医用加速器机房迷路辐射水平的测量与分析

目的 通过医用加速器机房迷路内辐射水平的测量与分析,为职业照射的控制提供科学依据,为理论模拟计算积累实验数据。方法 利用剂量率仪测量加速器机房迷路内的辐射水平,并对测量结果进行理论分析。结果 医用加速器机房出入口处杂散X-γ射线剂量率与机头朝向有关,并随照射野的减小而降低;杂散中子剂量率水平主要取决于加速器粒子的能量和输出剂量,随照射野的变化不明显。同时,医用加速器机房出入口处杂散X-γ射线和中子剂量率与医用加速器机房迷路的辐射防护设计密切相关。结论 合理改善医用加速器机房迷路的辐射防护设计是降低医用加速器机房出入口处X-γ射线和中子剂量率水平的行之有效的措施。     

医用电子直线加速器治疗室的中子防护屏蔽设计

目的 研究治疗室内中子来源和中子散射的实质,分析计算加速器机房治疗室及迷路内中子剂量的变化规律,进行屏蔽设计。方法 本文基于对一台运行中的15MeV加速器治疗平面上关注点的中子通量测量结果,参考NCRP 79号报告,进行屏蔽防护设计和计算。结果 医用加速器产生X射线达到一定能量时,光核反应中子是医用加速器机房中子污染的主要来源。在加速器治疗室出入口处,主要的辐射防护是通过散射进入迷路内侧入口的杂散中子及其俘获γ辐射。结论 中子污染与治疗作用无关,但却给相关人员增加了额外剂量负担,一定条件下,也可能产生中子照射危害。所以,应重视医用加速器产生的中子外照射危害,对医用加速器治疗室内中子污染需进行防护设计和评价。     

术中放疗加速器中子剂量当量率的测量研究

目的 测量并分析术中放疗加速器9和12 MeV电子线在手术室内引起的中子剂量当量率,与西门子Primus加速器相同电子线能量档产生的中子污染进行比较,为放射治疗引起的二次致癌风险提供数据参考。方法 利用中子探测仪测量术中放疗加速器在9和12 MeV电子线于机头两端、限光筒底端、患者治疗平面,以及其他关键位置产生的中子剂量当量率。取相似的位置在西门子Primus加速器上进行相同方法的测量。分析测量结果,并将两种加速器产生的中子进行比较。结果 手术过程中使用9和12 MeV的电子线治疗时会产生中子,对患者以及工作人员产生潜在的健康隐患。9 MeV时,术中放疗加速器机头两端以及限光筒底端两侧的中子剂量当量率分别为（51.8±3.1）、（45.2±1.5）、（70.5±4.9）和（68.2±3.3）μSv/h,比12 MeV产生的中子分别低5.9%、5.4%、17.8%和21.5%。手术室门内侧在9和12 MeV时产生的中子剂量当量率极低,可以忽略。西门子加速器出束9 MeV时,在相似测量点处产生的中子剂量当量率为（277.3±1.2）、（285.1±1.6）、（185.1±1.8）、（182.8±2.4）μSv/h,比12 MeV的分别低48.8%、47.6%、48.7%、和52.2%。能量达到12 MeV时,西门子Primus加速器产生的中子剂量当量率是术中加速器的10倍以上。结论 两种医用加速器12 MeV电子线产生的中子剂量当量率远高于9 MeV产生的中子,增加了患者第二原发癌的风险;传统医用加速器在相同能量档产生的中子剂量当量率远高于术中电子加速器,应采取适当的屏蔽防护。     

