放射物理

 

 

OP07 Benchmarking Tests of Contemporary SRS Platforms: Have Technological Developments Resulted in Improved Treatment Plan Quality? Ian PADDICK (London, United Kingdom)

OP07当代SRS平台的基准测试:技术的发展是否提高了治疗方案的质量?

介绍

SRS设备的技术进步是显著的，在适形性、梯度和精度方面的要求是最高的。在2016年英国基准研究(Eaton等) 6年后，新技术提出了这样一个问题:“技术的改进是否导致了治疗计划的相应改进?”（new technology poses the question “have technological improvements led to a corresponding improvement in treatment plans?”.）

这项基准研究（This benchmarking study）评估了以下被选为2022年“最先进技术水平（‘state of the art’）”平台的能力:装备Lightning逆向计划设计的Icon型伽玛刀(GK)，装备S7与M6 MLC的射波刀, BrainLab Elements (Elekta VersaHD和Varian TrueBeam)，装备HyperArc的 Varian Edge(6X-FFF和10X-FFF)， Zap-X。

方法和材料

从既往研究中提供6例(2例多发性转移，4例良性靶体)患者。为了反映每个患者治疗转移瘤数目增加的演变，增加了一个有14个靶体的病例。7例患者中的28个靶体积在0.02 - 7.2cc之间[Six cases (two multiple metastasis cases, four benign targets) were used from the previous study. In order to reflect the evolution of the increased number of metastases treated per patient, a case with 14 targets was added. 28 targets amongst the seven patients ranged from 0.02cc to 7.2cc in volume. ]。

参与中心收到了DICOMRT文件，其中包括每个治疗计划的图像、靶区轮廓和潜在的危及器官。他们被要求使用有经验的工作人员(定义为在相关平台上至少有两年的经验)，以他们的最好能力计划每次治疗。

虽然在当地的实践中允许一些变化(如;使用边缘扩展)，各组被要求为每个靶体指定剂量，并事先商定危及器官的耐受剂量。

两种方案之间的比较参数包括覆盖率、选择性、Paddick适形性指数(PCI)、梯度指数(GI)、R50%、效率指数、危及器官剂量、估计计划时间和估计治疗时间[Parameters used for comparison between the plans, included coverage, selectivity, Paddick Conformity Index (PCI), Gradient Index (GI), R50%, Efficiency Index, doses to OARs, estimated planning and estimated treatment time. ]。

结果

所有靶区的平均覆盖率范围从98.2% (Brainlab/Elekta)到99.7% (Hyperarc 6X)。PCI值范围从0.722 (Zap-X)到0.900 (CyberKnife)。GI的平均值从3.15 (Zap-X)(代表最陡的剂量梯度)到5.08 (HyperArc 10X)不等。GI呈现出射线束能量的变化趋势，射线束能量的最低值来自较低的能量平台(Zap-X;3 mv,GK;1.25MeV)和最高值来自最高能量(HyperArc 10X)。

R50%值(不包括病例7)结合适形性指数和梯度指数，最小平均值为3.65 (GK)，最大平均值为4.76 (Hyperarc 10X)。改良LINACs治疗时间最短。

结论

与早期的研究相比，新的设备似乎能提供更高质量的治疗。CyberKnife和Linac平台具有较好的适形性，而能量较低的平台具有较好的剂量梯度。

OP08 Evaluation of a novel dose optimization software Leksell Gamma Knife Lightning – comparison of treatment plans for 40 challenging clinical cases.

Josef NOVOTNY (Prague, Czech Republic)

OP08比较40个具有挑战性的临床病例评估新的剂量优化软件:Leksell伽玛刀Lightning-治疗计划。

目的:ICON 型Leksell伽玛刀的治疗规划有三种方法:1)手动计划，2)Icon逆向计划，3)Lightning剂量优化。在前两种方法中，在靶体积内放置合理数目的等中心，重叠相对较小。可以使用各种加权因子和混合等中心(4、8、16毫米准直器与挡块的混合)。相反，Lightning使用了非常多的等中心，并且有非常大的重叠。它可以被描述为由于个别等中心位置变化非常小的剂量分布的“绘制”[There are three approaches in treatment planning for Leksell Gamma Knife Icon: 1) manual, 2) inverse planner for Icon and 3) Lightning dose optimization. In the first two methods a reasonable number of isocenters are being placed inside the target volume with relatively small overlap. Various weighting factors and hybrid isocenters (mixture of 4, 8, 16 mm collimators together with blocks) can be used. In opposite the Lightning is using a very large number of isocenters with a very large overlap. It can be described as a “painting” of dose distribution due to very small position change in individual isocenters. ]。

