电子束照射野面积对中心轴剂量和输出因子的影响

目的探讨电子束照射野挡块对中心轴剂量和输出因子的影响.方法用瑞典Scaditronix公司生产的RFA-300型三维水箱及P型硅半导体探头对瓦里安2100C和2300C/D直线加速器的多种能量电子束进行了中心轴百分深度剂量(PDD)扫描,并测量了照射野输出因子.结果测得的PDD数据表明,电子束深度剂量对照射野铅挡大小有某种程度的依赖性,一般倾向是当照射野减小时表面剂量增大,治疗深度减小,最大剂量深度(R100)向表面移.这些变化在高能时最为明显.输出因子的测量结果说明,对不同能量电子束在不同限光筒条件下,输出因子随照射野铅挡大小改变的情况不尽相同.结论临床治疗时使用的限光筒大小要尽量与实际照射野面积接近,在使用铅挡构成很小的照射野(如<6cm×6cm)时,应实际测量输出因子,以减少剂量误差.     

Hi-ART螺旋断层治疗机剂量学参数的测量

目的 探讨Hi-ART螺旋断层治疗机照射野剂量学参数测量的内容和方法.方法 用断层治疗机专门配置的微型扫描水箱在治疗条件下测量了6 MV X线的百分深度剂量和射野离轴比,并与常规Primus加速器6 MV X线进行比较.根据AAPM TG51号报告用Tomotrometer剂量仪和A1SL电离室在源皮距85 cm、照射野40 cm×5 cm、1.5 cm深度条件下对断层治疗机进行输出剂量刻度,并对剂量线性和重复性进行测量分析.输出剂量率随机架角的变化分别用0.6 cm3电离室和Unidos剂量仪在直径为3 cm有机玻璃体模中测量和用治疗机自身的MVCT探测器测量.设置不同的照射范围,在固体水组织等效材料中对多叶准直器照射野输出因子进行测量.结果 Hi-ART断层治疗机6 MV X线百分深度剂量的最大剂量点在1.0 cm左右.Hi-ART断层治疗机和Primus 6 MV X线在源皮距85 cm、深度10 cm处的百分深度剂量分别为59.6%和64.7%.单个照射野内剂量分布是不均匀的,在人体左右方向剂量分布呈锥形,在人体头脚方向剂量分布和照射野的宽度有关,40 cm×5 cm照射野的输出剂量率为848.38 cGy/min.剂量仪的读数R和照射时间t的关系为R=-0.017+0.256t,线性相关系数为0.999.重复测量的输出剂量率的最大偏差为1.6%,标准偏差＜0.5%;输出剂量率随机架角度变化的最大偏差为1.1%,标准偏差＜0.5%.多叶准直器相邻叶片对单个叶片照射野的剂量贡献比较大,继续增加叶片数目输出因子基本保持不变.结论 Hi-ART断层治疗机的输出剂量率高,照射野剂量分布不均匀.独特的设计和剂量学特性使其剂量计算模型和调强实现方式更加简单、高效.     

X线立体定向治疗系统的剂量测量—小野剂量分布测量方法

目的 :本文叙述我院利用半导体探头测量X射线立体定向治疗的剂量学参数 ,并对其结果给予评价 ,说明小野剂量分布的特点。材料与方法 :由于半导体探头具有体积小 ,灵敏度高等优点 ,我们选择P型半导体探头 ,以测量准直器 5m～ 5 0mm直径照射野的百分深度剂量 (PDD) ,离轴比 (OAR)及射野输出因子 (Sc ,p) ,所得结果与其他测量方法诸如电离室 ,胶片等 ,以及有关文献报导进行比较。结果 : 1 0和 3 0准直器的PDD值 ,在 5cm～ 2 0cm深度范围内 ,测量值与文献 7报道值的差别在± 0 .6以内。将PDD转换为TMR(组织最大剂量比 ) ,外推法计算 6MV -X线零野的有效线性衰减系数为 0 .0 5 1 0cm- 1。测量射野输出因子 ,在常规用照射野范围 ,半导体探头与NE2 5 71电离室所得数值 ,偏差为± 0 .4%以内 ,但当射野直径小于电离室直径的 2倍时 ,偏差增大。而用半导体测量准直器直径在 1 2 .5～ 2 7.5所得值 ,与报导用MonteCarlo方法计算值基本相吻合。对于射野离轴比 ,半导体和我们自行设计的胶片法组出的结果 ,差别在 1mm以内 ;半导体所测得的照射野半影区宽度 (90 %～ 1 0 % )与报导值极其接近。结论 :对于小野 ,由于照射野边缘剂量梯度过大和缺少侧向电平衡 ,选用探头的大小和测量位置 ,是影响精确测量极为重要的因素。     

