）中构建-30cmx30crux30cm虚拟模体,计算设置”SSD=100cm。照射野大小10cmxl0cm,给予处方剂量200 cGy。分别计算在模体不同深度赋予模体不同CT值的机器输出剂量。结果：深度越大,计算结果随CT变化越大;同一深度。负CT值比正CT值影响要大。结论：在实际病人照射中对于肺部肿瘤或者纵隔穿过肺组织的肿瘤．用6MV或15MVX射线实施照射射线穿透肺组织大于3cm,如果不做组织不均匀性校正,6MVX射线计算结果会带来大于5％的误差,15MVX射线计算结果会带来大于2％的误差,穿透肺组织越深,误差越大;对于骨组织,因为骨组织厚度一般为1em-3em,如果不做组织不均匀性校正,计算结果仅会带来大约1％的误差;对于其它组织实际照射中,如果不做组织不均匀性校正计算结果也仅会带来1％一2％的误差。     

DPM蒙特卡罗剂量计算算法在均匀组织和非均匀组织剂量精确性的验证

验证DPM蒙特卡罗剂量计算算法预测均匀组织和非均匀组织剂量的精确性。DPM分别计算:①6 MeV单能光子3cm×3cm照射野和Varian 60℃加速器源水模体百分深度剂量曲线和10cm深度处离轴比;②6 MeV单能光子3cm×3cm、10cm×10cm照射野分别在水(6cm)/肺(6cm)/水(8cm)、水(6cm)/骨骼(2cm)/水(12cm)非均匀组织的百分深度剂量曲线;③6MeV单能光子6cm×6cm照射野人体头部和腹部组织在射野内和射野外的百分深度剂量曲线。比较DPM计算值与DOSXYZnrc/EGSnrc系统在相同条件下的计算值。结果显示二者计算值在水模中的误差在±3%以内,在非均匀组织中,除了个别点,误差都在±3%以内。DPM能够精确计算均匀组织和非均匀组织剂量。     

半导体和电离室探头在直线加速器数据测量中的比较与分析

目的：比较分析半导体探头和电离室探头在三维水箱测量中的差异，为能够提高数据测量精度从而实现治疗计划系统建立准确的计算模型提供依据：方法：在加速器8MV光子线下，使用0．13cm^3的指形电离室和半导体探头在三维水箱中分别测量照射野1cm×lcm，2cm×2cm，3cm×3cm，4cm×4cm，5cm×5cm，6cm×6cm，8cm×8cm，10cm×l0cm的总散射因子、百分深度剂量曲线、离轴比曲线，对测量结果进行比较和分析；结果：对于总散射因子，在较大照射野测量时结果一致，在小野测量时存在差异，1cm×lcm照射野的两者测量结果偏差15．32％；对于百分深度曲线，在建成区差异最大，各照射野的在水面处的测量结果均偏差10％以上：对于离轴比曲线，在半影区存在显著差异．半导体探头在最大剂量点深度测量的射野大小均小明显小于电离室测量的结果。结论：总散射因子，小照射野测量时建议使用半导体探头或者较小体积的电离室；百分深度剂量曲线，建议使用电离室探头；离轴比曲线，使用半导体探头可测量到较好的射野半影区。     

关于百分深度剂量计算组织最大剂量比的处理方法及数据对比

目的:验证百分深度剂量计算组织最大剂量比的计算方法,提供可靠的测试数据和计算数据,为该计算方法在临床放射治疗上的应用提供实验上的数据依据。方法:通过百分深度剂量、准直器散射因子、总散射因子计算组织最大剂量比。将计算的组织最大剂量比分别与测试值和RFAplus计算值进行比较,使用误差分布函数比较计算值的准确度及其分布。结果:采用两个例子分别模拟计算,第一个例子将实际测量的组织最大剂量比与模拟计算值比较,第二个例子将RFAplus计算的组织最大剂量比与模拟计算值比较,偏差范围在-2.0%～ 2.0%之间,表明模拟计算值与测量值、RFAplus计算值符合较好。结论:利用百分深度剂量计算组织最大剂量比是一个切实有效的方法,可以在临床上推广运用,免去测试组织最大剂量比的烦琐过程。     

楔形野剂量计算中的误差分析和修正

目的研究楔形野剂量计算中的误差,并探讨解决方法.材料与方法在10MV和6MVX线条件下,用NEFarmer25710.6cc指形电离室和三维水箱在水模中测出平野和楔形野的各种参数,并用二种方法计算剂量,结果与实侧值比较.结果实测数据显示Pdd和Scp在平野和楔形野情况下存在差异.楔形因子因此随深度而变化,变化程度受射线能量、楔形板规格影响.与实测值比较,用传统方法计算楔形野剂量的结果存在误差,误差大小与能量、野面积、深度有关.6MVX线、15×15野、20cm深度处的计算误差可达11%.而用改进的方法进行计算,可将误差控制在1%以内.结论由于忽略了Pdd等物理参数在楔形野条件下的变化,用传统方法计算楔形野剂量存在误差.为保证临床剂量计算的准确性,应在计算公式中加入修正因子.     

