利用百分深度剂量计算组织最大剂量比的准确性研究

目的：研究用测量的X线照射野百分深度剂量和体模散射输出因子计算组织最大剂量比的可行性。方法：用PTwmp3三维水箱分别测量Precise加速器的6MV和10MVX线的百分深度剂量、组织最大剂量比以及照射野输出因子。利用NE2570剂量仪和自制的圆柱形有机玻璃体模测量加速器准直系统散射输出因子。用VisualBasic6．0编程计算组织最大剂量比，并将组织最大剂量比的计算值和测量值进行比较。结果：组织最大剂量比和射线能量、照射野面积有关。6MV和10MVX线的组织最大剂量比的计算值和测量值的误差小于2％。结论：组织最大剂量比的计算值和测量值符合得很好，可以直接应用于吸收剂量计算。     

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。     

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。     

Monte Carlo方法计算旋转照射的剂量分布

本文应用Monte Carlo方法模拟计算在不同旋转角度照射的情况下组织的剂量分布,对所得百分深度剂量曲线和等剂量曲线分析讨论,结果表明,组织的剂量分布、最大剂量深度、治疗效果等方面都有很大的差别.     

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。     

三维治疗计划系统模拟全身照射的应用研究

目的:采用三维治疗计划系统(3D-TPS)模拟计算全身照射(TBI)的剂量分布.材料和方法:对于全身照射,设置源皮距(SSD)为380 cm,射野大小为40 cm×40 cm,光栏角度为45°,采用自制大水箱测量了直线加速器8 MV光子线水中的百分深度剂量(PDD)和离轴比(OAR).上述相同照射条件下,在3D-TPS中进行水体模的PDD和OAR的模拟计算并与之测量结果进行对比,确认3D-TPS是否能够模拟计算TBI的剂量分布.采用3D-TPS计算人形体模的TBI剂量分布,采用剂量胶片和热释光测量对计算结果进行了比较和确认.结果:对于水体模中的百分深度量和离轴比,3D-TPS的模拟计算结果与大水箱的测量结果最大误差分别为3%和6%左右.对于人形体模的模拟计算,3D-TPS的模拟计算结果与胶片和热释光的测量结果基本符合.结论:3D-TPS可以较准确地模拟计算全身照射的剂量分布.通过3D-TPS对每个特定病人制作相应补偿块,为更均匀剂量的全身照射治疗提供了可能.     

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。     

DPM蒙卡方法在放疗剂量计算中的应用研究

目的:探讨最新推出基于蒙特卡罗方法的DPM(dose planning method)程序在放疗剂量计算中的应用,研究DPM程序计算放疗剂量的准确性及其临床应用的可行性。方法:对DPM源文件编译形成四个可执行文件,使其能在Windows系统下运行。(1)通过借助蒙卡BEAMnrc程序模拟我院Varian Clinac 21EX直线加速器治疗头,得到其相空间文件,并计算出SSD=100cm处的相空间(Phase Space)数据。(2)使用BEAMDP程序对该相空间文件进行能谱分析,获取到6MV-X线能谱分布。(3)修改DPM源程序,使之能调用该能谱。(4)DPM计算出水模体内百分深度剂量并用MATLAB软件显示PDD曲线分布,与实际测量进行拟合。(5)DPM计算非均匀组织内方野剂量,相同条件下与实测量、TPS计算值进行了比较。结果:蒙卡DPM程序调用直线加速器能谱计算水模体内的PDD曲线与实测曲线的拟合完全吻合,证明了DPM程序调用能谱方法可行而且计算准确。DPM蒙卡程序在非均匀组织中的计算也是准确的。结论:DPM蒙卡方法可应用实现组织中放疗剂量计算的研究。     

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。     

全身放射治疗的剂量测量研究

目的:在全身放射治疗条件下,测量直线加速器空气中射线场均匀性,水模体内剂量分布情况,以及不同规格水模体的百分深度剂量值。方法:将加速器的源皮距(SSD)延长至450 cm,机架头旋转为90°,准直器开到最大,治疗头旋转为45°,形成菱形射野,使用剂量测量仪:PTW-UNIDOS,电离室:PTW 30001,测量Varian Clinac 2100C直线加速器的剂量值。结果与结论:加速器在空气中射线场剂量:T方向上总的平均值为5.147,绝对误差为5.8%,归一后相对误差达到;G方向上总的平均值为5.124,绝对误差为5.1%,归一后相对误差达到;此加速器的射线场均匀性可以用于全身放射治疗。水模体内剂量分布情况,在10 cm深度处,平均剂量值为8.960,归一数据中的绝对误差为;在20 cm深度处,平均剂量为6.381,从归一数据中的绝对误差为。     

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.     

