半野技术结合非共面照射技术在乳腺癌胸锁联合照射中的应用

目的：介绍一种乳腺癌适形放疗中新的射野衔接技术,以保证乳腺癌患者放疗时锁骨上区域与胸壁区域靶区剂量均匀衔接,并降低治疗计划设计与实施中的操作复杂度。方法：选取一例乳腺癌胸锁联合照射病人,锁骨上靶区采用半野照射技术,胸壁靶区采用非共面切线野照射技术,使上下两组照射野在射野衔接处相切。使用直线加速器6MV-X射线照射靶区,处方剂量设置为50Gy包绕95％靶区体积,使用治疗计划系统计算三维剂量分布。结果：半野照射技术结合非共面照射技术应用于乳腺癌胸锁联合照射时,在治疗计划系统上显示处方剂量在射野衔接处均匀衔接,50Gy处方剂量等剂量线平滑,剂量线未见明显的凹陷和突出现象,无剂量冷热点出现。结论：半野照射技术联合非共面照射技术用于乳腺癌胸锁联合照射。在TPS上演示显示使用该方法能够使相邻射野剂量均匀衔接,适用于胸壁部分靶区头脚方向长度大于20cm的患者放射治疗需求,且使用方法较传统方法更加简单易行,值得推广,临床实际使用中建议使用验证手段来保障该技术的可靠性。     

鼻咽癌放射治疗野中野设计的探讨

背景与目的：鼻咽癌放射治疗,面颈联合野已作为标准的射野设计,用一个中心半束射野解决了面颈联合野与颈部切线野的衔接问题,但面颈联合野内存在剂量冷点与热点,剂量均匀性差。因此本研究希望就鼻咽癌放射治疗面颈联合野中进行野中野的设计作一些探讨。方法：在设计半束照射面颈联合野时,使野的下部最大剂量保持在105%左右,再在野的上部加射野的设计方案。结果：根据治疗计划系统（treatment planning system,TPS）计算,由于是一个中心的半束射野,面颈联合野与颈部切线野的衔接处无冷热点,在面颈联合野内加入野中野和颈部切线野,故比用两个中心设计的,面颈联合野与颈部切线野,技术员摆位更简单,更准确。其剂量分布比单一的面颈联合野更均匀,更合理。95%的等剂量线包容颅底及鼻咽处肿瘤体积（gross tumor volume,GTV）时,下颈及口咽部最高点剂量为105%左右,而且高剂量区容积也小。结论：用一个中心半束面颈联合野中野和颈部切线野方法照射,比单一面颈联合野照射时,剂量分布更均匀,更合理,比两个中心面颈联合野与颈部切线野照射时,技术员摆位更简单,衔接处的剂量更准确。     

松果体区病灶直线加速器放射外科的剂量分布探讨

通过运用仿真人体模型对松果体区靶病灶直线加速器多聚弧照射的监测，论证直线加速器作为放射外科工具治疗颅内病变的科学性，探讨有关辐射参数对剂量分布的影响。作者认为，直线器放射外科可实现与伽玛马相似的剂量分布，同是神经外科安全有效的治疗技术，准直器孔径越小，照射范围越大，则剂量梯度即越大，剂量分布越理想；靶区最大剂量对上器孔径有相当高的依赖性，本文还就合理的照射弧范围及边缘处方剂量作了讨论。     

医用加速器光子角分布的蒙特卡罗研究

目的:在放射治疗中,入射光子的角分布对人体中的剂量分布等有直接的影响。为了进一步提高放射治疗的精度,分析医用直线加速器等中心平面上光子的角分布及影响因素。方法:蒙特卡罗程序(BEAMnrc)是建立在蒙特卡罗程序(EGSnrc)之上,是为了医学物理中模拟三维放射治疗开发的一个程序。使用蒙特卡罗程序BEAMnrc模拟电子和光子在加速器治疗头中的输运行为,在源皮距为100 cm的等中心平面处得到相空间文件,通过程序蒙特卡罗程序BEAMdp处理相空间文件统计光子的角分布。结果:通过对标称能量为6 MV的医用加速器的光子角分布的统计,发现不同大小的射野,只要中心区域一致,光子的角分布基本相同。对于不同的离轴区域,光子的角分布与该区域的锥形角度基本一致,光子的角分布可以由锥形发散束来近似估计。结论:医用直线加速器等中心平面上光子的角分布与其所在区域有关,次级准直器对光子的角分布影响很小。在放射治疗的剂量计算中,应仔细考虑光子角分布的影响,这样可以提高放射治疗的精度和患者的生存质量。     

