TrueBeam加速器6 MV非均整模式X射线的蒙特卡罗模拟

目的 对TrueBeam加速器6 MV非均整模式(FFF)X射线蒙特卡罗模拟,寻找最佳的模型参数,为进一步研究6 MV FFF X射线临床剂量学奠定模型基础。方法 借助BEAMnrc和DOSXYZnrc程序,调整入射电子束能量、径向强度分布及角度展宽等参数,对TrueBeam加速器6 MV FFF X射线4 cm×4 cm到40 cm×40 cm射野的百分深度剂量(PDD)和离轴比(OAR)曲线进行蒙特卡罗模拟,比较不同大小射野情况下模拟和测量结果的差异。结果 在入射电子能量为6.1 MeV、半高宽(FWHM)为0.75 mm和角度展宽为0.9°时,模拟结果与相应条件下实际测量结果最接近。不同射野的PDD和30 cm×30 cm及以下射野的OAR曲线与测量数据相比满足Local Dose的限制条件,剂量误差< 1%,位置误差< 1 mm;40 cm×40 cm射野的OAR满足剂量误差<1.5%,位置误差<1 mm的限制条件。结论 本模型模拟结果与实际测量结果一致性较好,可将模型参数用于6 MV FFF X射线临床剂量学研究。     

医用加速器射线参数对百分深度剂量影响的蒙特卡罗模拟

目的 通过蒙特卡罗方法模拟瓦里安IX 6 MV直线加速器治疗机头,得到不同射野下的最适电子线能量,研究径向强度分布对百分深度剂量的影响。方法 首先对所研究的每个射野,保持径向强度大小不变,改变电子线能量,将得到的百分深度剂量与测量值进行对比,得到该射野下的最适电子线能量。随后将电子线能量设置为得到的最适值,改变径向强度分布大小,研究其对百分深度剂量的影响。结果 对于4 cm×4 cm、10 cm×10 cm、20 cm×20 cm和30 cm×30 cm的射野,最适能量分别为5.9、6.0、6.3和6.4 MeV;改变径向强度分布对4 cm×4 cm、10 cm×10 cm射野下的百分深度剂量没有影响,对20 cm×20 cm和30 cm×30 cm的射野则有明显影响。结论 适用于不同射野的最佳能量略有不同,径向强度的改变对大野下的深度剂量有较明显影响。为提高模拟精度,电子线能量和径向强度分布的选取需要考虑射野大小的因素。     

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。     

基于卷积神经网络的立体闪烁光三维剂量重建研究

目的 基于卷积神经网络(convolutional neural networks, CNN)对多视角闪烁光处理,重建放射治疗中三维相对剂量分布。方法 利用互补金属氧化物半导体(CMOS)成像传感器捕获正交三视角的荧光图像,将荧光图像转化为三维图像,输入已训练的卷积神经网络中进行剂量重建,分别评估不同射野重建剂量的伽马通过率、均方误差(MSE)、百分深度剂量(PDD)曲线和横向剂量分布(CBP)曲线。卷积神经网络模型为3D-Unet,其预先在虚拟数据集上进行训练。结果 以50%最大剂量为阈值,3%/3mm为标准,所有射野重建分布中心层面伽马通过率和立体平均伽马通过率均超过90%,均方误差维持在1%以下。所有射野重建分布的PDD曲线均方误差在1‰以下,CBP曲线均方误差在1％以下。结论 本研究实现了一种基于深度学习的三维闪烁光重建方法,完善了基于塑料闪烁体的瞬时三维相对剂量验证。     

半野分段弧照射技术对乳腺癌放疗危及器官的保护作用

目的 利用半野的剂量分布特性和容积调强（VMAT）技术的特点,探索一种可以更好保护肺和心脏的新技术。方法 采用三维水箱测量对称野及半野的射野边缘剂量分布,并比较分析各自特征。回顾性选取50例左侧乳腺癌术后放疗患者,保乳术和根治术各25例,处方剂量50 Gy/25次,基于RayStation计划系统,分别采用对称野连续弧VMAT技术和半野分段弧VMAT技术进行计划设计,比较和分析靶区的剂量适合度、治疗效率,以及心脏、肺等危及器官的各种剂量数据。结果 半野的辐射野大小在水模内不随深度增加而增加,对称野则因张角因素射野逐步变大,30 cm处增大到约2 cm,而且半野的射野外剂量低于对称野,差值愈近射野边缘愈明显。与对称野连续弧计划相比,半野段弧VMAT计划能显著改善肺和心脏的受照射剂量,差异有统计学意义（t=-4.11、-4.42,P=0.00）,其中心脏整体结构的V₅、V₃₀、D_mean均值减少为52.5%、65.5%、47%,与靶区关系紧密的左侧冠状动脉前降支降幅超过20%,患侧肺V₅、V₁₀、V₂₀、D_mean的均值分别减少21.6%、24.8%、25.0%、23.2%,其他正常器官剂量均值,半野段弧计划同样优于连续弧计划。结论 对于乳腺癌放疗,半野与VMAT结合可以充分发挥半野和VMAT的优势,显著改善心脏、患侧肺、健侧乳腺等危及器官的受照射剂量。     