湖北省调强放疗多叶光栅野光子线束吸收剂量和二维剂量分布验证研究

目的 用热释光剂量计（TLD）和放射性免冲洗胶片测量调强放疗（IMRT）多叶光栅（MLC）野光子线束吸收剂量并验证二维剂量分布。方法 选择湖北省7家三级甲等医院的7台不同型号医用直线加速器,使用国际原子能机构（IAEA）提供的15 cm×15 cm×15 cm聚苯乙烯专用模体,TLD和放射性免冲洗胶片,在源皮距90 cm,照射深度10 cm,照射野5 cm×5 cm,6 MV X射线,6 Gy吸收剂量照射条件下制定IMRT计划并实施照射,比较TLD和胶片吸收剂量测量值与放疗计划系统（TPS）预估剂量之间的偏差。同时,使用医院配备的30 cm×30 cm均质固体模体,在模体表面下5 cm处放置25 cm×25 cm放射性免冲洗胶片,并将IMRT计划中单个射野移植到模体中胶片层面上并实施照射,通过胶片剂量分析系统验证二维剂量分布。结果 所检医用直线加速器中,1号加速器TLD吸收剂量相对偏差和胶片吸收剂量相对偏差分别为-8.5%和-1.9%;7号加速器TLD吸收剂量相对偏差和胶片吸收剂量相对偏差分别为5.4%和0.5%;其余加速器TLD和胶片吸收剂量相对偏差均在±5%范围以内。所有加速器的二维剂量分布通过率均在90%以上。结论 TLD和胶片核查调强放疗剂量质量方法,操作简单,科学性强,TLD和胶片便于邮件方式寄送,该方法可运用于对放疗机构调强放疗剂量大范围的质量核查。     

质子加速器治疗室辐射防护优化研究

目的 研究铁屏蔽体在主防护墙中不同深度对防护墙外周围剂量当量率的影响。方法 采用FLUKA蒙特卡罗模拟程序构建了质子治疗室的模型,治疗室的屏蔽体由混凝土和钢构成。分别模拟220和250 MeV的质子照射水模体,以获得不同情况下的周围剂量当量率分布。结果 随着嵌入防护墙的铁屏蔽体深度的变化,两种模拟条件下质子治疗机房主防护墙外30 cm处的周围剂量当量率发生显著变化,最大周围剂量当量率（220 MeV：3.42 μSv/h,250 MeV：6.39 μSv/h）比最小周围剂量当量率（220 MeV：1.75 μSv/h,250 MeV：3.32 μSv/h）高2倍。结论 在质子治疗加速器的设计中,应仔细评估铁屏蔽体在主防护墙中的位置。     

河南省调强放疗光子线束吸收剂量和二维剂量分布验证研究

目的 用热释光剂量计（TLD）和胶片测量调强放疗（IMRT）光子线束吸收剂量和二维剂量分布。方法 采用非概率抽样方法,在河南省选择5家三级甲等医院的8台可开展IMRT的医用加速器,TLD放入国家原子能机构（IAEA）提供的聚苯乙烯固体模体（15 cm×15 cm×15 cm）中,经CT扫描,影像传给放射治疗计划系统（TPS）制定放疗计划,源皮距90 cm,深度10 cm,照射野5 cm×5 cm,6 MV X射线,计算吸收剂量6 Gy和相应的监督单位（MU）,实施IMRT计划照射模体,测量TLD吸收剂量,同样方法测量胶片吸收剂量。医院的均质固体模体,尺寸30 cm×30 cm,厚度20 cm,25 cm×25 cm的胶片放在模体中,源皮距95 cm,深度5 cm,实施IMRT计划。结果 调查的8台医用加速器中,有7台加速器的TLD吸收剂量相对偏差符合要求,1台加速器不符合要求;胶片吸收剂量相对偏差全部符合要求;7台加速器的二维剂量分布通过率符合要求,1台加速器不符合要求。结论 TLD和胶片用于核查调强放疗多叶光栅野吸收剂量和二维剂量分布,方法简单,可操作性强,适合在我省医院大范围实施IMRT剂量质量核查。     