全脑全脊髓照射技术的改进

目的:介绍全脑全脊髓放射治疗的一种新技术。方法:患者采用仰卧位,在整体定位板上做颈肩和体膜固定,行CT扫描定位,将图像传输治疗计划系统,进行三维重建。按照全脑全脊髓照射的要求勾画靶区,设计治疗计划,调整剂量分布。治疗前行CBCT扫描,进行在线的体位验证。结果:通过计划系统进行剂量计算,可以直观显示靶区的剂量分布并加以调整,计划照射野衔接处没有明显的剂量冷点和热点出现体位验证结果符合临床要求;通过CBCT在线验证,保证位置准确。结论:全脑全脊髓放射治疗采用了仰卧位热塑膜固定,较传统的俯卧位使患者更舒适,治疗过程中体位容易保持,确保治疗的准确;CT模拟定位方法,较传统的模拟机定位简单易行,且定位精确;用计划系统计算剂量分布并进行调整,使靶区剂量分布均匀,避免照射野衔接处剂量分布出现冷、热点。     

国产医用直线加速器开展立体定向放射治疗的可行性研究

目的:从剂量输出、机械性能和辐射性能方面比较两种进口医用电子直线加速器和3种国产医用电子直线加速器的各项参数指标,对比分析国产医用直线加速器的技术和性能是否达到开展立体定向放射治疗的基本标准。 方法:选取两种开展过立体定向放射治疗的进口医用电子直线加速器和3种装机量较大的国产医用电子直线加速器。利用电离室和静电计在水模体上测量加速器的剂量输出性能;利用坐标纸、前指针、刻度尺等工具测量加速器机械精度;通过PIPSpro5.3.1和doselab图形分析软件测量加速器辐射性能和到位精度,从而分析固体水和EBT免冲洗胶片记录辐射野。 结果:以AAPM TG-142和中华人民共和国医药行业标准YY0832.2-2015为参考,建立一套完整的针对国产电子医用直线加速器的评价标准。检测发现国产加速器输出剂量精度、重复性、线性较高,旋转机架、准直器和治疗床辐射野等中心精度大部分小于1 mm,铅门和多叶准直器平均到位精度小于0.5 mm,两种国产加速器端对端偏差结果小于5%,说明国产加速器基本性能较好。 结论:部分国产加速器从剂量输出和治疗精度方面已达到开展立体定向放射治疗的基本要求,但开展立体定向治疗需要相关放疗单位投入更多的人力和相应设备做好加速器的质量保证和质量控制工作。     

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。     

三维适形放射治疗位置与剂量精度的验证

目的:验证三维适形放射治疗位置与剂量精度.方法:采用胶片法和电离室法,对所使用的治疗计划系统和加速器距离精度、靶中心位置精度及等中心吸收剂量进行实测验证.结果:治疗计划系统所显示图像的距离精度＜1%,靶中心位置精度＜2mm,等中心吸收剂量精度＜1%.结论:所验证的三维适形放疗位置与剂量精度符合临床要求,验证方法可行.     