材料与方法:选择40例患者(10例脑膜瘤、10例听神经鞘瘤、10例垂体腺瘤、10例转移瘤)进行比较。选择治疗体积较大的患者(1.8 - 23.0 cm³，中位7.6 cm³)和有挑战性的病例。采用靶区平均剂量、靶区覆盖率、选择性、梯度指数、Shaw适形性指数、12Gy体积、80%和90%等剂量线体积、视神经、脑干和耳蜗的最大受照剂量、耳蜗和垂体的平均受照剂量、射线束照射时长和使用等中心点数目等参数评估新治疗方案的疗效。测量了Lightning计算治疗方案的时间[ Following parameters were used to assess benefits in new treatment planning approach: target mean dose, target coverage, selectivity, gradient index, Shaw conformity index, volume of 12 Gy, volume of 80% and 90% isodose, maximal dose to optic nerve, brainstem and cochlea, mean dose to cochlea and pituitary, beam-on time and number of isocenters used. Time for Lightning to calculate the treatment plan was also measured.   ]。

结果:使用Lightning法计算所有病例的时间极短(14 ~ 108 s，中位数35 s)。在相同的靶体积覆盖范围(中位值0.99)下，Lightning总是使用更多的等中心点(16 - 86，中值41)来实现目标。下面的百分比是给出的中位数，以比较Lightning与以前的方法。靶区平均剂量降低5.7%，选择性提高8.9%，梯度指数提高0.2%，Shaw适形指数提高8.7%，12 Gy体积减少5.2%，体积80%和90%等剂量分别提高9.1%和5.0%。关键结构的受照剂量改善了12.3%(视神经)，8.9%(耳蜗)，5.0%(垂体)。Lightning的射线束开启时长减少14.4%[Extremely short time (14 – 108 s, median 35 s) was observed for calculation of all cases when using Lightning. With the same target volume coverage (median 0.99), Lightning used always more (16 - 86, median 41) isocenters to achieve the goal. Following percentages are given for medians to compare Lightning with former methods. Target mean dose was reduced by 5.7%, selectivity improved by 8.9%, gradient index improved by 0.2 %, Shaw conformity index improved by 8.7%, volume of 12 Gy was reduced by 5.2%, volume of 80% and 90% isodose increased by 9.1% and 5.0%, respectively. Doses to critical structures improved by 12.3% (optic), 8.9% (cochlea), 5.0% (pituitary). Beam-on time was reduced in the case of Lightning by 14.4%.]。

结论:在实际应用中，Lightning剂量优化软件优于以往的方法。在剂量学特性方面，它甚至能产生无法更好的计划，关键结构受照剂量较低，但也能产生射线束开启时长更短的计划[Practically in all studied parameters Lightning dose optimization software was superior to former methods. It is capable to generate not even better plans in terms of dosimetry characteristics, lower doses to critical structures but also plans with shorter beam-on time.]。

OP22 Appliance of CBCT of Leksell Gamma Knife Icon for improving accuracy of stereotactic radiosurgery.

Viacheslav RAK, Greg KOYNASH (Moscow, Russia)

OP22应用ICON Leksell伽玛刀CBCT提高立体定向放射外科的准确性。

G型Leksell框架是立体定向放射外科中著名的固定装置。有观点认为基于基准的CT配准的准确性优于MRI。一些中心只使用MRI扫描进行立体定向放射外科，没有任何临床问题。然而，不同型号的MRI扫描仪和协议具有不同程度的几何畸变，因此1毫米的精度是世界范围内公认的。新型的伽玛刀Icon性内置锥形束CT模块，具有质量保证工具。CBCT的温和校准会导致更高水平的预期精度(小于0.1 mm)。因此，在开始治疗前，可以确定每个患者的MRI畸变。由于固定螺钉过紧导致的框架部分位移和对角框架变形，以及成像误差，都可能导致治疗过程中准确性的丧失（The partial displacement of the frame and in opposite frame deformation due to overtightening the fixation screws in addition to imaging error, can lead to lost of accuracy during treatment. ）。

