数值方法分析电子线笔形束模型的能量展宽函数

目的:探讨用离轴比曲线分析电子束照射野笔形束模型能量展宽函数的方法.方法:用PTW mp3三维水箱测量Synergy加速器所有电子束能量、限光筒、空气间隙在不同深度的射野离轴比曲线.用数值分析方法对射野离轴比曲线进行分析,得到电子线照射野笔形束模型能量展宽函数σp(z)随电子束标称能量、限光筒大小和限光筒底端面到体模表面空气间隙变化的规律.将计算得到的σp(z)输入到PLATO治疗计划系统,计算吸收剂量,并与相同条件下用0.6 cc电离室剂量仪测量的结果进行比较.结果:能量展宽σp(z)随深度增加而变大,接近电子最大射程末端,很快减小,呈液滴状分布.能量展宽和电子的标称能量以及限光简大小有关,这主要是电子在体模中的单次和多次散射作用引起的.能量展宽随限光筒低端面到体模表面的空气间隙线性变化.标准条件下吸收剂量的计算值和测量值很接近,最大误差小于±5%.结论:电子束照射野笔形束模型充分考虑电子在体模内的作用特点和过程,是比较好的计算模型.用射野离轴比数据分析电子束照射野笔形束模型的特征参数.结果准确可靠.     

高能X线和电子束吸收剂量的测量与计算

目的：对医用直线加速器高能X线和电子束的吸收剂量进行测量与计算，保证输出剂量的准确性。方法：以水模体作为介质，用IAEA277报告及剂量学原理进行吸收剂量的测量与计算。结果：在有效测量点的吸收剂量值与标准计算值之间的误差在0．17％～0．54％之间。小于国家标准2．0％的要求。结论：高能X线与电子束的吸收剂量测量结果能够满足临床剂量学的要求。     

电子束环形限光筒剂量学特性的测量

目的:研究一种新型不锈钢壁环形限光筒的剂量学特性.方法:用PTW mp3三维水箱在标称源皮距下测量环形限光筒的百分深度剂量和照射野离轴比,用PTWUNIODS剂量仪和0.6 CC电离室测量射野输出因子、有效源皮距和X线污染,并将测量结果与6 cm×6 cm限光筒中制作的相同孔径的圆形铅块做比较.结果:环形限光筒电子线照射野的百分深度剂量、离轴比和有效源皮距与电子束能量、限光筒型号密切相关.有效射程和治疗深度随限光筒孔径增大而增大.限光简直径越小,电子线能量越大,表面剂量越高.环形限光筒的输出因子和相同孔径的圆形铅块不同,随能量变化较大.x线污染比较小.结论:环形限光筒的剂量学特性能满足临床要求,对于小肿瘤具有很好的适形效果,适合表浅的淋巴结预防照射、口腔粘膜肿瘤和某些管腔肿瘤治疗,也可以用于术中照射.     

等效方野剂量问题的探讨

在肿瘤放疗计划的设计过程中,经常会遇到矩形照射野,我们总把它们等效为方野来进行计算、等方野边长、即可查表,又可由公式L=ZWH/(W+H)来获取(L、W、H、分别为方野的边长、矩野的长和宽)文章通过对矩野及其等效方野在吸收剂量和等剂量曲线方面的实际测试,认为在吸收剂量方面,矩野和其等效方野是等效的,而它们在等剂量曲线方面,尽管很相近却不能等效,前者在临床测置中给调机绘图带来极大的方便、而后者告知我似。在使用等剂量曲线时,都必须实际测试。     

高能电子束和光子束混合治疗的临床应用

目的:利用MM50高能电磁扫描电子束的剂量学特性,进行电子束与光子束混合治疗在临床上的应用研究。探讨和评价在胸部肿瘤治疗中靶区和肺及脊髓的剂量分布的改善情况。方法:首先使用绝对剂量仪和三维水箱等设备对MM50高能电子束和光子束物理及剂量特性进行深入的测量,并对数据进行各种分析和处理。然后使用计划系统布野并获得病人的剂量分布,对靶区和重要器官的受量进行比较分析。结果:采用电子线/光子线混合照射的治疗方法与传统的对穿野治疗相比,靶区和重要器官如肺和脊髓的吸收剂量分布得到很大改善。结论:MM50高能电子束和光子束混合治疗在临床上有较高的应用价值。这种技术对纵膈等胸部肿瘤治疗时,可以大大减轻肺部的受量,因而降低放射性肺炎的发生率。     