四川省7台加速器调强放射治疗多叶光栅叶片到位精确度验证方法研究

目的 用放射性免冲洗胶片验证调强放射治疗（IMRT）多叶光栅（MLC）叶片到位精确度方法研究。方法 选择瓦里安、医科达、西门子3个厂家的医用电子直线加速器共7台,用25 cm×25 cm的放射性免冲洗胶片放在30 cm×30 cm、厚3.0 cm的均质固体模体上,厚度2.0 cm的固体模体板覆盖在胶片上面,经CT扫描,影像传给放射治疗计划系统（TPS）制定治疗计划,多叶光栅形成5条条状栅栏野,能量6 MV X射线束,每条栅栏野长3 cm,宽0.6 cm,每条条状野间隔3 cm,在最大剂量点处,胶片到源距离100 cm,每条栅栏野给出监督剂量250 MU。照射后邮寄到国际原子能机构（IAEA）剂量学实验室测量和计算。结果 6台加速器胶片测量与TPS计划每条栅栏野MLC条状位置偏差符合IAEA要求的±0.5 mm,1台加速器偏差不符合要求。7台加速器胶片测量每对与每条多叶光栅叶片位置偏差均在IAEA要求0.5 mm以内,符合要求。6台加速器胶片测量每对与每条所有MLC叶片实际宽度差值在0.75 mm范围内,1台加速器为-0.8 mm,不符合要求。6台加速器胶片测量每条多叶光栅叶片实际宽度标准偏差在0.3 mm范围内,符合要求。1台加速器为0.4 mm,不符合要求。结论 用放射性免冲洗胶片验证调强放射治疗多叶光栅片到位精确度的方法简单,快速精确,建议广泛应用到临床。     

BeamModulator新型多叶准直器半影的测量

目的 探讨医科达Synergy-S直线加速器配备的微型多叶准直器的半影特性。 方法 利用PTW MP3 三维水箱和PinPoint电离室分别在水中和空气中测量6、10和18 MV X线的射野离轴比曲线,数据处理后得到半影,分析半影随射线能量、模体深度以及叶片位置的变化。结果 6 MV X线在空气中的半影比水中最大剂量深度处的半影小2 mm;叶片端面的半影比叶片侧面的半影大1 mm左右。微型多叶准直器的半影大小与射线能量、模体深度以及叶片位置均有关。相同照射条件下,射线能量从6 MV提高到18 MV,半影增加1~1.5 mm;模体深度从d_max增加至10 cm,半影增加了1.5 mm;叶片位置不同,半影相差1.5~2 mm。结论 叶片的半影与其机械设计与使用条件密切相关。吸收剂量计算和治疗计划设计时需要充分考虑多叶准直器的半影特性。     

一种侧卧位X射线分次全身照射技术及剂量监测结果分析

目的 报道一种侧卧位前后对穿X射线分次全身照射技术,并对照射中的实时剂量监测结果进行分析。方法 采用Varian Trilogy医用电子直线加速器10 MV X射线,行水平野对穿全身照射,源到模体表面距离390 cm,测量X射线全身照射条件下的射野百分深度剂量、离轴剂量分布及绝对剂量输出。对10例患者采用侧卧位前后对穿野分次全身照射。照射处方剂量1200 cGy/6次,共3 d,体中线剂量率约5.0 cGy/min。治疗时利用多通道半导体剂量计实时监测患者剂量准确性及剂量分布均匀性,采用固体水进行剂量非均匀性补偿。结果 治疗条件下模体测量射野离轴剂量分布均匀性<±5.0%,最大剂量点处绝对剂量输出为0.0721 cGy/MU。10例患者均能够顺利完成侧卧位治疗,各个部位监测总剂量偏离处方剂量-4.9%~6.7%,平均监测剂量均匀性<5.0%。结论 侧卧位X射线全身分次照射技术患者耐受性好,照射过程中实时监测剂量,采用固体水进行剂量非均匀性补偿,能够保证患者接受准确均匀的剂量分布,方法简便易行。     