本研究的目的是在综合CBCT模块的帮助下评估MRI基准确定的立体定向空间的几何偏差（The purpose of the research is to assess geometrical deviation of the stereotactic space defined by MRI fiducials with the help of integrated CBCT module. ）。

我们分析了110例患者的3 tesla MRI和CBCT的平均和轴向差异。中位X、Y、Z线性位移( linear shift)分别为0.05 mm、-0.05 mm和0.6 mm。中位X、Y和Z轴向位移（axial shift）分别为0.72º、0.01º和-0、11º。中位最大移位位移（Median maximal shift displacement ）为1,09 mm。80.9%的病例需要基于CBCT的定义。在19.1%的患者中发现了可接受的位移。作出决定的原因是覆盖范围缺失(低于95%)，进行功能性放射外科治疗，超出关键结构的耐受剂量或位移超过0.5 mm。1例后侧螺钉移位，最大螺钉移位4.54 mm[CBCT-based definition was needed in 80.9% cases. Acceptable shift was found in 19,1% cases. The reasons for a decision were loss of coverage (below 95%), excess of tolerated dose to critical structures or shift more than 0.5 mm for functional radiosurgery. In one case, there was a shift of posterior screw with maximal shot displacement of 4.54 mm. ]。

相关分析显示，前立柱长度与x轴移位(p=0.02)、x轴旋转(p=0.003)、y轴旋转(p=0.001)呈正相关。后联合的z坐标与最大靶点位移呈强负相关(p=0.000006)。我们还发现MRI平均基准误差与x偏移(p=0.04)、MRI最大基准误差与y偏移(p=0,04)之间也存在正相关。计算出的位移在短柱组(29%)和长柱组(71%)之间没有显著差异[between length of anterior post and X-shift (p=0.02), X-rotation (p=0.003), Y-rotation (p=0.001). Strong negative correlation was shown between Z-coordinate of posterior commissure and maximal shot shift (p=0.000006). We have found also positive correlations between mean MRI fiducial error and X-shift (p=0.04), maximal MRI fiducial error and Y-shift (p=0,04). The calculated shift did not significantly differ between groups with short (29%) or long (71%) posts of the frame. ]。

综上所述，计算出的立体定向空间偏差与框架构型、目标z坐标等因素有关。应用CBCT可防止立体定向框架的局部移位，减少MRI畸变和框架变形对治疗准确性的影响[the calculated deviation of stereotactic space can depend on a number of factors like configuration of frame and Z-coordinate of target. Appliance of CBCT can prevent partial displacement of the stereotactic frame, reduce the impact of MRI distortion and frame deformation on accuracy of treatment.]。

 

 

以下未翻译

 

OP09 Implementation of IAEA TRS 483 in small field dosimetry of Leksell Gamma Knife Icon – transition from IAEA TRS 398 to IAEA TRS 483.

Josef NOVOTNY (Prague, Czech Republic)

OP09原子能机构TRS 483在ICON 型Leksell伽玛刀标小野剂量测量中的应用——从原子能机构TRS 398过渡到原子能机构TRS 483。

OP09 Implementation of IAEA TRS 483 in small field dosimetry of Leksell Gamma Knife Icon – transition from IAEA TRS 398 to IAEA TRS 483.

 

Purpose: Traditional dosimetry calibration of small Leksell Gamma Knife (LGK) beams was based on IAEA TRS 398 protocol. New IAEA TRS 483 protocol is available since 2017. Contrary to TRS 398, new small field TRS 483 protocol takes into account non-standard conditions e.g. very small field size, specific geometry, phantom used for measurement and etc. The purpose of this study was to perform transition from TRS 398 to TRS 483.

Materials and Method: Two Elekta plastic spherical phantoms were used: 1) acrylonitrile butadiene styrene (ABS) and 2) Solid Water (SW). Special inserts were made in each phantom to accommodate PTW 31010 Semiflex ion chamber with sensitive volume 0.125 cm3 (used for absolute dose calibration) and PTW 60019 microDiamond detector with sensitive volume 0.004 mm3 (used for output factors (OF) measurement). PTW Unidos electrometer was used for both absolute and relative dosimetry. Both TRS 398 and TRS 483 protocols and both ABS and SW phantoms were used for absolute and relative dosimetry.