鼻咽癌半束照射的剂量分布

目的 比较鼻咽癌半束和全束照射技术射野衔接处的剂量分布以及脑和肺的受量。方法 应用Varian 600CD直线加速器，在固体水模体中采用胶片黑度法测量鼻咽癌面颈联合野和下颈切线野射野衔接处的剂量分布。应用Helax TMS-3D治疗计划系统，根据实际病例的螺旋CT模拟定位资料，在数字重建图像上设计面颈联合野和下颈切线野，分别计算半束和全束照射技术条件下脑组织和肺的受量，比较两者受照射的剂量体积直方图(DVH)及脑组织受照射的最大剂量、最小剂量、中位剂量、平均剂量、25％受较高照射剂量脑组织的下限剂量(D25％)以及受量超过75％肿瘤剂量的脑体积(V75%)。结果 两种照射技术射野衔接处均无漏照及低剂量区情况，半束照射和全束共线照射分别有约4mm、10mm的剂量重叠区，两野衔接处剂量最高点高出剂量归一点分别为28％和117％。两种照射技术脑及肺受照射的DVH相似，脑受照射的最小剂量、最大剂量、中位剂量、平均剂量、D25％、V75％及破裂孔处剂量均以全束照射略高，但相差幅度均不超过1％。结论 鼻咽癌应用面颈联合野和下颈切线野放射治疗时，与全束照射技术相比，半束照射技术照射野衔接处的剂量重叠区较小，脑和肺的照射体积和剂量没有增加。     

电子束全身照射的实施和剂量学研究

自行设计了全身电子线照射(TSEI)的患者固定用标准体位架,以及可以人工360度旋转的特制旋转盘。分别应用剂量仪、仿真石蜡固体体模、固体水,柯达慢感光胶片,胶片分析仪等多种剂量测试工具,对开展该技术所需的临床剂量学参数进行系统的数据采集,并加以分析。总结出一套对临床行之有效的照射剂量参数,并已应用于临床,为逐步开展此项工作的临床机构提供参考。     

X (γ)射线全身照射过程中患者全身剂量分布的蒙特卡罗模拟

目的 对比实际测量结果探究利用蒙特卡罗方法模拟患者在实际X (γ)射线全身照射过程中全身剂量分布的可行性。方法 利用MCNPX构建准确的医科达Synergy加速器6 MV治疗头蒙卡模型,根据CT值与物质密度的关系将ATOM物理体模的CT转换为用于MCNPX计算的体素模型,模拟患者在X (γ)射线全身照射过程中常用的水平照射方式中全身的剂量分布,并将模拟结果与热释光剂量计在ATOM物理体模内不同位置处的测量值进行对比分析其差异。结果 标准源皮距下6 MV加速器治疗头模型在水模体中计算的百分深度剂量曲线和离轴剂量曲线与医院的实际测量值差异性均<2%,其中10 cm×10 cm射野下的最大剂量点深度约为1.5 cm,与实际测量值相符。全身照射中体模内不同位置处剂量的模拟结果与热释光剂量计测量值的最大差异性约为4%,MCNPX的模拟结果与热释光的测量结果基本符合。结论 MCNPX较精确地模拟计算患者全身照射的剂量分布,蒙特卡罗模拟为全身照射过程中患者全身剂量的均匀性优化提供了可能。     