Determination of zero-field size percent depth doses and tissue maximum ratios for stereotactic radiosurgery and IMRT dosimetry: comparison between experimental measurements and Monte Carlo simulation

In this study, zero-field percent depth dose (PDD) and tissue maximum ratio (TMR) for 6 MV x rays have been determined by extrapolation from dosimetric measurements over the field size range 1 x 1-10 x 10 cm2. The key to small field dosimetry is the selection of a proper dosimeter for the measurements, as well as the alignment of the detector with the central axis (CAX) of beam. The measured PDD results are compared with those obtained from Monte Carlo (MC) simulation to examine the consistency and integrity of the measured data from which the zero-field PDD is extrapolated. Of the six most commonly used dosimeters in the clinic, the stereotactic diode field detector (SFD), the PTW Pinpoint, and the Exradin A14 are the most consistent and produce results within 2% of each other over the entire field size range 1 x 1-40 x 40 cm2. Although the diamond detector has the smallest sensitive volume, it is the least stable and tends to disagree with all other dosimeters by more than 10%. The zero-field PDD data extrapolated from larger field measurements obtained with the SFD are in good agreement with the MC results. The extrapolated and MC data agree within 2.5% over the clinical depth range (dmax-30 cm), when the MC data for the zero field are derived from a 1 X 1 cm2 field simulation using a miniphantom (1 x 1 x 48 cm3). The agreement between the measured PDD and the MC data based on a full phantom (48 x 48 x 48 cm3) simulation is fairly good within 1% at shallow depths to approximately 5% at 30 cm. Our results seem to indicate that zero-field TMR can be accurately calculated from PDD measurements with a proper choice of detector and a careful alignment of detector axis with the CAX.     

Modelling of electron contamination in clinical photon beams for Monte Carlo dose calculation

The purpose of this work is to model electron contamination in clinical photon beams and to commission the source model using measured data for Monte Carlo treatment planning. In this work, a planar source is used to represent the contaminant electrons at a plane above the upper jaws. The source size depends on the dimensions of the field size at the isocentre. The energy spectra of the contaminant electrons are predetermined using Monte Carlo simulations for photon beams from different clinical accelerators. A 'random creep' method is employed to derive the weight of the electron contamination source by matching Monte Carlo calculated monoenergetic photon and electron percent depth-dose (PDD) curves with measured PDD curves. We have integrated this electron contamination source into a previously developed multiple source model and validated the model for photon beams from Siemens PRIMUS accelerators. The EGS4 based Monte Carlo user code BEAM and MCSIM were used for linac head sinulation and dose calculation. The Monte Carlo calculated dose distributions were compared with measured data. Our results showed good agreement (less than 2% or 2 mm) for 6, 10 and 18 MV photon beams.     

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.     

A differential method for inhomogeneity correction on dose in a photon beam

For a uniform slab of inhomogeneity in a supervoltage beam, correction factors can be calculated from the Batho equation. In this report, we present a method for calculating the effect of an annular inhomogeneity, concentric about the beam axis, upon the dose at a point on the axis and below the annulus. A derivation of the equation required in the calculation for supervoltage radiation is given. Results from measurements made in 60Co beams for polystyrene foam, cedar, and aluminum annuli, all having 3.0 x 2.0 cm2 in cross section but with different inside diameters, are compared with correction values calculated by the method. For situations where the annulus is just submerged in the phantom, measured and calculated values are in good agreement. For a general situation, two calculation types are proposed and the data show that in general the measured scatter perturbation lies between the calculated values of the two types. Application of our technique predicts a sign reversal in the scatter perturbation due to an inhomogeneity. This reversal has previously been observed and reported and is also demonstrated in our measurements.     

Dose computations for asymmetric fields defined by independent jaws

Asymmetric fields defined by independent jaws can be used to split a beam or to match adjacent fields. We have extended a method originally developed for symmetric fields to calculate the dose for asymmetric fields. The dose to a point is computed as the product of the tissue maximum ratio (TMR), the off center ratio (OCR), and the inverse square factor. The TMR is computed from the measured central axis depth doses for symmetric fields. The OCR is obtained by multiplying the primary OCR (POCR) and the boundary factors (BF's) for the four jaws. The POCR's and BF's were derived from measured beam profiles, which include the effect of off-axis beam quality variations. Using this method, the beam profiles and isodose distributions for asymmetric fields of a 6-MV accelerator were calculated and compared with the measured data. The agreement is within experimental errors both in the penumbra region and along the central ray of the asymmetric field.