Varian加速器不同的射野形成方式对射野剂量学参数的影响

目的 探讨Varian加速器不同射野形成方式对射野剂量学参数的影响,为治疗计划系统（TPS）数据建模提供理论依据。方法 在准直器（JAW）、多叶光栅（MLC）和准直器跟随多叶光栅（JAW+MLC）3种射野的形成方式下,分别测量百分深度剂量（PDD）、射野离轴量（OAR）及射野总散射因子（Scp）,并对实测数据进行分析比较。结果 3种射野形成方式对中心轴的百分深度剂量影响很小;在加速器的左右方向和枪靶方向,MLC形成的射野均较JAW形成射野大,在左右方向最大可达2.9 mm。在枪靶方向,最大可达1.7 mm。在左右方向MLC形成的射野测量曲线的半影较在相同射野大小JAW形成射野的半影大。在枪靶方向MLC形成的射野测量曲线的半影较在相同射野大小JAW形成射野的半影小。在两个方向 JAW+MLC形成射野与JAW形成射野大小与半影均无明显差异。结论 射野的不同形成方式对射野大小、半影、总散射因子有影响,建议做调强放射治疗（IMRT）时,在TPS数据建模过程中,应对MLC射野的剂量参数进行关注。     

动态钨门技术在中段食管癌螺旋断层调强放疗中的应用

目的 评价螺旋断层调强放疗（TOMO）设备升级后,能否用5.0 cm动态钨门替代2.5 cm固定钨门治疗中段食管癌。方法 对中国医学科学院北京协和医学院肿瘤医院收治的10例局部晚期根治性中段食管癌患者进行研究。在TOMO计划系统分别设计2.5 cm固定钨门（FJ2.5）、2.5 cm动态钨门（DJ2.5）和5.0 cm动态钨门（DJ5.0）计划。比较3种计划的靶区适形度指数（CI）、均匀性指数（HI）和危及器官（OAR）受量以评价计划质量;比较出束时间和机器跳数以评价效率。结果 3种计划的靶区CI和HI均满足临床要求。与DJ5.0计划相比,FJ2.5计划的双肺V₅和平均剂量、正常组织V₅、V₁₀和平均剂量均增加,差异有统计学意义（t=9.751、4.163、11.840、10.321、3.745,P<0.05）,DJ2.5计划的心脏V₃₀、V₄₀、平均剂量和最大剂量、正常组织V₂₀和平均剂量均降低,差异有统计学意义（-2.454、-3.275、-4.192、-6.435、-4.139、-6.431,P<0.05）。与DJ2.5计划相比,FJ2.5计划的双肺V₅、V₂₀、V₃₀和平均剂量、心脏V₃₀和平均剂量、脊髓和脊髓计划体积（PRV）最大剂量、正常组织V₅、V₁₀、V₂₀和平均剂量均增加,差异有统计学意义（t=8.289、6.142、3.137、8.895、3.597、4.565、3.782、5.429、16.421、12.496、8.286、11.933,P<0.05）。与FJ2.5和DJ2.5计划相比,DJ5.0计划的平均出束时间分别缩短43.9%和42.8%,平均机器跳数分别减少42.8%和43.8%。结论 若综合考虑计划质量和执行效率,建议采用5.0 cm动态钨门技术用于中段食管癌螺旋断层调强放疗,不但可以有效缩短治疗时间、提高射线利用率,而且与2.5 cm固定钨门技术相比双肺和正常组织保护更好。若只考虑计划质量,建议采用2.5 cm动态钨门技术,其计划质量好。     