加速器机载kV锥形束CT有效剂量的分析

目的 分析医用直线加速器机载kV锥形束CT扫描过程中患者的有效剂量随扫描条件的变化。方法 用PTW TM30009电离室分别在T40017头模和T40016躯干模体中,改变XVI锥形束CT的管电压、毫安秒、准直器以及机架旋转范围等参数测量加权CT剂量指数,计算相应的剂量长度乘积和有效剂量。结果 kV锥形束CT的加权剂量指数和有效剂量随管电压呈二次方变化,随毫安秒线性变化,与准直器以及机架旋转范围密切相关。临床常用条件下,kV锥形束CT单次扫描的剂量长度乘积和有效剂量低于参考剂量水平。结论 锥形束CT扫描过程中患者接受的有效剂量与扫描条件密切相关。锥形束CT扫描时,应该根据患者的解剖部位合理选择成像参数,最大限度减少患者接受剂量。     

The measurement of photoneutrons in the vicinity of a Siemens Primus linear accelerator.

This study involves the measurement of photoneutron contamination emitted from a Siemens Primus medical linear accelerator by using BD-PND bubble detectors. Various bubble detectors were arranged around the linac head with the interval of I m and at the same height as the isocenter to measure the dose equivalent distribution in the treatment room. The measurements were performed for 15 MV X-rays with 40 x 40cm2 and 0 x 0cm2 fields and for 15,18, and 21 MeV electrons with 25 x 25 cm2 electron cone. Neutron dose equivalent rate at the points of measurement in the treatment room decreased with increasing distance to the isocenter. The maximum neutron dose equivalents were at the isocenter, and the values for 15MV 40 x 40 and 0 x 0 cm2 were 1843+/-90 and 169.9+/-59.9 microSv per Gy X-ray, respectively. The values for 15, 18 and 21 MeV electrons with 25 x 25 cm2 cones were 100.0+/-20.4, 262.7+/-61.2 and 349.0+/-29.6 microSv per Gy electron, respectively. The neutron contamination of electrons less than 12 MeV was below the detection limit.     

A novel end-to-end test for combined dosimetric and geometric treatment verification using a 3D-printed phantom

The dosimetric and geometric accuracy are important components to ensure safe patient treatment in radiation therapy. Therefore, these components must be checked during quality control. This work presents a possible solution for the determination of the geometric isocenter deviation in the entire treatment chain. Additionally, the dose measurement of the established end-to-end test workflow measured in the same procedure as the geometric deviation is described. An in-house designed end-to-end test phantom went through the entire procedure of a standard patient treatment and the dosimetric and geometric accuracy were determined. At 3 linear accelerators (linac), the phantom was positioned either with cone beam computed tomography or with surface guidance. In this position, a Winston-Lutz test was performed and the deviations of the gantry, collimator and couch isocenter measurements to the phantom position were determined. Additionally, a dose measurement in the phantom was performed and compared to the dose predicted in the treatment planning system. To validate the results obtained with the in-house designed phantom, comparative measurements with commercial phantoms were performed. According to the performed end-to-end test, 2 out of the 3 linacs showed isocenter variations larger than 1 mm for collimator and gantry rotations and larger than 2 mm for couch rotations. With an isocenter variation of less than 1 mm for collimator and gantry rotations, 1 linac fulfilled the tolerance for stereotactic treatments without couch rotation. With couch rotation, an isocenter variation of less than 2 mm was detected at this linac, which fulfilled the tolerance for IMRT treatments. The mean dose deviation between measurement and treatment planning system was 1.82% ± 1.03%. The results acquired with the UMM phantom did not show statistically significant deviations to those acquired with relevant other commercial phantoms. The novel end-to-end test procedure allows for a combined dosimetric and geometric treatment evaluation. Besides the commonly performed dose end-to-end test the geometric isocenter deviation within a patient treatment workflow was evaluated and categorized for IMRT or SBRT.     