乳腺癌术后胸壁照射技术剂量分布的研究

目的：对乳腺癌术后胸壁放射治疗几种常用照射技术的剂量分布特点进行研究。方法：对乳腺癌手术后患者和做了标记的测量体模，按放疗体位做CT扫描，CT影像经网络送入三维TPS，在TPS上设计4种照射方案，并在人体模上进行模拟照射（参考剂量1Gy)，用热释光剂量仪进行实际测量。结果：实验结果表明除电子弧形旋转照射外，其它3种照射技术的胸壁剂量都较均匀（胸壁平均剂量＞0．90Gy)。电子线弧形旋转照射 和X线双切线影＋内乳区电子线野技术由于在内乳区设野保证了内乳区有充足的剂量（内乳点剂量＞0．90Gy)，但是后者在2个野交界处易形成剂量冷热点。单纯X线切线野和适形野技术在内乳区可产生欠剂量情况，体积剂量直方图显示电子线旋转照射技术肺部受高剂量照体积最小，适形野技术也使肺部受高剂量照射体积明显减小。在体表加盖1．0－1．5cm的组织等效填充物后，4种照射技术的皮肤剂量可提高到0．90Gy以上。结论：乳腺癌胸壁照射技术有不同的剂量分布特点，在临床应用时应视患者具体情况选择使用。     

鼻咽癌常规放疗中面颈联合野和下颈野衔接的探讨

目的：比较鼻咽癌常规放射治疗中面颈联合野和下颈切线野不同衔接方法及间距的衔接平面的剂量分布。方法：利用CMS计划系统来计算面颈联合野和下颈切线野在机架角0度方向上（即颈前部）共线衔接、间隔5mm衔接、重叠1mm和2mm衔接以及机架角在90度或270度方向上（即颈侧方向上）共线衔接平面的剂量分布。结果：两者在0度方向上共线衔接、间隔5mm、重叠1mm和2mm衔接以及在90度或270度方向上共线衔接时，衔接平面最高剂量分别是参考剂量的103％、54％、114％、124％、73％，而在0度方向上共线衔接、重叠1mm和2mm衔接平面的95％剂量曲线分别位于皮下2．5cm、0．8cm、0．4cm。结论：鼻咽癌常规放射治疗中面颈联合野和下颈切线野全束照射时推荐在0度方向上1mm的重叠衔接。     

移动式术中放疗Mobetron加速器的测试

目的 测试移动式术中放疗Mobetron加速器,分析它的电子束剂量学特点.方法 测量移动式术中放疗Mobetron加速器电子束的剂量学特点,并与西门子常规加速器电子束进行比较.Mobetron加速器配置有4、6、9、12 MeV电子束.测量项目包括垂直于水模体表面的中心轴百分深度剂量和平行于水模体表面的射野离轴比、输出因子、限光简外漏射剂量、铅挡块对电子束的衰减、输出量校准.使用的测量仪器包括三维水箱、静电计、0.6 cm3Farmer电离室、平行板电离室和固体水.测量时将不同端面和直径限光筒依次与加速器机头连接,并使端面与模体表面相切.结果 除12 MeV外其他能量的表面剂量均低于90%,相同能量下术中加速器表面剂量明显高于常规加速器剂量.对10 cm直径、0°倾斜角的限光筒四档能量的最大剂量深度依次为0.7、1.3、2.0、2.2 cm,治疗深度依次为1.0、1.8、2.7、3.6 cm;对0°限光筒治疗时只需选直径比瘤床大1cm的筒即可.由于斜端面的限光筒照射野平坦度和对称性明显变差,限光筒尺寸的选择要依据等剂量分布图.四档能量的限光筒外1 cm处漏射线分别为1.2%、5.1%、10.0%、9.1%,全挡时铅挡厚度分别为1.5、3.0、4.5、6.0 mm.结论 通过测试了解了Mobetron加速器性能特点并获得了临床应用和日常质量保证所需数据.     