用多叶准直器实现适形放射治疗

适形放射疗法是一种先进的放射疗法。它通过旋转照射或静态多射野照射使得高剂量区剂量分布的形状在三维方向上与靶区（病灶）的实际形状一致，而尽可能地降低靶区周围健康组织的照射量，从而大大提高放射治疗的治疗增益比，提高单次照射所能给出的处方剂量，达到更好的治疗效果。本文系统地阐述了用多叶准直器实现适形放疗的理论与技术。     

用蒙特卡洛方法模拟和评价电离室矩阵探测器的定位灵敏度

目的:由于阵列式平板探测器内每个电离室单元探测器结构和尺寸的差异,构成了不同厂家生产的探测器阵列在放射治疗质量控制中的地位,各自的优势和不足。本文通过Monte Carlo(MC)模拟对不同面积的准直器遮挡入射束所形成的剂量场分布,并和实验测量的结果进行比较,用于评价不同厂家生产的方形探测器电离室矩阵与圆形探测器阵列的对照射野定义的位置灵敏度。方法:用BEAMnrc软件对电子直线加速器机头建模并模拟6 MV X射线的注量场分布,得到PHASE SPACE(相空间)文件,用DOSXYZnrc计算水模体中探测器的剂量分布范围与实际被照射面积的对应关系,从而评价PTW公司的SEVEN29电离室矩阵(方形探测器)与IBA公司的MatriXX电离室矩阵(圆形探测器)对位置的探测敏感度。结果和结论:PTW公司的SEVEN29和IBA公司的MatriXX两种阵列中单个探测器的定位灵敏度在2%的误差范围内基本一致,在放射治疗中凡是涉及对位置的精度检测的操作过程中,这两种电离室矩阵都可以任意选用,其位置的空间定位精度在1 mm以下。     

交替半身放疗的剂量学探讨

目的：探讨在加速器上通过延长源皮距来实现交替半身照射的方法。方法：将自制治疗床置治疗室地板上。源皮距延长到213cm，得到足够大且均匀的照射野，在长方形水箱中测量射野平坦度和TPR数据，避免长源皮距引起的剂量改变。用实测入射面剂量和出射面剂量的方法，验证病人体中面吸收剂量的准确性。结果：在有效照射范围内射野均匀性小于4％，患者体中面的吸收剂量率为10cGy／min，吸收剂量的准确性优于2％。讨论：交替半身放疗要求病人有准确的吸收剂量和均匀的剂量分布，且危及器官的受照剂量要准确控制，我们认为选择10cGy／min的吸收剂量率较为适宜。更优化的剂量率选择有待于今后的进一步研究。     

Targeting accuracy of an image guided gating system for stereotactic body radiotherapy

Recently, a commercial system capable of x-ray image guided patient positioning and respiratory gated delivery has become available. Here we describe the operational principles of this system and investigate its geometric targeting accuracy under controlled conditions. The system tracks breathing via infrared (IR) detection of reflective markers located on the patient's abdomen. Localization kilovoltage (kV) x-rays are triggered from within the gated delivery window portion of the breathing trace and after positioning, the tumour will cross the linac isocentre during gated delivery. We tested geometric accuracy of this system by localizing and delivering gated fields to a moving phantom. Effects of phantom speed, gating window location, timing errors and phantom rotations on positioning and gating accuracy were investigated. The system delivered gated fields to both a moving and static phantom with equal accuracy. The position of the gating window affects accuracy only to the extent that an asymmetric breathing motion could affect dose distribution within its boundaries. Positioning errors were found to be less then 0.5 +/- 0.2 mm for phantom rotations up to 5 degrees. We found and corrected a synchronization error caused by a faulty x-ray duration setting and detected a 60 +/- 20 ms time delay in our linear accelerator.     

Modeling photon output caused by backscattered radiation into the monitor chamber from collimator jaws using a Monte Carlo technique