Accuracy of absorbed dose calculation and measurement of scatter factors with different depth of mini-phantoms using a pinpoint ionization chamber

The output factor of high-energy X-ray machines varies with collimation. According to Khan's theory, collimator and phantom scatter factors contribute to total scatter factor. For precise X-ray irradiation, the two factors need to be taken into consideration. To obtain proper factors, we made two original polystyrene cylindrical mini-phantoms. These phantoms are both 4 cm in diameter and have a pinpoint ion chamber placed at a depth of 5 cm and 10 cm, respectively. Using a 6 MV X-ray machine, collimator scatter factors were calculated for various field arrangements (i.e., field sizes ranging from 4 cm x 4 cm to 40 cm x 40 cm at isocenter). To determine if calculated values were appropriate, we measured point doses of 20 X-ray irradiation patterns using a Farmer-type ion chamber with a water equivalent phantom at depths of 5 cm and 10 cm, respectively. Two hundred MUs were irradiated to the above-mentioned depths for each field. Based on the measured doses, variations were obtained for four calculation methods. Accounting for 1) secondary collimator (jaw) setting, 2) blocked field (multi-leaf collimator) setting, 3) Khan's theory using a 5 cm mini-phantom, and 4) Khan's theory using a 10 cm mini-phantom. Dose variations in each method of calculation were as follows: 1) +0.3 to +10.2% (mean, +2.0 to +3.2%) , 2) -2.3 to 0.0% (mean, -0.8 to -0.6%), 3) 0.0 to +1.5% (mean, +0.1 to +0.3%), 4) 0.0 to +1.4% (mean, -0.1 to +0.1%).     

Project for the development of the linac based NCT facility in University of Tsukuba

A project team headed by University of Tsukuba launched the development of a new accelerator based BNCT facility. In the project, we have adopted Radio-Frequency Quadrupole (RFQ)+Drift Tube Linac (DTL) type linac as proton accelerators. Proton energy generated from the linac was set to 8 MeV and average current was 10 mA. The linac tube has been constructed by Mitsubishi Heavy Industry Co. For neutron generator device, beryllium is selected as neutron target material; high intensity neutrons are generated by the reaction with beryllium and the 80 kW proton beam.Our team chose beryllium as the neutron target material. At present beryllium target system is being designed with Monte-Carlo estimations and heat analysis with ANSYS. The neutron generator consists of moderator, collimator and shielding. It is being designed together with the beryllium target system. We also acquired a building in Tokai village; the building has been renovated for use as BNCT treatment facility. It is noteworthy that the linac tube had been installed in the facility in September 2012.In BNCT procedure, several medical devices are required for BNCT treatment such as treatment planning system, patient positioning device and radiation monitors. Thus these are being developed together with the linac based neutron source. For treatment planning system, we are now developing a new multi-modal Monte-Carlo treatment planning system based on JCDS. The system allows us to perform dose estimation for BNCT as well as particle radiotherapy and X-ray therapy. And the patient positioning device can navigate a patient to irradiation position quickly and properly. Furthermore the device is able to monitor movement of the patient׳s position during irradiation.     

The munich fission neutron therapy facility MEDAPP at the research reactor FRM II

Purpose

At the new research reactor FRM II of the Technical University of Munich (TUM), the facility for Medical Applications (MEDAPP) was installed where fast neutrons are available as a beam for medical use.

Material and Methods

Thermal neutrons induce fission in a pair of uranium converter plates and generate fast neutrons which are guided to the patient by a beam tube. The maximum opening of the multi leaf collimator (MLC) is 30 × 20 cm² W × H. The beam is characterized by neutron-photon mixed beam phantom dosimetry. Specific safety measures are outlined.

Results

The neutron and gamma dose rates are 0.52 Gy/min and 0.20 Gy/min, respectively, in 2 cm depth of a water phantom. The half maximum depth of the neutron dose rate in water is 5.4 cm (mean neutron energy 1.9 ± 0.1 MeV). Conformity with the European Medical Devices Directive (MDD) 93/42/EEG, was proven so that MEDAPP has a CE mark and since February 2007 also the license for clinical operation.