卧位全身电子线照射技术

对于表浅的恶性皮肤淋巴瘤等疾病,采用电子线全身照射能得到较好的疗效。从五十年代开始到现在已发展多种照射技术。我们采用一种简单的病人卧位的全身照射方法,用4MeV能量的电子束照射病人,治疗中光阑开到最大而且不加任何限光筒,SSD=158cm。病人采用仰卧和俯卧两种体位,每种体位机架从45°和315°两个方向照射,人体从头侧到足侧按光野结合,根据病人身长用了3～4个照射野相接。治疗常规是     

Impact of cutout off axis on electron beam dosimetric parameters

Dosimetric changes caused by the positional uncertainty of centering a small electron cutout to the machine central axis (CAX) of the linear accelerator (linac) were investigated. Six circular cutouts with 4?cm diameter were made with their centres shifted off by 0, 2, 4, 6, 8 and 10?mm from the machine CAX. The 6?3?6?cm(2) electron applicator was used for the measurement. The percentage depth doses (PDDs) were measured at the Machine CAX and also with respect to cutout centre for 6, 9, 12, 16 and 20?MeV electron beams. The in-line and cross-line profiles were measured at the depth of maximum dose (R100). The relative output factor (ROF) was measured at the reference depth. All the measurements were made at nominal source to surface distance (100?cm SSD) as well as at extended SSDs (100, 102, 106 and110?cm). When the cutout centre was shifted away from the machine CAX for low energy beams the depth of 100% dose (R(100)), the depth of 90% dose (R(90)) and the depth of 80% dose (R(80)) had no significant change. For higher energies (>9?MeV) there was a reduction in these dosimetric parameters. The isodose coverage of the in-line and cross-line profile was reduced when the cutout centre was shifted away from the machine CAX. At extended SSDs the dosimetric changes are only because of geometric divergence of the beam and not by the positional uncertainty of the cutout. It is important for the radiation oncologist, dosimetrist, therapist and physicist to note such dosimetric changes while using the electron beam to the patients.     

Use of an electron reflector to improve dose uniformity at the vertex during total skin electron therapy

PURPOSE: The vertex of the scalp is always tangentially irradiated during total skin electron therapy (TSET). This study was conducted to determine the dose distribution at the vertex for a commonly used irradiation technique and to evaluate the use of an electron reflector, positioned above the head, as a means of improving the dose uniformity. METHODS AND MATERIALS: Phantoms, simulating the head of a patient, were irradiated using our standard procedure for TSET. The technique is a six-field irradiation using dual angled electron beams at a treatment distance of 3.6 meters. Vertex dosimetry was performed using ionization methods and film. Measurements were made for an unmodified 6 MeV electron beam and for a 4 MeV beam obtained by placing an acrylic scattering plate in the beam line. Studies were performed to examine the effect of electron scattering on vertex dose when a lead reflector, 50 x 50 cm in area, was positioned above the phantom. RESULTS: The surface dose at the vertex, in the absence of the reflector, was found to be less than 40% of the prescribed skin dose. Use of the lead reflector increased this value to 73% for the 6 MeV beam and 99% for the degraded 4 MeV beam. Significant improvements in depth dose were also observed. The dose enhancement is not strongly dependent on reflector distance or angulation since the reflector acts as a large source of broadly scattered electrons. CONCLUSION: The vertex may be significantly underdosed using standard techniques for total skin electron therapy. Use of an electron reflector improves the dose uniformity at the vertex and may reduce or eliminate the need for supplemental irradiation.     

Radiation therapy for skin cancer near the eye: kilovoltage x-rays versus electrons.

When skin cancer near the eye is irradiated, a corneal shield is placed between the lids and globe to protect ocular structures. The effectiveness of the shield was evaluated with 250 kVp x-ray and 6-20 MeV electron beams. To simulate the clinical situation, a face phantom was constructed out of solid pieces of water-equivalent epoxy. In the region of the eye the phantom was milled to the exact contour of a human face. The phantom was used to reconstruct the setup that had been used to treat a patient with a 1-cm basal cell carcinoma of the mid portion of the lower lid. A medium-sized corneal shield (2-mm-thick lead plated with 0.1 mm gold) was placed on the eye portion of the phantom. A contoured lead (6 mm thick) face mask was placed on the surface of the phantom to define a 3-cm diameter radiation field that included only the inferior hemisphere of the shield. The doses that the cornea, lens, and retina would receive beneath the midpoint of the inferior hemisphere of the shield were measured using thermoluminescent and film dosimetry. With 6 to 8 MeV electrons, the corneal dose was 2 to 4 times higher than with 250 kVp x-rays. Corneal and lens doses rose rapidly with increasing electron beam energy such that with greater than 8 MeV the shield would provide relatively poor ocular protection. A scanning ion chamber and film dosimetry were used to determine the isodose profiles of 250 kVp x-ray and 6 MeV electron beams for a 3-cm diameter field collimated on the surface. With 250 kVp x-rays the 95% isodose area was 32% wider than with 6 MeV electrons. The ease of shielding and the ability to minimize field size argue in favor of kilovoltage x-rays for early-stage skin cancer near the eye.     