电子束窄条野宽度对有效治疗深度的影响

目的：探讨电子束窄条野宽度对治疗深度的影响。方法：用辐射野扫描系统对Varian 2300C／D直线加速器多种能量电子束窄条野进行中心轴深度剂量扫描，分析R85参数与标准方野的偏差。结果：（1）所有窄条野的R85值均小于相应的标准方野，而相同能量下，宽度为2cm的窄条野明显大于宽度为3cm的窄条野。（2）电子束能吊对窄条野有一定影响，表现为同一窄条野R85值与标准方野的偏差，随着能帚的增加而增大。结论：窄条野的窄边宽度和射线的能量，对有效治疗深度有一定影响，临床在进行射野设计、能量选择和剂景计算时应予以考虑。     

自制术中放疗限光筒装置的物理学特点

目的：分析自制术中放疗限光筒剂量特点,材料与方法：不同则电子线限筒是用５ｍｍ厚有机玻璃制作而成。剂量仪使用法码２５７０型,电离室用ＮＥ２５７１,０．６ＣＣ,水模中测量射野因子,三维扫描仪在水模中实测射束中心轴百分深度量、ＯＡＲ、均匀性等,水模为５０ｃｍ＊５０ｃｍ＊５０ｃｍ。结果：当高于１２ＭｅＶ电子束时,表面剂量均超过９０９％,筒径从１１－６ｃｍ时表面剂量呈下降趋势,小于６ｃｍ时有升高迹象,导致剂     

正交电子束和X射线照射野衔接的剂量学分析

正交电子野和光子野衔接区域,一定会有剂量热点和冷点出现,剂量分布不均匀程度与治疗机的物理参数直接相关.本文通过测量了Elekta Precise 治疗机和Elekta Synergy 治疗机X射线和电子线的部分剂量学参数,对正交电子束和X(γ)射线照射野的衔接区域内的剂量分布的不均匀程度进行了定量分析,提出用扩展光子野半影的方法来降低剂量分布的不均匀程度,比较了不同治疗机条件下衔接区域内的剂量分布.结果表明,无论是在未扩展光子野半影的情况下,还是在扩展了光子野半影的情况下,与使用Elekta Precise 治疗机相比,使用光子射野半影较小的Elekta Synergy 治疗机,电子野与光子野衔接区域内的剂量不均匀程度更强.     

高能电子线放射治疗的剂量学参数测量与分析

目的：对高能电子线总输出因子、百分深度剂量、深度剂量分布的剂量学参数进行测量并分析讨论。方法：在Varian23EX直线加速器上,利用9606剂量测量仪和0．6cc指型电离室测量不同能量、不同限光筒及不同射野下的输出剂量并作归一,得到我们所要的剂量学参数,然后分析数据。结果：总输出因子在不同能量下与正方形射野边长的关系可满足等式：y=a·e^bx＋c·e^dx。水模体百分剂量分布中,6MeV电子线各限光筒的90％、85％等剂量深度基本不变,9MeV-15MeV下90％、85％等剂量深度随着限光筒尺寸增大而变深。对于水模体的深度剂量分布情况,6MeV和12MeV能量的10cmx10cm、15cmxl5cm限光筒均整区内对称点的最大相对剂量差分别都为0．04％、O．03％。结论：通过测量掌握实际照射中的剂量学特点．对于电子线剂量的准确计算以及临床计划制定具有很大的参考价值。     

X线照射野笔形束模型及其特征参数的测量分析

本文介绍了Ｘ线照射野笔形束模型的理论。用测量数据得到了笔形束模型的特征参数。对Ｘ线照射野的主要剂量学参数的测量值和计算值进行了比较，结果发现它们是一致的。     

Calculation of electron beam dose distributions for arbitrarily shaped fields

A method for the calculation of absorbed dose distributions of arbitrarily shaped electron beams is presented. Isodose distributions and output factors of treatment fields can be predicted with good accuracy, without the need for any dose measurement in the actual field. A Gaussian pencil beam model is employed with two different pencil beams for each electron beam energy. The values of the parameters of the pencil beam dose distributions are determined from a set of measurements of broad beam distributions; in this way the influence of electrons scattered by the applicator walls is taken into account. The dose distribution of electrons scattered from high atomic number metal frames, which define the treatment field contour at the skin, is calculated separately and added. This calculation is based on experimentally derived data. The method has been tested for beams with 6, 10, 14 and 20 MeV electron energy. The distance between calculated and measured isodose lines with values between 10 and 90% is under 0.3 cm. The difference between calculated and measured output factors does not exceed 2%.     