Results: The optimal conditions for dose rate measurement are in SW phantom and following TRS 483 protocol. SW phantom is almost water equivalent (and thus very small corrections need to be applied), better mimics real clinical situation (patient fixation in treatment position) and due to longitudinal ion chamber orientation minimizing stem effect. Other results showed following deviations compare to SW and TRS 483: -1.97%, -0.55% and -0.37% for ABS phantom and TRS 398, for ABS phantom and TRS 483 and for SW phantom and TRS 398, respectively. OF measurements with microDiamond in ABS phantom for 8 mm collimator showed -0.1% and 0.6% deviation to Monte-Carlo calculated vendor default values when using TRS 398 and TRS 483, respectively, for 4 mm 2.1% and 1.4% deviation for TRS 398 and TRS 483, respectively. OF measurements with microDiamond in SW phantom for 8 mm collimator showed -1.5% and -1.0% deviation when using TRS 398 and TRS 483, respectively, for 4 mm 2.1% and 1.4% deviation for TRS 398 and TRS 483, respectively.

Conclusions: Re-calibration of LGK Icon was made based on TRS 483 protocol which better reflects small field dosimetry conditions. Relatively small (within 2%) deviations to existing calibration and default OF values were observed.

OP10 Gantry triggered x-ray verification of patient positioning during single-isocenter stereotactic radiosurgery using ExacTrac Dynamic: increasing certainty of lesion localization.

Adrián GUTIÉRREZ (Brussels, Belgium)

在单等中心立体定向放射外科中使用ExacTrac Dynamic  Gantry触发x线检查患者定位:增加病灶定位的确定性。

B16 ORAL PRESENTATIONS

Introduction and purpose

Single-isocenter linac-based stereotactic radiosurgery (SRS) has emerged as a dedicated treatment option for multiple brain metastases. To do so, image-guidance for patient positioning and motion management is becoming very important. The purpose of this study was to analyze the translational and rotational intra-fraction errors during SRS, by applying surface-guidance coupled with gantry triggered stereoscopic x-ray verifications during the arc delivery. The benefits of such a positioning system were also assessed.

Materials and methods

Treatments were planned with non- coplanar dynamic conformal arcs for 24 patients corresponding to 93 brain lesions. Intra-arc positioning errors were measured using stereoscopic x-rays (ExacTrac Dynamic, BrainLAB, Munchen, Germany), triggered in the middle of every treatment arc (234 arcs in total). Couch corrections above 0. mm and 0.5° are always applied. Intra-arc positioning data was analyzed and compared to those of a previous study in our department, where intra-fraction stereoscopic x-rays were only taken after each couch rotation.

Results and discussion

Intra-arc errors ranged between 0 mm and 1.64mm for translations and 0° and 0.88° for rotations (Figure 1). Total 3D displacement ranged between 0.03 mm and 1.64mm. 95th percentiles of errors across all arcs delivered were 0.58mm, 0.47mm and 0.32mm for longitudinal, lateral and vertical displacements, and 0.46°, 0.27° and 0.43° for roll, pitch and yaw rotations respectively. Mean errors across all patients were 0.18mm, 0.07mm and 0.16mm for longitudinal, lateral and vertical displacements, and 0.13°, 0.12° and 0.11° for roll, pitch and yaw rotations (Table 1). 6 out of 24 patients showed at least one arc above the correction thresholds (0.7mm for translations, 0.5° for rotations), corresponding to 17 treatment arcs (7% of delivered beams). When compared to inter-beam errors measured after table rotation, the mean errors measured were considerably smaller (Figure 2), ranging from 38.2% (lateral) to 80% (longitudinal) reduction.

Conclusions

Gantry triggered x-ray verification provides information of the real position of the patient during irradiation and allows verification of the couch corrections performed before every arc. When comparing inter-arc and intra-arc positioning errors, we could identify table rotation as an important source of patient motion. A beam-off strategy is to be considered when measured intra-arc errors are out of tolerance, as the frequency of corrections would not increase treatment times considerably. Intra-arc monitoring and correction with stereoscopic x-rays increases the certainty of lesion localization, making a 0 mm margin strategy possible.