Dosimetry of rotational partial-skin electron irradiation.

BACKGROUND AND PURPOSE: Often, the most appropriate treatment for superficially and extensively spreading tumors of the skin is to use electron irradiation at enlarged distances. Rotational skin electron irradiation is a proven method for the treatment of the entire skin surface. We here report modifications of this technique in the set-up of partial-skin electron irradiation and the results of dosimetric examinations with regard to optimal shielding, dose profiles and depth dose curves under various irradiation conditions. MATERIALS AND METHODS: Irradiation was performed using electron beams with nominal energies of 6 MeV from a linear accelerator. The phantom was located on a rotating platform at a source-surface distance SSD=300 cm. A horizontal slit aperture (height: 32 cm) within a 2 cm thick polymethylmethacrylate (PMMA) shielding plate near the phantom was used to define the size of the irradiated region. Influences on dose distributions due to scattering processes on the PMMA edges were investigated using a flat ionization chamber and films. Absolute dose measurements and film calibration were made with the flat chamber. The quality of bremsstrahlung radiation behind the shielding was determined with a thimble ionization chamber in the phantom. RESULTS AND CONCLUSIONS: The results of rotational partial-skin electron irradiation reveal some of the investigated shielding geometries to be optimal. Depth dose distributions and dose rates correspond to the results obtained in total skin electron rotational irradiation. It is possible to apply the dose superficially in the first millimeters of the skin; the dose maximum is located at a depth of 0-2 mm, the 80% isodose at 9 mm. The amount of bremsstrahlung contamination is 2.5%. The local amount of absorbed dose per monitor unit depends strongly on patient/phantom cross-section geometry. At our institute, rotational partial-skin electron irradiation was implemented into clinical routine in 1997.     

电子线斜入射对剂量分布影响的分析

目的　量化分析不同能量电子线在斜入射情况下对剂量分布的影响。方法　在水模体中 ,测量束流中心轴上不同相对剂量值的深度 ;并测量 80 %和 5 0 %等剂量线的倾斜角度 ;将电子线在不同斜入射角度时的这些测量数据与垂直入射时的测量数据进行比较。结果　①电子线斜入射角度越大 ,最大剂量点深度和 90 %、80 %、5 0 %及 10 %的深度越小 ;②能量越高 ,斜入射对最大剂量点深度和 90 %、80 %、5 0 %及 10 %的深度影响越大 ;③斜入射对相对剂量分布的影响还与射野大小有关 ,射野越大 ,最大剂量点深度和 90 %、80 %、5 0 %及 10 %的深度变化越小 ;④在相同的斜入射条件下 ,电子线能量的改变比射野大小的改变对相对剂量分布的影响要大 ;⑤在斜入射时 ,80 %和 5 0 %等剂量线会向有空气隙的一侧倾斜 ,并且倾斜角度要比斜入射角度大。结论　斜入射不仅使最大电离深度值减小 ,而且使电子线穿透能力减弱 ;电子线穿透能力的减弱程度与电子线能量和射野大小有关 ;临床上要充分考虑在斜入射时 80 %和 5 0 %等剂量线的横向移动 ,否则很容易造成肿瘤靶区的漏照射 ,从而导致肿瘤的局部复发。     