Dose per monitor unit in photon fields generated by clinical linear accelerators can be affected by the backscattered radiation into the monitor chamber from collimator jaws. Thus, it is necessary to account for the backscattered radiation in computing monitor unit setting for a treatment field. In this work, we investigated effects of the backscatter from collimator jaws based on Monte Carlo simulations of a clinical linear accelerator. The backscattered radiation scored within the monitor chamber was identified as originating either from the upper jaws (Y jaws), or from the lower jaws (X jaws). From the results of Monte Carlo simulations, ratios of the monitor-chamber-scored dose caused by the backscatter to the dose caused by the forward radiation, R(x,y), were modeled as functions of the individual X and Y jaw positions. The amount of the backscattered radiation for any field setting was then computed as a compound contribution from both the X and Y jaws. The dose ratios of R(x,y) were then used to calculate the change in photon output caused by the backscatter, Scb(x,y). Results of these calculations were compared with available measured data based on counting the electron pulses or charge from the electron target of an accelerator. Data from this study showed that the backscattered radiation contributes approximately 3% to the monitor-chamber-scored dose. A majority of the backscattered radiation comes from the upper jaws, which are located closer to the monitor chamber. The amount of the backscatter decreases approximately in a linear fashion with the jaw opening. This results in about a 2% increase of photon output from a 10 cm x 10 cm field to a 40 cm x 40 cm field. The off-axis location of the jaw opening does not have a significant effect on the magnitude of the backscatter. The backscatter effect is significant for monitor chambers using kapton windows, particularly for treatment fields using moving jaws. Applying the backscatter correction improves the accuracy of monitor-unit calculation using a model-based dose calculation algorithm such as the convolution method.     

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.     

Modeling the extrafocal radiation and monitor chamber backscatter for photon beam dose calculation

A simple analytical approach has been developed to model extrafocal radiation and monitor chamber backscatter for clinical photon beams. Model parameters for both the extrafocal source and monitor chamber backscatter are determined simultaneously using conventional measured data, i.e., in-air output factors for square and rectangular fields defined by the photon jaws. The model has been applied to 6 MV and 15 MV photon beams produced by a Varian Clinac 2300C/D accelerator. Contributions to the in-air output factor from the extrafocal radiation and monitor chamber backscatter, as predicted by the model, are in good agreement with the measurements. The model can be used to calculate the in-air output factors analytically, with an accuracy of 0.2% for symmetric or asymmetric rectangular fields defined by jaws when the calculation point is at the isocenter and 0.5% when the calculation point is at an extended SSD. For MLC-defined fields, with the jaws at the recommended positions, calculated in-air output factors agree with the measured data to within 0.3% at the isocenter and 0.7% at off-axis positions. The model has been incorporated into a Monte Carlo dose algorithm to calculate the absolute dose distributions in patients or phantoms. For three MLC-defined irregular fields (triangle shape, C-shape, and L-shape), the calculations agree with the measurements to about 1% even for points at off-axis positions. The model will be particularly useful for IMRT dose calculations because it accurately predicts beam output and penumbra dose.     

Comparison of ionization chambers of various volumes for IMRT absolute dose verification

IMRT plans are usually verified by phantom measurements: dose distributions are measured using film and the absolute dose using an ionization chamber. The measured and calculated doses are compared and planned MUs are modified if necessary. To achieve a conformal dose distribution, IMRT fields are composed of small subfields, or "beamlets." The size of beamlets is on the order of 1 x 1 cm2. Therefore, small chambers with sensitive volumes < or = 0.1 cm3 are generally used for absolute dose verification. A dosimetry system consisting of an electrometer, an ion chamber, and connecting cables may exhibit charge leakage. Since chamber sensitivity is proportional to volume, the effect of leakage on the measured charge is relatively greater for small chambers. Furthermore, the charge contribution from beamlets located at significant distances from the point of measurement may be below the small chambers threshold and hence not detected. On the other hand, large (0.6 cm3) chambers used for the dosimetry of conventional external fields are quite sensitive. Since these chambers are long, the electron fluence through them may not be uniform ("temporal" uniformity may not exist in the chamber volume). However, the cumulative, or "spatial" fluence distribution (as indicated by calculated IMRT dose distribution) may become uniform at the chamber location when the delivery of all IMRT fields is completed. Under the condition of "spatial" fluence uniformity, the charge collected by the large chamber may accurately represent the absolute dose delivered by IMRT to the point of measurement. In this work, 0.6, 0.125, and 0.009 cm3 chambers were used for the absolute dose verification for tomographic and step-and-shoot IMRT plans. With the largest, 0.6 cm3 chamber, the measured dose was equal to calculated within 0.5%, when no leakage corrections were made. Without leakage corrections, the error of measurement with a 0.125 cm3 chamber was 2.6% (tomographic IMRT) and 1.5% (step-and-shoot IMRT). When doses measured by a 0.125 cm3 chamber were corrected for leakage, the difference between the calculated and measured doses reduced to 0.5%. Leakage corrected doses obtained with the 0.009 cm3 chamber were within 1.5%-1.7% of calculated doses. Without leakage corrections, the measurement error was 16% (tomographic IMRT) and 7% (step-and-shoot IMRT).     