OP11 Evaluation of the timing and quality of a reference beam model-based “short” commissioning.

Giacomo REGGIORI (Milan, Italy)

OP11 基于参考射线束模型的“短”调试的时间和质量评估。

OP11 Evaluation of the timing and quality of a reference beam model-based “short” commissioning.

Purpose and objective

Commissioning measurements are time-consuming and require high precision in execution. Reference Beam Models (RBM) consist of predefined Pencil Beam and Monte Carlo dose profiles that may dramatically reduce the number of measurements necessary to commission a beam. The purpose of this work was to evaluate the accuracy and robustness of using the RBMs offered by BrainLab®(Munich, Germany) with the treatment planning system (TPS) Elements® for multiple brain metastases.

Materials and method

The 6MV and 10MVFFF beams of a TrueBeamSTX Linac were considered. The Linac was equipped with a HD120 MultiLeafCollimator (MLC) whose central leaves have a width of 2.5mm at isocenter.

A Beamscan water tank (PTW, Freiburg, Germany) was used with a SSD=900. Absolute dose was measured at isocenter with a Farmer-type calibrated ion chamber for a 10x10cm2 field. Profiles and PDDs were measured for 4 different MLC-defined square fields ranging from 5x5 to 220x220mm2. Output Factors were measured for the same fields and in the same set-up. A PTW MicroDiamond detector and a 0.125cc PTW Semiflex 3D ion chamber were used for all measurements. A comparison between these measurements and calculations performed in a virtual water phantom with MC-Elements and Acuros algorithms were performed.

Once the TPS was configured, some “simple” plans (i.e. without MLC) and 5 patients were planned with the Multiple Brain Metastases module and delivered. The dose distribution was verified with three different methods. The 2D fluence distribution was evaluated with Portal Dosimetry. The log-file reconstructed 3D dose distribution was evaluated with an indipendent algorithm (M3D, Mobius). The measured 3D dose distribution was evaluated with the octavius detector.

Results

The total time required for the commissioning measurements was less than 6 hours. The best agreement between measured and modeled values both for OFs and profiles was obtained selecting a spot size of 0.4mm and 0.Xmm for 6MV and 10MVFFF beams respectively (figure 1). Calculated OFs were within 1.6% for all field sizes except for the 5x5mm were it was 4.8% (figure 2). The 3%-3mm 3DGamma >96.3% (96.3%-99.8%) for the “simple” plans. Gamma values for the 5 clinical plans were 99.5%-100% for Portal Dosimetry, 99.8%-100% for the M3D calculation and 97.3%-99.1% for the Octavius4D measurements.

Conclusion

Machine commissioning times are dramatically reduced and compatible with clinical practice. The configuration and selection of the RBM is simple and intuitive. Good agreement between measured and calculated dose distributions was observed down to very small field sizes.

OP12 A comparative dosimetric study of Pencil Beam, Acuros XB, and Monte Carlo algorithms for stereotactic body radiosurgery of lung lesions.

Javad RAHIMIAN (Los Angeles, USA),

Pencil Beam、Acuros XB和蒙特卡罗算法在立体定向体部放射外科治疗肺部病变中的剂量比较研究。

（略）

OP19 - WITHDRAWN - Image-guided margin assessment to LINAC-based radiosurgery for single and multiple brain metastases based on post-treatment CBCT shifts.

Tatsiana REYNOLDS (St Paul, USA)

OP19 - withdraw -对基于LINAC的放射外科治疗单个和多个脑转移瘤基于治疗后CBCT偏移的图像引导边缘扩展评估。

目的:

使用非共面HyperArc™(Varian)技术的单等中心基于LINAC的立体定向放射治疗(SRS)的目标是为脑转移瘤(BM)患者提供高精度治疗，同时保留正常脑组织，以避免放射性坏死等并发症。基于LINAC的SRS是可取的，因为患者舒适和较短的治疗时间。

计划靶体积(PTV)边缘扩展对于靶向肿瘤体积(GTV)和避免几何缺失是至关重要的。增加PTV边缘扩展可能会增加放射性坏死的风险。因此，设置适当的PTV边缘扩展对于SRS治疗至关重要。