单中心上下半野照射鼻咽癌颈部剂量分布的研究

目的：对单个等中心上下半野照射鼻咽癌颈部剂量分布特征进行研究。方法：在加速器上用6MV X射线分别按常规技术和单个等中心上下半野技术方法对剂量体模进行模拟照射，用热释光和胶片剂量仪测出面颈野和颈锁野衔接层面相关剂量及剂量重叠情况。对用2种照射技术治疗的2组患者定时拍摄验证片比较摆位重要性。结果：热释光测量结果显示单中心技术在照射野衔接层面平均剂量（1．01Gy)接近剂量DT1 Gy，而常规分野技术在照射野衔接层面平均剂量（1．09－1．13Gy)有10％左右的超出，胶片显示单中心技术照射在颈部的等剂量分布较均匀合理，无明显高剂量区出现。验证片证实单中心技术的照射野重叠（1mm)和摆位偏移（0.5mm)很小，分别优于常规分野技术（6－14mm和3mm）。结论：单中心上下半野照射技术在照射野衔接处可以得到比较准确和均匀的剂量分布，减少了摆位误差并有较好的摆位重复性，是一种值得推广的照射技术。     

A semi-empirical model for the generation of dose distributions produced by a scanning electron beam

There are linear accelerators (Sagittaire and Saturne accelerators produced by Compagnie Generale de Radioiogie (CGR/MeV) Corporation) which produce broad, flat electron fields by magnetically scanning the relatively narrow electron beam as it emerges from the accelerator vaccum system. A semi-empirical model, which mimics the scanning action of this type of accelerator, was developed for the generation of dose distributions in homogeneous media. The model employs the dose distributions of the scanning electron beams. These were measured with photographic film in a polystyrene phantom by turning of the magnetic scanning system. The mean deviation of calculated from measured dose distributions is about 0.2 % , a few points have deviations as large as 2–4 % inside of the 50 % isodose curve, but less than 8 % outside of the 50 % isodose curve. The model has been used to generate the electron beam library required by a modified version of a commercially-available computerized treatment-planning system. (The RAD-8 treatment planning system was purchased from the Digital Equipment Corporation. It is currently available from Electronic Music Industries (EMI), Ltd.)     

Dose distribution in total skin electron beam irradiation using the six-field technique

Total skin low energy electron beam irradiation is used to treat superficially widespread skin lesions such as cutaneous T-cell lymphoma. Total skin irradiation involves delivering an adequate dose at a depth of 0.25 to 1.0 cm, while sparing underlying tissue. The dose distributions obtained when using a modified Stanford six-field technique depend upon the beam energy, the beam angle, the diameter and shape of the body part, and other variables. The dose distribution uniformity of six pairs of angulated electron beams has been studied as a function of beam energy, the gantry angle, +/- theta, above and below the horizontal and the diameter of a cylindrical polystyrene phantom. Depth doses and dose uniformity for single and multiple fields have been measured as a function of beam energy, phantom diameter and position.     

Electron Wedges for Radiation Therapy

Purpose: Brain tumors can be advantageously treated with electron over photon radiation, by exploiting the rapid fall-off in dose with depth. This advantage could be further enhanced by utilizing multiple electron beams. However, in some beam configurations, wedged dose profiles would be necessary for the dose uniformity. Unlike photons, shaped pieces of material placed in electron beam severely degrade the energy, give additional scattering and, therefore, are suboptimal. The purpose of this study was to create wedged electron fields, using intensity modulation. The combination of electron wedges enables a more uniform coverage of brain tumors with a reduced dose to normal tissue.Methods and Materials: Intensity modulation was performed for 10 to 50 MeV electrons using a narrow scanning elementary beam of a racetrack Microtron accelerator, delivering radiation pulses with coordinates and intensities prescribed by a custom scan matrix. Dispensing more pulses (or longer pulses) within the field to increase the local dose, one can sharpen the penumbra at depth and generate wedged dose distributions of arbitrary angle as well as many other desired profiles. We modulated the electron beams, measured dose distributions using film in an anthropomorphic phantom, and compared the results with conventional techniques.Results: Intensity modulation of electron beams decreases the 50–90% penumbra at depth by 40% and increases the flatness by 80%. Wedged profiles at depth can be created for any angle up to about 70°, depending on the beam energy. Multiple modulated electron beams give smaller 20–70% but larger 70–100% isodose regions than photon beams.Conclusions: Electron beams can improve dose distributions in brain compared to the same number of photon beams, reducing the 20–70% isodoses region in normal tissue by 30%. Intensity modulation significantly improves the dose distribution from combined electron beams providing a sharper penumbra, better conformity, and reduced margin.