医用直线加速器的X射线剂量的实验研究

本文介绍了用于测量电子直线加速器剂量的电离室和胶片探测器。用这些探测器测量了电子直线加速器在方型和圆形野情况下的几种重要的剂量分布,如百分深度剂量、组织最大比、离轴比等。文中介绍了用这些探测器测量这些参数时的特点。     

应用辐射直接显色剂量胶片测量高能光子线的表面和建成区剂量

目的：准确测量高能光子射线剂量建成区的剂量分布，评估三维水箱扫描深度剂量曲线在表浅部位的误差。方法：使用辐射直接显色胶片（EBT胶片）测量加速器6MV光子线由体模表面到最大剂量深度区间的建成剂量分布，并与传统的电离室和半导体探头三维水箱扫描百分深度剂量曲线在该区间的剂量分布进行比较。结果：在接近最大剂量深度的区间（0．6cm-Dmax），EBT胶片与三维水箱扫描测量结果非常接近，差别小于2％；在电离室和半导体探头的有效测量深度至0．6cm深度区间，对不同射野大小，EBT胶片测量值大于两种三维水箱测量值5％～10％；在小于电离室和半导体探头的有效测量深度的区间，EBT胶片的测量值与水箱扫描结果比较差别最大分别达到22.7％（半导体探头）和49.3％（电离室）。结论：EBT胶片可以用于准确测量表面和建成区剂量分布，三维水箱扫描得到的PDD曲线应该进行建成区修正。     

组织最大剂量比的数学解析形式

作者根据ML—20MDX直线加速器的6MV、10MVX射线和文献上发表的4MV、8MVX射线的TMR值,用泰勒级数展开的数学方法进行了拟合,提出了一个较好的数学解析式。 TMR=k(d) M(d)·/n(W 2)对4×4CM~2到30×30CM~2照射野范围,深度小于20CM时,TMR计算值与测量值的拟     

医用直线加速器输出剂量稳定性分析

目的：分析医用直线加速器输出剂量稳定性及其影响因素。方法：采用SPSS15.0统计分析软件,统计2009年每日治疗病人前监测6 MV、15 MVX射线,和9 MeV、12 MeV电子线输出剂量数据,分析医用直线加速器不同能量输出剂量的稳定性及其影响因素,提出加速器输出剂量质量保证的相关措施。结果：4档能量中的3档能量（9 MeV,12 MeV,15MV）输出剂量K-S检测双尾渐进概率P值分别为0.428、0.933、0.355均大于显著性水平0.05,符合正态分布。由于加速器微波源输出不稳定,6 MV输出剂量1月到3月,从98.4%连续不断漂移上升到102.5%。6 MV K-S检测双尾渐进概率P值是0.012小于显著性水平0.05,不符合正态分布。结论：直线加速器输出剂量的稳定性是肿瘤放射治疗治疗质量保证的重要方面。每日治疗肿瘤病人前监测和直线加速器输出剂量,分析直线加速器输出剂量的稳定性,有助于降低加速器系统误差,提高患者治疗剂量的精度。     

Dose calculation for asymmetric photon fields with independent jaws and multileaf collimators

We have developed a simple method for dose calculation in dual asymmetric open and irregular fields with four independent jaws and multileaf collimators. Our calculation method extends the scatter correction method of Kwa et al. [Med. Phys. 21, 1599-1604 (1994)] based on the principle of Day's equivalent-field calculation. The scatter correction factor was determined by the ratio of the derived doses of a smaller asymmetric open field or irregular field to a larger symmetric field. The algorithm with the scatter correction method can be calculated from output factors, tissue maximum ratios, and off-axis ratios for conventional symmetric fields. The doses calculated by this method were compared with the measured doses for various asymmetric open and irregular fields. The agreement between the calculated and measured doses for 4 and 10 MV photon beams was within 0.5% at the geometric center of the asymmetric open fields. For the asymmetric irregular fields with the same geometrical center, agreement within 1% was found in most cases.     