本研究的目的是基于治疗后锥束CT (CBCT)偏移及其对单个和多个脑转移瘤患者靶区覆盖的剂量影响，提供图像引导的边缘扩展评估。

材料/方法:

对55例脑转移瘤患者117个脑部病变进行回顾性分析。所有患者均使用Encompass支持装置(Qfix)固定，并计划使用HyperArc技术。方案包括52个单发BM方案，17个多发BM方案(病灶数量2 ~ 7个)。所有多发BM靶区都在规划等中心6cm以内。在治疗后CBCT基础上，总共评估了120个单发BM组分和72个多发BM。使用MIM软件评估由于分割内运动导致的靶区覆盖率损失。将治疗后CBCT的转换应用于计划CT，并评估PTV/GTV的剂量学覆盖率。

结果:

为了评估因分割内运动导致的靶区覆盖损失，我们考虑了117例单发BM。其中25例(21%)患者边缘扩展0- 1mm，92例(79%)患者PTV边缘扩展为2mm。在0- 1mm边缘扩展的患者中，PTV和GTV的靶覆盖率明显下降。PTV和GTV的靶覆盖率损失分别高达40%和28%，PTV和GTV的靶覆盖率损失分别为10.57±8.80%和6.51±8.16%。相比之下，2毫米的计划显示最大PTV靶覆盖率损失为16%，平均为4.14±3.34%。GTV损失最大者1%，平均值为0.04±0.11%。对于所有多发性转移的BM患者，使用2毫米边缘扩展，没有明显的GTV覆盖率损失。

结论:

该研究表明，基于治疗后CBCT偏移分析的2毫米边缘扩展足以单个等中心治疗单发和多发脑转移瘤患者。

OP20 Dosimetric impact of setup errors in single-isocenter VMAT radiosurgery for multiple brain metastases.

Valeria FACCENDA (Monza, Italy)

OP20单等中心VMAT放射外科治疗多发性脑转移瘤中设置误差的剂量影响

目的

在立体定向放射外科治疗(SRS)和分割立体定向放射治疗(fSRS)治疗多个脑转移瘤(BM)中，采用单等中心体积拉弧治疗(VMAT)，分次定位误差可能影响靶区覆盖。本研究旨在调查几何和剂量测量精度在这类应用。

材料和方法

对28例(79 BM)采用单等中心共面FFF-VMAT技术治疗的患者进行分析。PTV由2mm各向同性GTV展开定义。用锥形束CT (CBCT)评估治疗前设置误差，用机器人六割自由度治疗床校正。每次分割的分割内误差通过治疗后CBCT测量，并应用于定位CT。使用摩纳哥蒙特卡洛TPS重新计算了涉及平移和旋转(Fx-plan)的计划。比较原始方案和Fx方案的剂量学参数，进行wilcox - mann - whitney检验(alpha=0.05)。研究了两种方案的BM体积、最大维数、等中心距离和重心漂移与靶区覆盖差的关系。

结果

治疗后三维误差中位数为0.4 mm(0.1-1.5)，最大旋转误差中位数为0.3°(0.1-1.2)。因此，BM重心偏移中值在原始图和Fx图之间为0.5 mm(0.1-2.7)。GTV中位容积为0.16 cc (0.01 ~ 3.91)， PTV中位容积为0.72 cc(0.12 ~ 7.46)。BM最大尺寸中值为9.4 mm(2.9 ~ 24.0)，距等中心距离中位值为5.11 cm(0.89 ~ 7.52)。只有2个BM(1例患者)的GTV D95%降低>4%，而在61个病灶(17例患者)中观察到Fx计划的覆盖率降低>1%。PTV D95%平均下降1.4%，31例(16例)PTV出现1%的剂量减少。Fx -plan观察到的脑V12Gy (SRS)和V20Gy (fSRS)平均增加0.4%(-0.6-3.6)。剂量学比较无统计学意义(p>0.05)。靶区覆盖率的差异与BM体积、最大维数和距离等中心之间的相关性不佳，但与BM重心漂移之间的线性回归可以接受:GTV和PTV D95%变化的R2=0.45和R2=0.50。

结论

由于最优的患者设置，以及完整的六自由度校正，安全的PTV边缘扩展，和快速的射线束传输，残留设置和患者运动误差对多个转移瘤病例的剂量效应是可以忽略不计的。这些发现证实了该治疗技术可能降低PTV边缘扩展。

OP21 Improved Small Field Dosimetry for Radiosurgery Planning through Optimized MLC Modeling.