放疗中呼吸运动对不同形状射野剂量分布的影响

目的 利用剂量胶片分析研究放射治疗中呼吸运动对靶区剂量分布的影响。方法应用可内置胶片的QUASAR多功能呼吸运动体模,在运动和静止状态下分别照射正方形、圆形、椭圆形、哑铃形和凹形5种形状的模拟射野。比较其剂量分布的差别。用平板体模在位移为0、0.5、1.0、1.5和2.0cm时分别照射圆形、椭圆形、正方形射野,比较不同靶区位移大小对剂量分布的影响。应用的比较方法包括等剂量线、γ值和NAT比较法。提出面积变化因子F_s(运动状态下面积/静止状态面积或位移不为0时的面积/位移为0时的面积。以F_S90、F_S50、F_S25分别代表90%、50%、25%剂量曲线包围的面积在不同状态下的比值)。结果 与静态下相比,水平运动状态下的F_S90减小,F_S25增大。位移越大,它们偏离程度越大。垂直运动状态,正方形和哑铃形射野的F_s有变化,其余变化很小。γ值和NAT比较:各射野的水平运动状态和静止状态比较,P_γ＜60%和P_NAT＜75%;正方形、圆形、凹形和哑铃形照射野的垂直运动状态与静态下相比,P_γ＜85%;平板体模验证中,P_γ和P_NAT随着位移的增大而减小。结论 呼吸运动对靶区放射治疗的剂量分布的影响表现为沿运动方向高剂量区域内收,低剂量区扩大,这种影响随着肿瘤位移的增大而增大。     

手工计算非对称动态楔形野处方剂量的校正

目的 研究非对称射野情况下使用动态楔形板时手工计算处方剂量的校正。方法 利用VarianEclipse治疗计划系统和23EX加速器的数据计算射野分别为6cm×6cm、8cm×8cm、10cm×10cm、12cm×12cm、14cm×14cm、16cm×16cm、18cm×18cm、20cm×20cm的处方剂量。计算时保持射野不变,非楔方向为对称,改变楔形方向的准直器大小,使射野的几何中心与等中心的距离以1cm为步长递增。动态楔形板度数取10°、15°、20°、25°、30°、45°和60°,能量取6和10MV。根据计算结果模拟出射野几何中心与等中心的距离与校正因子之间的关系曲线图。选择有代表性的角度和射野,利用该校正因子对手工计算所得到的结果进行校正,并进行实际测量,验证结果是否在误差允许范围内。结果 射野大小对校正因子的影响很小,所以取不同射野时的平均值作为实际计算时使用的校正因子。不做校正时,能量为6MVX线的情况下,30°楔形板最大误差可达18%,45°楔形板最大误差可达30%,与实际所需要的处方剂量相差很多,校正以后测量结果的误差范围分别为-1.8%～0.09%和-1.8%～-0.25%,该误差大小可以接受。结论 在非对称动态楔形野的情况下,手工计算时采用对称野的楔形因子得到的处方剂量与实际治疗时应该使用的处方剂量有很大差别,采用校正因子校正后,误差缩小到临床能够接受的范围。     

螺旋断层放射治疗室内泄漏辐射与散射辐射监测与评价研究

目的 检测螺旋断层放射治疗(TOMO)室内泄漏辐射与散射辐射的剂量水平,揭示TOMO室内与常规放射治疗不同的剂量分布规律,为TOMO放射防护提供科学依据。方法 以Tomotherapy Hi-Art TOMO装置为出束治疗设备,在模拟治疗照射条件下累积出束100 Gy,使用GR-200A型LiF(Mg,Cu,P)热释光剂量计(TLD),测量治疗床平面不同方向和TOMO室内空间泄漏辐射和散射辐射的空气比释动能,求出各测量位点的泄漏辐射比和散漏(散射+泄漏)辐射比(辐射比为测量点的辐射剂量与等中心点处标准治疗的输出剂量之比)。结果 TOMO室内泄漏辐射与散射辐射的剂量水平均基本以旋转等中心和治疗床纵轴呈左右对称分布,治疗设备前方的辐射水平明显高于设备后方;治疗床平面上,距等中心100 cm处的泄漏辐射比最高值仅为1.3×10^-4,距等中心300 cm处的泄漏辐射比均不高于2.0×10^-5;距等中心200~300 cm处的散射辐射仅为相应位置泄漏辐射的25%~30%,室内散射辐射相对于泄漏辐射衰减更快。结论 TOMO室内泄漏和散射辐射水平远低于常规放射治疗室内的辐射水平。     

The effect of a metal hip prosthesis on the radiation dose in therapeutic photon beam irradiations.