诊断用螺旋CT在适形放疗模拟定位中的应用分析

目的:分析诊断用螺旋CT用于放疗模拟定位时各种因素对放疗精度的影响,探索降低系统误差、提高放疗几何精确度的方法和措施.方法:在放疗模拟定位过程中,使用诊断用螺旋CT对70例肿瘤患者进行扫描,制定三维治疗计划得到患者正、侧位DDR射野方向照片,在X线模拟定位机下进行对比验证.结果:头颈部肿瘤靶中心点偏差小于3mm,胸部肿...     

Measurement of the dose equivalent of leakage radiation through an isocentric gantry used for neutron therapy

The leakage radiation through the shielding on an isocentric gantry of a neutron therapy machine was measured with a Rossi-type proportional counter. The dose equivalent of the leakage radiation was determined at two positions: (1) in the plane of the patient and (2) in the plane of the target. The dose equivalent of the leakage radiation is approximately the same as the leakage of a high-energy x-ray linac.     

Characterization of an add-on multileaf collimator for electron beam therapy

An add-on multileaf collimator for electrons (eMLC) has been developed that provides computer-controlled beam collimation and isocentric dose delivery. The design parameters result from the design study by Gauer et al (2006 Phys. Med. Biol. 51 5987-6003) and were configured such that a compact and light-weight eMLC with motorized leaves can be industrially manufactured and stably mounted on a conventional linear accelerator. In the present study, the efficiency of an initial computer-controlled prototype was examined according to the design goals and the performance of energy- and intensity-modulated treatment techniques. This study concentrates on the attachment and gantry stability as well as the dosimetric characteristics of central-axis and off-axis dose, field size dependence, collimator scatter, field abutment, radiation leakage and the setting of the accelerator jaws. To provide isocentric irradiation, the eMLC can be placed either 16 or 28 cm above the isocentre through interchangeable holders. The mechanical implementation of this feature results in a maximum field displacement of less than 0.6 mm at 90 degrees and 270 degrees gantry angles. Compared to a 10 x 10 cm applicator at 6-14 MeV, the beam penumbra of the eMLC at a 16 cm collimator-to-isocentre distance is 0.8-0.4 cm greater and the depth-dose curves show a larger build-up effect. Due to the loss in energy dependence of the therapeutic range and the much lower dose output at small beam sizes, a minimum beam size of 3 x 3 cm is necessary to avoid suboptimal dose delivery. Dose output and beam symmetry are not affected by collimator scatter when the central axis is blocked. As a consequence of the broader beam penumbra, uniform dose distributions were measured in the junction region of adjacent beams at perpendicular and oblique beam incidence. However, adjacent beams with a high difference in a beam energy of 6 to 14 MeV generate cold and hot spots of approximately 15% in the abutting region. In order to improve uniformity, the energy of adjacent beams must be limited to 6 to 10 MeV and 10 to 14 MeV respectively. At the maximum available beam energy of 14 MeV, radiation leakage results mainly from the intraleaf leakage of approximately 2.5% relative dose which could be effectively eliminated at off-axis distances remote from the field edge by adjusting the jaw field size to the respective opening of the eMLC. Additionally, the interleaf and leaf-end leakage could be reduced by using a tongue-and-groove leaf shape and adjoining the leaf-ends off-axis respectively.