Lauren WEINSTEIN (South San Francisco, USA)

OP21通过优化MLC建模改进小野放射外科计划剂量。

目的:优化多叶准直器(MLC)参数是放射外科手术规划中射线束精确建模的关键，特别是在涉及非常小野、高调制和/或非均匀介质的野的规划中。在与Brainlab(德国慕尼黑)的合作中，我们展示了改进的MLC建模如何在Elements(德国Brainlab)中使用的Pencil Beam (PB)和Monte Carlo (MC)模型的测量和计算剂量之间产生更大的一致性。

Purpose: Optimized multi-leaf collimator (MLC) parameters are essential for accurate beam modeling in radiosurgery planning, particularly in plans that involve very small fields, fields with high modulation and/or heterogenous medium.In collaboration with Brainlab (Munich, Germany), we demonstrate how improved MLC modeling yields greater consistency between measured and calculated dose for the Pencil Beam (PB) and Monte Carlo (MC) models used in Elements (Brainlab, Germany). 
Methods: MLC parameters that define the tongue and groove (TnG) effect and transmission through rounded leaf tips were determined from 32 asynchronous sweeping gap fields, comprised of 8 TnG ratios for four different leaf gaps (Hernandez, 2017). Measurements were performed with 6FFF on a HDMLC Truebeam (Varian, USA). Dose was measured in water using an Exradin A12 (Standard Imaging, USA) positioned at isocenter at a depth of 10 cm and an SSD of 90 cm.Brainlab’s analysis of our measured data yielded updated MLC parameters for both PB (Dynamic Leaf Shift (DLS) and TnG) and MC (Radiological Leaf Shift (RLS) and TnG) models. Validation of these models were performed using multiple plans, with differing complexities, optimized and calculated in each planning Element – Multiple Metastases, Cranial SRS and Spine SRS.   29 PB and MC plans were calculated using a 1 mm dose grid and 1% uncertainty (MC). Validations were performed with Gafchromic XD film (Ashland, USA) in multiple heterogeneous and homogenous phantoms, using ExacTrac (Brainlab, Germany) for positioning. FilmQAPro (Ashland, USA) was used to compare calculated and measured dose. 
Results: For PB, DLS and TnG were changed from 0.12 mm and 0.49 mm to 0.18 mm and 0.32 mm, respectively. Based on our measurements, Brainlab modified their MC model (version 3.0) to allow adjustment of RLS and TnG, which was not configurable in earlier versions (2.5 or earlier).RLS and TnG were determined to be 0.25 mm and 0.7 mm, respectively. Excellent agreement between calculated and measured dose was observed for all plans. Average gamma score of >98% + 2% for PB and >98% + 1.8% for MC using 2%/1mm criteria. Plans calculated with MC 2.5 showed marked improvement in gamma scores when recalculated with MC 3.0, with up to a 68% higher gamma score (3%/1mm) for a highly complex plan. 
Conclusion: Accurate modeling of MLC can be achieved using asynchronous sweeping gap measurements. Improved Elements' beam models are critical to achieving excellent agreement between measurement and calculation, even for very complex and/or small fields.

OP23 A novel methodology for dosimetry audits focused on intracranial stereotactic radiosurgery applications.

Evangelos PAPPAS (ATHENS, Greece)

OP23 A novel methodology for dosimetry audits focused on intracranial stereotactic radiosurgery applications.

侧重于颅内立体定向放射外科应用的新剂量审计方法。

 

With contemporary SRS, the interlinked dosimetry- and geometry-related treatment parameters, require a high-degree of accuracy and precision. This translates into the need for reduced uncertainties at each step of this complex procedure. This work presents an innovative phantom-based audit methodology that, combining results from different dosimetry methods, evaluates all stages of the radiotherapy chain, serving as an ideal tool to promote best practice and assure high-quality treatments.