A model to determine the initial phase space of a clinical electron beam from measured beam data

Advanced electron beam dose calculation models for radiation oncology require as input an initial phase space (IPS) that describes a clinical electron beam. The IPS is a distribution in position, energy and direction of electrons and photons in a plane in front of the patient. A method is presented to derive the IPS of a clinical electron beam from a limited set of measured beam data. The electron beam is modelled by a sum of four beam components: a main diverging beam, applicator edge scatter, applicator transmission and a second diverging beam. The two diverging beam components are described by weighted sums of monoenergetic diverging electron and photon beams. The weight factors of these monoenergetic beams are determined by the method of simulated annealing such that a best fit is obtained with depth-dose curves measured for several field sizes at two source-surface distances. The resulting IPSs are applied by the phase-space evolution electron beam dose calculation model to calculate absolute 3D dose distributions. The accuracy of the calculated results is in general within 1.5% or 1.5 mm; worst cases show differences of up to 3% or 3 mm. The method presented here to describe clinical electron beams yields accurate results, requires only a limited set of measurements and might be considered as an alternative to the use of Monte Carlo methods to generate full initial phase spaces.     

Measurement of absorbed dose with a bone-equivalent extrapolation chamber

A hybrid phantom-embedded extrapolation chamber (PEEC) made of Solid Water and bone-equivalent material was used for determining absorbed dose in a bone-equivalent phantom irradiated with clinical radiation beams (cobalt-60 gamma rays; 6 and 18 MV x rays; and 9 and 15 MeV electrons). The dose was determined with the Spencer-Attix cavity theory, using ionization gradient measurements and an indirect determination of the chamber air-mass through measurements of chamber capacitance. The collected charge was corrected for ionic recombination and diffusion in the chamber air volume following the standard two-voltage technique. Due to the hybrid chamber design, correction factors accounting for scatter deficit and electrode composition were determined and applied in the dose equation to obtain absorbed dose in bone for the equivalent homogeneous bone phantom. Correction factors for graphite electrodes were calculated with Monte Carlo techniques and the calculated results were verified through relative air cavity dose measurements for three different polarizing electrode materials: graphite, steel, and brass in conjunction with a graphite collecting electrode. Scatter deficit, due mainly to loss of lateral scatter in the hybrid chamber, reduces the dose to the air cavity in the hybrid PEEC in comparison with full bone PEEC by 0.7% to approximately 2% depending on beam quality and energy. In megavoltage photon and electron beams, graphite electrodes do not affect the dose measurement in the Solid Water PEEC but decrease the cavity dose by up to 5% in the bone-equivalent PEEC even for very thin graphite electrodes (<0.0025 cm). In conjunction with appropriate correction factors determined with Monte Carlo techniques, the uncalibrated hybrid PEEC can be used for measuring absorbed dose in bone material to within 2% for high-energy photon and electron beams.     

The use of an extra-focal electron source to model collimator-scattered electrons using the pencil-beam redefinition algorithm