Conclusion

The clinical neutron irradiations of malignant tumors, which were performed at the former research reactor FRM until 2000, can be continued at FRM II under improved conditions. First patients were irradiated in June 2007.     

放疗工作环境辐射对Exradin W1闪烁体探测器测量立体定向治疗计划的影响

目的 研究放疗工作环境辐射对塑料闪烁体探测器Exradin W1进行立体定向放射治疗（SRT）计划绝对剂量验证的影响。方法 将立体验证模体（SDVP）的电子计算机断层（CT）影像扫描后导入计划系统，利用自制档铅分别在屏蔽或不屏蔽光电组件的条件下进行3 cm×3 cm至20 cm×20 cm的方形梯度射野照射、虚拟靶体积（PTV）非共面弧照射以及10例容积调强弧形立体定向放射治疗（VMAT SRT）临床计划验证，记录各测量值并对比分析环境辐射在不同条件下对剂量测量的影响。结果 光电组件的噪声效应随开放射野面积增大而增大，随光电组件与等中心距离增大而减小；非共面弧对光电组件噪声效应贡献随射野增大而增大，最大可达4.16%；临床SRT计划验证测量时，屏蔽前与屏蔽后与治疗计划系统（TPS）相对误差分别为（1.39±1.05）%和（0.59±1.03）%，差异具有统计学意义（t=-5.343，P<0.05）；与A16小空气电离室实测结果相对误差分别为（1.22±1.56）%和（0.42±1.42）%，W1测量误差明显减少，差异具有统计学意义（t=-5.414，P<0.05）。结论 Exradin W1探测的测量结果与电离室及计划系统的计算结果一致度较好，但其准确度易受放疗工作环境辐射的影响。测量非共面照射时应将光电组件尽量摆放在远离辐射等中心位置，并予以适当遮挡或屏蔽，可有效提高测量准确性和稳定性，为临床精准放射治疗提供有力保障。     

Investigation of photoneutron dose equivalent from high-energy photons in radiotherapy.

Spatial distribution of photoneutron dose equivalent during radiotherapy at different beam size, depth, and distance from a 15 MV linear accelerator was investigated with bubble detectors in a water phantom. The photoneutron dose equivalent was mainly from fast neutrons, and decreased with distance at a fixed field and with depth. Besides, photoneutron dose equivalent was slightly affected by beam size due to the variation of tungsten area exposed in the beam direction and photoneutrons occurred at the jaws. Fast photoneutron dose equivalent of shallow critical organs was represented still considerably outside the beam size.     

Interaction between the biological effects of high- and low-LET radiation dose components in a mixed field exposure

Abstract

Purpose: The relative biological effectiveness of two epithermal neutron sources, a reactor based source at Studsvik, Sweden, and a proton accelerator-based source in Birmingham, UK, was studied in relation to the proportional absorbed dose distribution as a function of neutron energy. Evidence for any interactions between the effects of biological damage induced by high- and low-linear energy transfer (LET) dose components, in this ‘mixed field’ irradiation, was also examined

Materials and methods: Clonogenic survival in Chinese Hamster-derived V79 cells was used to assess biological effectiveness in this study. Cells were irradiated in suspension at 4°C at depths of 20, 35, 50 and 65 mm in a water phantom. This prevented the repair of sublethal damage, predominantly that produced by both incident and induced γ-rays in the field, over the variable periods of exposure required to irradiate cells with the same total absorbed dose. Cell survival, as a function of the absorbed radiation dose and depth in the phantom, was compared with Monte Carlo N-Particle (MCNP) calculations of the proportional absorbed dose distribution as a function of neutron energy for the two sources.