Prostate and cervical cancer patients are often treated with external X-ray beams of bi-lateral incidence. Such treatment may incur some dose effect that cannot be predicted precisely in commercial treatment planning systems (TPS) for patients having undergone total hip replacement. This study performs a Monte Carlo (MC) simulation and an analytical calculation (convolution superposition algorithm which is implemented in ADAC TPS) of a 6 MV, 5 x 5 cm2 X-ray beam incident into water with the existence of hip prosthesis, e.g. Ti6A14V and CoCrMo alloy. The results indicate that ADAC TPS cannot precisely account for the scatter and backscatter radiation that a metal hip prosthesis causes. For percent depth dose (PDD) curves, the maximum underdosage of ADAC TPS up to 5mm above the interface between dense material and water is 5%, 20% and 27% for PDD(Bone), PDD(Ti) and PDD(Co), respectively. The dose re-buildup, which occurs behind the hip region, becomes more and more obvious for denser medium existed in water. Increasing inhomogeneity also enhances the underdosage of ADAC for greater depth (> 10cm), as the figures of nearly 2% in PDD(Bone), PDD(Ti) and 4-5% in PDD(Co) reveal. Overestimating the attenuated power of high-density non-water material in ADAC TPS causes this underdosage. For dose profiles, no significant differences were found in Profile(Bone) at any depth. Profile(Ti) reveals that MC slightly exceeds ADAC at off-axis position 1.0-2.0 cm. Profile(Co) reveals this more obviously. This finding means that scatter radiation from these denser materials is significant and cannot be predicted precisely in ADAC.     

Effect of a carbon fiber tabletop on the surface dose and attenuation for high-energy photon beams

Purpose  The dose changes in the buildup region and beam attenuation by a carbon fiber tabletop were investigated for 6-and 18-MV photon beams. Materials and methods  Measurements were performed for 2 × 2 cm to 40 × 40 cm field sizes. The surface dose and percentage depth doses (PDD) were measured by a Markus parallel plate chamber. Attenuation measurements were made at the cylindrical phantom for 180° rotation of the beam. Results  A carbon fiber tabletop increases the surface dose from 7.5% to 63.0% and from 4% to 43% for small fields at 6 and 18 MV, respectively. The increase was nearly fivefold for the 10 × 10 cm field and nearly twofold for the 40 × 40 cm field. Beam attenuation of the tabletop varies from 3.0% to 5.6% for 180° and 120° gantry angles for 6 MV. Conclusion  The carbon fiber tabletop significantly decreases the skin-sparing effect. The dosimetric effect of the tabletop may be higher, especially for the intensity-modulated radiation therapy depending on the beam orientation. Attenuation should be considered and corrected such as any material under the patient at the treatment planning stage.     

Verification of a simple point dose calculation method for a dual energy linear accelerator with asymmetric jaws

Certain fundamental dosimetrical parameters involving the applications of asymmetric jaws were investigated. The nominal accelerating potentials (NAPs) were found to decrease from 5.1 to 4.2 and from 18.0 to 13.4 for the 6 and 18 MV beams, respectively, as the off-axis distance (OAD) increases from 0.0 to 15.0 cm. The relative beam intensity increases from 1.00 to 1.07 at OAD of 15.0 cm for the 6 MV beam, and to 1.02 at OAD of 7.0 cm for the 18 MV beam. The percentage depth doses (PDDs) for half-blocked fields of 4 × 4 cm, 10 × 10 cm and 20 × 20 cm were found to deviate from those of corresponding symmetric fields by less than 2% down to the depth of 35.0 cm. The field size factor (FSF) for the asymmetric field from 4 × 4 cm to 20 × 20 cm deviates less than 1.0% from those of the corresponding symmetric fields. The equivalent square concept was found to be applicable to asymmetric fields within 1% error if the jaw exchange effect is taken into consideration. The measured point doses for half-blocked fields of 4 × 4 cm, 10 × 10 cm and 20 × 20 cm for both 6 and 18 MV were within 3% of the calculated dose based on a published dose calculation method which employs symmetric field beam parameters, such as field size factor (FSF), percentage depth dose (PDD), and off-axis correction factors (OAFs). The efficacy of this point dose calculation method is discussed.     