Currently, the pencil-beam redefinition algorithm (PBRA) utilizes a single electron source to model clinical electron beams. In the single-source model, the electrons appear to originate from a virtual source located near the scattering foils. Although this approach may be acceptable for most treatment machines, previous studies have shown dose differences as high as 8% relative to the given dose for small fields for some machines such as the Varian Clinac 1800. In such machines collimation-scattered electrons originating from the photon jaws and the applicator give rise to extra-focal electron sources. In this study, we examined the impact of modeling an additional electron source to better account for the collimator-scattered electrons. The desired dose calculation accuracy in water throughout the dose distribution is 3% or better relative to the given dose. We present here a methodology for determining the electron-source parameters for the dual-source model using a minimal set of data, that is, two central-axis depth-dose curves and two off-axis profiles. A Varian Clinac 1800 accelerator was modeled for beam energies of 20 and 9 MeV and applicator sizes of 15 x 15 and 6 x 6 cm2. The improvement in the accuracy of PBRA-calculated dose, evaluated using measured two-dimensional dose distributions in water, was characterized using the figure of merit, FA3%, which represents the fractional area containing dose differences greater than 3%. For the 15 x 15 cm2 field the evaluation was restricted to the penumbral region, and for the 6 x 6 cm2 field the central region of the beam was included as it was impacted by the penumbra. The greatest improvement in dose accuracy was for the 6 x 6 cm2 applicator. At 9 MeV, FA3% decreased from 15% to 0% at 100 cm SSD and from 34% to 4% at 110 cm SSD. At 20 MeV, FA3% decreased from 17% to 2% at 100 cm SSD and from 41% to 10% at 110 cm SSD. In the penumbra of the 15 x 15 cm2 applicator, the improvement was less, but still significant. At 9 MeV, FA3% changed from 11% to 1% at 100 cm SSD and from 10% to 12% at 110 cm SSD. At 20 MeV, FA3% decreased from 12% to 8% at 100 cm SSD and from 14% to 5% at 110 cm SSD. Results demonstrate that use of a dual-source beam model can provide significantly improved accuracy in the PBRA-calculated dose distribution that was not achievable with a single-source beam model when modeling the Varian Clinac 1800 electron beams. Time of PBRA dose calculation was approximately doubled; however, dual-source beam modeling of newer accelerators (e.g., the Varian Clinac 2100) may not be necessary because of less impact of collimator-scattered electrons on dosimetry.     

Absorbed dose from secondary electrons in high energy photon beams.

The absorbed dose in high energy photon beams due to scattered electrons from the irradiated air volume and from beam-shaping platforms has been calculated using the Fermi-Eyges theory of multiple scattering. The results are presented as lateral surface absorbed dose distributions across the field for three different radiation qualities, namely 60Co, 6 MV and 21 MV X-rays. For 60Co the relative absorbed dose due to electrons expelled in air reaches a value as high as 30% of the absorbed dose at dose maximum at a field size 40 X 40 cm2 and an SSD of 100 cm. The absorbed dose from electrons emanating from beam-shaping platforms contribute significantly to the absorbed dose at the surface when the platform is placed closer than 20--40 cm from the surface for field sizes greater than 10 X 10 cm2 to 40 X 40 cm2 respectively.     

Calculational methods for estimating skin dose from electrons in Co-60 gamma-ray beams

Several methods have been employed to calculate the relative contribution to skin dose due to scattered electrons in Co-60 gamma-ray beams. Either the Klein-Nishina differential scattering probability is employed to determine the number and initial energy of electrons scattered into the direction of a detector, or a Gaussian approximation is used to specify the surface distribution of initial pencil electron beams created by parallel or diverging photon fields. Results of these calculations are compared with experimental data. In addition, that fraction of relative surface dose resulting from photon interactions in air alone is estimated and compared with data extrapolated from measurements at large source-surface distance (SSD). The contribution to surface dose from electrons generated in air is 50% or more of the total skin dose for SSDs greater than 80 cm.     

An experimental and Monte Carlo investigation of the energy dependence of alanine/EPR dosimetry: II. Clinical electron beams

The energy dependence of alanine/EPR dosimetry for 8, 12, 18 and 22 MeV clinical electron beams was investigated by experiment and by Monte Carlo simulations. Alanine pellets in a waterproof holder were irradiated in a water phantom using an Elekta Precise linear accelerator. The dose rates at the reference point were determined following the TG-51 protocol using an NACP-02 parallel-plate chamber calibrated in a (60)Co beam. The EPR spectra of irradiated pellets were measured using a Bruker EMX 081 EPR spectrometer. Experimentally, we found no significant change in alanine/EPR response to absorbed dose-to-water over the energy range 8-22 MeV at an uncertainty level of 0.6%. However, the response for high-energy electrons is about 1.3 (+/-1.1)% lower than for (60)Co. The EGSnrc Monte Carlo system was used to calculate the ratio of absorbed dose-to-alanine to absorbed dose-to-water and it was shown that there is 1.3 (+/-0.2)% reduction in this ratio from the (60)Co beam to the electron beams, which confirms the experimental results. Alanine/EPR response per unit absorbed dose-to-alanine was also investigated and it is the same for high-energy electrons and (60)Co gamma-rays.     