 

The phantom used was a 3D-printed head phantom, accommodating inserts for film, OSL, and gel dosimeters, calibrated at an SSDL. The user received an explicit, for the practice to be audited, RTstructure set, and was challenged to achieve a specific level of accuracy. Following the patient SRS treatment local protocol, the phantom treatment was simulated, planned, and exported to the delivery platform by the staff members who are normally involved at each step of the treatment chain. To assess whether QA results met the pre-defined standards, the latest recommendations of AAPM-RSS Medical Physics Practice Guideline 9.a. for SRS-SBRT were adopted for film dosimetry. A linac-based single-isocenter multi-focal SRS treatment was evaluated. 3 similar VMAT plans were generated, one for each detector type, taking into account the calibration dose range of each detector. Localization was performed with a kV CBCT. 6D corrections were applied prior to delivery. The OSL and film dosimeters were unloaded for analysis, and the phantom incorporating the irradiated gel-filled cylinder was MR scanned for the dose read-out 24 hours post-irradiation at a fully characterized MR scanner.

 

Results from one selected center audited has not indicated any concerns regarding the local practices for the specific aspects of dosimetry for intracranial SRS. Measured and calculated dose distributions were spatially co-registered and compared. Calculations were experimentally validated within uncertainties. The maximum deviation between measurements and TPS calculations for OSL dosimetry was 4.08%. The 3D GI of the film plane was 99.17% and the total spatial offsets of the planned and the corresponding gel-measured distributions for the targets involved were 0.77mm, 0.45mm and 0.81mm, respectively. Further work is required for the full characterization of OSLDs response to reduce the experimental uncertainties.

 

Novel dosimetry audit techniques allow the multi-step evaluation of the radiotherapy treatment chain. To keep up with the clinical need and novel equipment future developments will be focused on aspects such as treatment planning based on MR images and online intrafraction replanning strategies, as these are being increasingly applied into routine clinical services.

OP24 Iba myqa srs detector for cyberknife radiosurgery quality assurance.

Francesco PADELLI (Milano, Italy),

用于射波刀放射外科质量保证的Iba myqa srs探测器。

OP24 Iba myqa srs detector for cyberknife radiosurgery quality assurance.

 

Background and Aims

 

Dose administration accuracy in radiosurgery (RS) treatments is of paramount importance to guarantee both the clinical outcome and the absence of severe toxicities. A comprehensive delivery quality assurance (DQA) program is therefore mandatory. In this study we evaluated the IBA myQA SRS® (IBA Dosimetry, Germany) high-resolution solid-state detector in a new context of RS delivered using CyberKnife® (Accuray, US) 6 MV robotic linac. The detector’s performance was investigated in periodic machine DQA and patient-specific treatments verification.

 

 

 

Methods

 

MyQA SRS [Figure 1] is composed of a 140×120 mm CMOS matrix with 400 um resolution, allocated in a cylindrical ABS phantom topped by a hemispheric cap. Dose calibration was ensured delivering 500 cGy to the matrix central area by an ad-hoc optimized plan.

 

System performance evaluation included: periodic dosimetry tests (dose linearity and reproducibility, output factors, off-axis-ratios) [Figure 2], detector angular response and dose rate dependence (in the clinically useful source-to-surface range between 650 mm and 1200 mm), and variable aperture IRIS® collimator field size measurement.

 

For patient-specific DQA, the system performance was studied for various RS intracranial targets considering complete optimized plans and plans corrected taking into account the device angular response (delivered after removal of beams above a threshold angle selected according to the angular dependence analysis). An evaluation by 3% 1 mm Gamma Index was performed [Figure 3].

 

 

 

Results

 

Detector response for periodic DQA tests was always found to be in accordance with the authors center’s commissioning data. Dependence from dose rate was confirmed, corroborating the manufacturer requirement of a dose calibration specific for each dose rate of interest. Field dimensions for the IRIS collimator were consistent with commissioning values, with an accordance within 0.4 mm. Finally, angular dependence tests resulted in a signal decay greater than 5% when beams outside a ±50° amplitude cone with respect to the patient’s anterior-posterior direction were delivered.

 

Concerning patient-specific QA, >50° angled beams elimination from treatment delivery led to an improvement in Gamma Index passing rates ranging between +3% and +115%, depending on target and plan characteristics.