Conversion of measured percentage depth dose to tissue maximum ratio values in stereotactic radiotherapy

For many treatment planning systems tissue maximum ratios (TMR) are required as input. These tissue maximum ratios can be measured with a 3D computer-controlled water phantom; however, a TMR measurement option is not always available on such a system. Alternatively TMR values can be measured 'manually' by lowering the detector and raising the water phantom with the same distance, but this makes TMR measurements time consuming. Therefore we have derived TMR values from percentage depth dose (PDD) curves. Existing conversion methods express TMR values in terms of PDD, phantom scatter factor (Sp), and inverse square law. For stereotactic treatments circular fields ranging from 5-50 mm (19 cones) are used with the treatment planning system XKnife (Radionics). The calculation of TMR curves for this range is not possible with existing methods. This is because PDD curves of field sizes smaller than 5 mm (smallest cone size) are needed, but these cones are not provided. Besides, for field sizes smaller than 40 mm, the phantom scatter factor is difficult to determine and will introduce significant errors. To overcome these uncertainties, an alternative method has been developed to obtain TMR values from PDD data, where absolute doses are expressed in terms of PDD, total scatter factor and inverse square law. For each depth, the dose as a function of field size is fitted to a double exponential function. Then the TMR is calculated by taking the ratio of this function at the depth of interest and the reference depth, for the correct field size. For all 19 cones the total scatter factor and PDDs have been measured with a shielded diode in water for a 6 MV photon beam. Calculated TMR curves are compared with TMR values measured with a diode. The agreement is within 2%. Therefore this relatively simple conversion method meets the required accuracy for daily dose calculation in stereotactic radiotherapy. In principle this method could also be applied for other small field sizes such as those formed with a mini multileaf collimator.     

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.     

Establishing an Initial Electron Beam Model with Monte Carlo Simulation for a Single 6 MV X-ray Medical Linac Based on Particle Dynamics Characteristics

Objective：In this study, we try to establish an initial electron beam model by combining Monte Carlo simulation method with particle dynamic calculation （TRSV） for the single 6 MV X-ray accelerating waveguide of BJ-6 medical linac. Methods and Materials ： 1. We adapted the treatment head configuration of BJ-6 medical linac made by Beijing Medical Equipment Institute （BMEI） as the radiation system for this study. 2. Use particle dynamics calculation code called TRSV to drive out the initial electron beam parameters of the energy spectrum, the spatial intensity distribution, and the beam incidence angle. 3. Analyze the 6 MV X-ray beam characteristics of PDDc , OARc in a water phantom by using Monte Carlo simulation （ BEAMnrc, DOSXYZnrc） for a preset of the initial electron beam parameters which have been determined by TRSV, do the comparisons of the measured results of PDDm, OARm in a real water phantom, and then use the deviations of calculated and measured results to slightly modify the initial electron beam model back and forth until the deviations meet the error less than 2%. Results：The deviations between the Monte Carlo simulation results of percentage depth doses at PDDc and off-axis ratios OARc and the measured results of PDDm and OARm in a water phantom were within 2%. Conclusion：When doing the Monte Carlo simulation to determine the parameters of an initial electron beam for a particular medical linac like B J-6, modifying some parameters based on the particle dynamics calculation code would give some more reasonable and more acceptable results.     

Using Monte Carlo simulations to commission photon beam output factors--a feasibility study

This study investigates the feasibility of using Monte Carlo methods to assist the commissioning of photon beam output factors from a medical accelerator. The Monte Carlo code, BEAMnrc, was used to model 6 MV and 18 MV photon beams from a Varian linear accelerator. When excellent agreements were obtained between the Monte Carlo simulated and measured dose distributions in a water phantom, the entire geometry including the accelerator head and the water phantom was simulated to calculate the relative output factors. Simulated output factors were compared with measured data, which consist of a typical commission dataset for the output factors. The measurements were done using an ionization chamber in a water phantom at a depth of 10 cm with a source-detector distance of 100 cm. Square fields and rectangular fields with widths and lengths ranging from 4 cm to 40 cm were studied. The result shows a very good agreement (< 1.5%) between the Monte Carlo calculated and the measured relative output factors for a typical commissioning dataset. The Monte Carlo calculated backscatter factors to the beam monitor chamber agree well with measured data in the literature. Monte Carlo simulations have also been shown to be able to accurately predict the collimator exchange effect and its component for rectangular fields. The information obtained is also useful to develop an algorithm for accurate beam modelling. This investigation indicates that Monte Carlo methods can be used to assist commissioning of output factors for photon beams.     

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.     