Dosimetry characteristics of degraded electron beams investigated by Monte Carlo calculations in a setup for intraoperative radiation therapy

Degraded electron beams, as used for intraoperative radiation therapy (IORT) or similar complicated dosimetric situations, have different characteristics compared to conventional electron therapy beams. If international dosimetry protocols are applied in a direct manner to such degraded beams, uncertainties will be introduced in the absorbed dose determination. The Monte Carlo method has been used to verify experimentally determined relative absorbed dose distributions and output factors in an IORT geometry. Monte Carlo generated dose distributions are mostly within +/-2% or +/-2 mm of measured data. The simulated output variation between the IORT cones (relative output factors) are mostly within 2% of measured values. By comparing IORT and conventional electron beam characteristics (e.g. energy spectra, angular distributions and the contributions of different system components to these quantities) limitations and uncertainties of commonly used dosimetric techniques in IORT electron fields are quantified. The intraoperative treatment field contains a larger amount of scattered electrons, which leads to a broader energy spectrum as well as a wider angular distribution of electrons at the phantom surface. The dose from the scattered electrons can contribute up to 40% of the total dose at a depth of dose maximum, compared to approximately 10% for standard beams. A study of the energy spectra at the reference depth reveals that an uncertainty of the order of 1% can be introduced if ionization chamber based dosimetry is used to determine output factors for the investigated IORT system. We recommend that relative absorbed dose distributions and output factors in IORT electron beams and for similar complicated dosimetric situations should be determined with detectors having a small energy and angular dependence (e.g. diamond detectors or p-Si diodes).     

术中放疗深度剂量、表面剂量及漏射线的剂量研究

目的：研究术中放疗深度剂量、表面剂量及漏射线的剂量的测量方法，总结临床应用经验。方法：使用IC-15电离室和WEUHOFER WP700蓝水箱测量加速器电子束术中放疗限光筒中心轴百分深度剂量和表面剂量：采用Farmer剂量仪2570及有机玻璃小水箱，测量剂量输出因子及限光筒外漏射线。结果：6MeV和9MeV表面剂量分别为85．9％、87．2％。12MeV、16MeV、20MeV限光筒外1cm处漏射线分别达到6．81％、6．10％、6．85％。结论：术中放疗是一种复杂的治疗技术，在临床辐射剂量学上有其独特性。术中放疗表面剂量应该满足90％，建议增加填充物，如盐水纱布等，提高表面剂量。限光筒外的泄漏射线必须小于中心轴最大剂量的5％，做好肿瘤周围正常组织的辐射防护很重要。     

Monte Carlo investigation of electron beam output factors versus size of square cutout.

A major task in commissioning an electron accelerator is to measure relative output factors versus cutout size (i.e., cutout factors) for various electron beam energies and applicator sizes. We use the BEAM Monte Carlo code [Med Phys. 22, 503-524 (1995)] to stimulate clinical electron beams and to calculate the relative output factors for square cutouts. Calculations are performed for a Siemens MD2 linear accelerator with beam energies, 6, 9, 11, and 13 MeV. The calculated cutout factors for square cutouts in 10 X 10 cm2, 15 X 15 cm2, and 20 X 20 cm2 applicators at SSDs of 100 and 115 cm agree with the measurements made using a silicon diode within about 1% except for the smallest cutouts at SSD= 115 cm where they agree within 0.015. The details of each component of the dose, such as the dose from particles scattered off the jaws and the applicator, the dose from contaminant photons, the dose from direct electrons, etc., are also analyzed. The calculations show that inphantom side-scatter equilibrium is a major factor for the contribution from the direct component which usually dominates the output of a beam. It takes about 6 h of CPU time on a Pentium Pro 200 MHz computer to simulate an accelerator and additional 2 h to calculate the relative output factor for each cutout with a statistical uncertainty of 1%.