Results: In terms of the dose-related reduction in clonogenic cell survival, the epithermal neutron source at Studsvik was more biologically effective than the Birmingham source at all depths considered in the phantom. Although the contribution from the high-LET dose component was greater for the Studsvik source at 20 mm depth in the phantom, at greater depths the dose contribution from the high-LET dose component at Studsvik overlap with those for the Birmingham source. However, the most striking difference is in the fast neutron component to the dose of the two sources, neutron energies > 1 MeV were only associated with the Studsvik source. The relative biological effectiveness (RBE) of both sources declined slightly with depth in the phantom, as the total high-LET dose component declined. The maximum source RBE for Studsvik was 2.70 ± 0.50 at 20 mm; reduced to 2.10 ± 0.35 at depths of 50 and 65 mm. The corresponding values for Birmingham were 1.68 ± 0.25 and 1.31 ± 0.19, all values relate only to the surviving fraction of V79 cells at 37%, since RBE values are only applicable to the selected endpoint. Based on a dose reduction factor (DRF) of 1.0 for the total low-LET component to the absorbed dose, the RBE values for the high-LET dose component (fast neutrons and induced protons from the nitrogen capture reaction) was 14.5 and 7.05 for the Studsvik and Birmingham neutron sources, respectively. This is well outside the range of RBE historically reported values for V79 cells for the same level of cell survival for fast neutrons. The calculation of RBE values, based on the proportional absorbed dose distribution as a function of neutron energy, from historical data, and using a RBE of 1.8 for the dose from the nitrogen capture reaction, suggests RBE values for the total high-LET dose component of 3.1–2.8 and 2.5–2.0 for Studsvik and Birmingham, respectively, values again declining with depth in the phantom.

Conclusions: The overall biological effectiveness of the mixed field irradiation from an epithermal neutron sources depends on the composition and quality of the different dose components. The experimentally derived RBE values for the total high-LET dose components in these ‘mixed field’ irradiations are well in excess of historical data for fast neutrons. The difference between the historically expected and the observed RBE values is attributed to the interactions between the damage produced by high- and low-LET radiation.     

基于电子射野影像系统与加速器日志文件重建模体内剂量的初步比较

目的 研究基于电子射野影像系统（EPID）与加速器日志文件（dynalogs file）重建模体内剂量的差异性。方法 收集12例盆腔患者的容积旋转调强（VMAT）计划,将计划信息复制到“Cheese”模体上重新计算剂量,而后在瓦里安加速器（RapidArc）上执行,“Cheese”模体置于等中心处获取射野影像（EPI）,将EPI传入EPIgray软件中重建剂量。同时利用Mobius软件调用加速器日志文件,实现对模体计划剂量的重建。以A1SL型号的电离室和配套的剂量仪测量整个计划执行结束后射野等中心（电离室中心）处剂量值,在计划系统（TPS）中读取电离室敏感体积体内的平均剂量值（设置电离室中心与等中心重合）。结果 电离室测量值与TPS中读取的等中心处剂量值相比,两者偏差为1.31%。两种方式重建的射野等中心的剂量分别与电离室测量数值相比,差异均无统计学意义（P>0.05）。结论 两种重建体内剂量的方法均能为VMAT在体剂量验证提供参考。     

Build-up and depth-dose characteristics of different fast neutron beams relevant for radiotherapy.

Build-up and central axis depth-dose curves have been obtained for d(50) + Be and d + T neutron beams. Measurements carried out with the collimator opening covered with a layer of lead showed that for all three neutron beams the entrance dose is approximately 60% of the dose at the maximum. Consequently the skin-sparing properties of these neutron beams will be approximately equal and comparable to those for electron beam therapy. Central axis depth-dose curves have been established for d(50) + Be neutrons at 129 cm SSD, for p(42) + Be neutrons at 125 cm SSD and d + T neurtons and 60Co gamma rays at 80 cm SSD. The 50% dose values in a water phantom are at depths of 12.7 cm, 12.0 cm, 9.7 cm and 12.7 cm respectively, for field sizes of approximately 15 cm x 20 cm. Insertion of a 6 cm thick nylon filter in the p(42)+Be beam increases this value from 12.0 cm to 13.5 cm. The gamma component for the d+T neutron beam is higher than for the cyclotron beams.