Build-up and surface dose measurements on phantoms using micro-MOSFET in 6 and 10 MV x-ray beams and comparisons with Monte Carlo calculations

This work is intended to investigate the application and accuracy of micro-MOSFET for superficial dose measurement under clinically used MV x-ray beams. Dose response of micro-MOSFET in the build-up region and on surface under MV x-ray beams were measured and compared to Monte Carlo calculations. First, percentage-depth-doses were measured with micro-MOSFET under 6 and 10 MV beams of normal incidence onto a flat solid water phantom. Micro-MOSFET data were compared with the measurements from a parallel plate ionization chamber and Monte Carlo dose calculation in the build-up region. Then, percentage-depth-doses were measured for oblique beams at 0 degrees-80 degrees onto the flat solid water phantom with micro-MOSFET placed at depths of 2 cm, 1 cm, and 2 mm below the surface. Measurements were compared to Monte Carlo calculations under these settings. Finally, measurements were performed with micro-MOSFET embedded in the first 1 mm layer of bolus placed on a flat phantom and a curved phantom of semi-cylindrical shape. Results were compared to superficial dose calculated from Monte Carlo for a 2 mm thin layer that extends from the surface to a depth of 2 mm. Results were (1) Comparison of measurements with MC calculation in the build-up region showed that micro-MOSFET has a water-equivalence thickness (WET) of 0.87 mm for 6 MV beam and 0.99 mm for 10 MV beam from the flat side, and a WET of 0.72 mm for 6 MV beam and 0.76 mm for 10 MV beam from the epoxy side. (2) For normal beam incidences, percentage depth dose agree within 3%-5% among micro-MOSFET measurements, parallel-plate ionization chamber measurements, and MC calculations. (3) For oblique incidence on the flat phantom with micro-MOSFET placed at depths of 2 cm, 1 cm, and 2 mm, measurements were consistent with MC calculations within a typical uncertainty of 3%-5%. (4) For oblique incidence on the flat phantom and a curved-surface phantom, measurements with micro-MOSFET placed at 1.0 mm agrees with the MC calculation within 6%, including uncertainties of micro-MOSFET measurements of 2%-3% (1 standard deviation), MOSFET angular dependence of 3.0%-3.5%, and 1%-2% systematical error due to phantom setup geometry asymmetry. Micro-MOSFET can be used for skin dose measurements in 6 and 10 MV beams with an estimated accuracy of +/- 6%.     

A comparison of calculated and measured data for total body irradiation by 10 MV x-rays

This paper describes the rationale for using computed rather than measured data as a reference in the dosimetry of total body irradiation. The proof of this statement rests on a comparison of measured dosimetric values for large fields at extended distances with those derived by simple recalculation from the data measured at the isocentre. The depth doses and dose rates were experimentally obtained for 10 MV x-rays at distances of 100, 200 and 300 cm for collimated square fields with sides ranging from 5 to 30 cm. Phantoms of different volumes and shapes, including a RANDO phantom, and a large (50 cm x 50 cm x 50 cm), intermediate (25 cm x 25 cm x 25 cm), and 'mini-phantom' (build-up cap, 4.6 cm diameter) were used. Comparison of the measured and computed data for the largest collimated field shows that (i) calculated dose rates do not differ by more than +1% from the measured data, phantom size and shape having no effect on the results, (ii) depth doses measured in a patient-like RANDO phantom are a maximum of 2% higher than the calculated data but are also 2% lower than the depth doses measured in a standard water tank. For the radiation fields and treatment distances commonly employed in total body irradiation, we conclude that the calculated data can serve as reference values for dosimetry because they have the same inherent uncertainty as the data measured in non-patient-like phantoms.     

Measurement of in-air output ratios using different miniphantom materials

In-air output ratios, S(c), were measured using miniphantoms made of PMMA (thickness 2.4-24 g cm(-2)), graphite (1.8-26.5 g cm(-2)), copper (1.6-23.3 g cm(-2)) and lead (2.3-21.6 g cm(-2)), for collimator settings of 3 x 3 to 40 x 40 cm(2), and x-ray energies of 6 MV and 15 MV, respectively. The effects of the miniphantom on S(c) were quantified as correction factors as functions of collimator setting, material types and miniphantom thickness for each photon energy to correct the measured values. For miniphantoms with sufficient thickness to eliminate electron disequilibrium, the total correction factors can be expressed as multiplications of three factors: the attenuation correction factor, the mass energy absorption correction factor and the phantom scatter correction factor. This formalism implies that the collimator setting dependence of the correction factor is mainly caused by the energy spectrum shift. The narrow-beam attenuation coefficients in various phantom materials for different collimator settings were determined in narrow-beam geometries using a specially constructed collimator mounted on the tray holder of the accelerator. We have determined that the maximum total correction factor is approximately 1.01. For miniphantoms made of PMMA, graphite, copper and lead, at the miniphantom thickness of 10 g cm(-2), the maximum total correction factors are 1.002, 1.003, 1.005, 1.007, and 1.002, 1.003, 1.008 and 1.009 for 6 MV and 15 MV, respectively.