A comparison of small-field tissue phantom ratio data generation methods for an Elekta Agility 6 MV photon beam

Tissue-phantom ratios (TPRs) are a common dosimetric quantity used to describe the change in dose with depth in tissue. These can be challenging and time consuming to measure. The conversion of percentage depth dose (PDD) data using standard formulae is widely employed as an alternative method in generating TPR. However, the applicability of these formulae for small fields has been questioned in the literature. Functional representation has also been proposed for small-field TPR production. This article compares measured TPR data for small 6 MV photon fields against that generated by conversion of PDD using standard formulae to assess the efficacy of the conversion data. By functionally fitting the measured TPR data for square fields greater than 4 cm in length, the TPR curves for smaller fields are generated and compared with measurements. TPRs and PDDs were measured in a water tank for a range of square field sizes. The PDDs were converted to TPRs using standard formulae. TPRs for fields of 4 × 4 cm² and larger were used to create functional fits. The parameterization coefficients were used to construct extrapolated TPR curves for 1 × 1 cm², 2 × 2-cm², and 3 × 3-cm² fields. The TPR data generated using standard formulae were in excellent agreement with direct TPR measurements. The TPR data for 1 × 1-cm², 2 × 2-cm², and 3 × 3-cm² fields created by extrapolation of the larger field functional fits gave inaccurate initial results. The corresponding mean differences for the 3 fields were 4.0%, 2.0%, and 0.9%. Generation of TPR data using a standard PDD-conversion methodology has been shown to give good agreement with our directly measured data for small fields. However, extrapolation of TPR data using the functional fit to fields of 4 × 4 cm² or larger resulted in generation of TPR curves that did not compare well with the measured data.     

EBT GAFCHROMICTM film dosimetry in compensator-based intensity modulated radiation therapy

The electron benefit transfer (EBT) GAFCHROMIC films possess a number of features making them appropriate for high-quality dosimetry in intensity-modulated radiation therapy (IMRT). Compensators to deliver IMRT are known to change the beam-energy spectrum as well as to produce scattered photons and to contaminate electrons; therefore, the accuracy and validity of EBT-film dosimetry in compensator-based IMRT should be investigated. Percentage-depth doses and lateral-beam profiles were measured using EBT films in perpendicular orientation with respect to 6 and 18 MV photon beam energies for: (1) different thicknesses of cerrobend slab (open, 1.0, 2.0, 4.0, and 6.0 cm), field sizes (5×5, 10×10, and 20×20 cm²), and measurement depths (D_max, 5.0 and 10.0 cm); and (2) step-wedged compensator in a solid phantom. To verify results, same measurements were implemented using a 0.125 cm³ ionization chamber in a water phantom and also in Monte Carlo simulations using the Monte Carlo N-particle radiation transport computer code. The mean energy of photons was increased due to beam hardening in comparison with open fields at both 6 and 18 MV energies. For a 20×20 cm² field size of a 6 MV photon beam and a 6.0 cm thick block, the surface dose decreased by about 12% and percentage-depth doses increased up to 3% at 30.0 cm depth, due to the beam-hardening effect induced by the block. In contrast, at 18 MV, the surface dose increased by about 8% and depth dose reduced by 3% at 30.0 cm depth. The penumbral widths (80% to 20%) increase with block thickness, field size, and beam energy. The EBT film results were in good agreement with the ionization chamber dose profiles and Monte Carlo N-particle radiation transport computer code simulation behind the step-wedged compensator. Also, there was a good agreement between the EBT-film and the treatment-planning results on the anthropomorphic phantom. The EBT films can be accurately used as a 2D dosimeter for dose verification and quality assurance of compensator-based C-IMRT.     

N-isopropylacrylamide gel dosimeter to evaluate clinical photon beam characteristics

The introduction of beam intensity control concept in current radiotherapy techniques has increased treatment planning complexity. Thus, small-field dose measurement has become increasingly vital. Polymer gel dosimetry method is widely studied. It is the only dose measurement tool that provides 3D dose distribution. This study aims to use an N-isopropylacrylamide (NIPAM) gel dosimeter to conduct beam performance measurements of percentage depth dose (PDD), beam flatness, and symmetry for photon beams with field sizes of 3×3 and 4×4 cm². Computed tomography scans were used to readout the gel dosimeters. In the PDD measurement, the NIPAM gel dosimeter and Gafchromic^TM EBT3 radiochromic film displayed high consistency in the region deeper than the build-up region. The gel dosimeter dose profile had 3% lower flatness and symmetry measurement at 5 cm depth for different fields compared with that of the Gafchromic^TM EBT3 film. During gamma evaluation under 3%/3 mm dose difference/distance-to-agreement standard, the pass rates of the polymer gel dosimeter to the TPS and EBT3 film were both higher than 96%. Given that the gel is tissue equivalent, it did not exhibit the energy dependence problems of radiochromic films. Therefore, the practical use of NIPAM polymer gel dosimeters is enhanced in clinical dose verification.