Progress toward a microradiation therapy small animal conformal irradiator

Microradiation therapy (microRT) systems are being designed to provide conformal radiation therapy to small animals enabling quantitative radiation response evaluation. We used a Monte Carlo approach to estimate the radiation dose distributions from proposed blueprints and developed a beam model to aid in the microRT system design process. This process was applied to a prototype irradiator that uses a small (3 mm long and 3 mm in diameter), cylindrical, high-activity 192Ir source delivering the radiation beam using custom-fabricated tungsten collimators. The BEAMnrc Monte Carlo code was used to simulate dose distributions from these prototype collimators. Simulations were performed at three source-to-surface distances (50, 60, and 70 mm), and with five circular field sizes (5, 7.5, 10, 12.5, and 15 mm). A dose to a 50 X 50 X 50 mm3 water phantom with 1 X 1 X 1 mm3 voxel spacing was computed. A multiparameter dose calculation algorithm was developed to efficiently and accurately calculate doses for treatment planning exercises. The parametrization was selected so that the parameters varied smoothly as a function of depth, source-to-surface distance, and field size, allowing interpolation for geometries that were not simulated using the Monte Carlo simulation. Direct comparison of the model with the Monte Carlo simulations showed that the variations were within 5% error for field sizes larger than 10 mm, and up to 10% for smaller field sizes.     

Dosimetric characterization of a bi-directional micromultileaf collimator for stereotactic applications

A 6 MV photon beam from Linac SL75-5 has been collimated with a new micromultileaf device that is able to shape the field in the two orthogonal directions with four banks of leaves. This is the first clinical installation of the collimator and in this paper the dosimetric characterization of the system is reported. The dosimetric parameters required by the treatment planning system used for the dose calculation in the patient are: tissue maximum ratios, output factors, transmission and leakage of the leaves, penumbra values. Ionization chambers, silicon diode, radiographic films, and LiF thermoluminescent dosimeters have been employed for measurements of absolute dose and beam dosimetric data. Measurements with different dosimeters supply results in reasonable agreement among them and consistent with data available in literature for other models of micromultileaf collimator; that permits the use of the measured parameters for clinical applications. The discrepancies between results obtained with the different detectors (around 2%) for the analyzed parameters can be considered an indication of the accuracy that can be reached by current stereotactic dosimetry.     

MicroRT-small animal conformal irradiator

A novel small animal conformal radiation therapy system has been designed and prototyped: MicroRT. The microRT system integrates multimodality imaging, radiation treatment planning, and conformal radiation therapy that utilizes a clinical 192Ir isotope high dose rate source as the radiation source (teletherapy). A multiparameter dose calculation algorithm based on Monte Carlo dose distribution simulations is used to efficiently and accurately calculate doses for treatment planning purposes. A series of precisely machined tungsten collimators mounted onto a cylindrical collimator assembly is used to provide the radiation beam portals. The current design allows a source-to-target distance range of 1-8 cm at four beam angles: 0 degrees (beam oriented down), 90 degrees, 180 degrees, and 270 degrees. The animal is anesthetized and placed in an immobilization device with built-in fiducial markers and scanned using a computed tomography, magnetic resonance, or positron emission tomography scanner prior to irradiation. Treatment plans using up to four beam orientations are created utilizing a custom treatment planning system-microRTP. A three-axis computer-controlled stage that supports and accurately positions the animals is programmed to place the animal relative to the radiation beams according to the microRTP plan. The microRT system positioning accuracy was found to be submillimeter. The radiation source is guided through one of four catheter channels and placed in line with the tungsten collimators to deliver the conformal radiation treatment. The microRT hardware specifications, the accuracy of the treatment planning and positioning systems, and some typical procedures for radiobiological experiments that can be performed with the microRT device are presented.     

An x-ray image guidance system for small animal stereotactic irradiation

An x-ray image-guided small animal stereotactic irradiator was developed and characterized to enable tumor visualization and accurate target localization for small field, high dose irradiation. The system utilizes a custom collimation system, a motorized positioning system (x, y, θ), a digital imaging panel and operating software, and is integrated with a commercial x-ray unit. The essential characteristics of the irradiator include small radiation fields (1-10 mm), high dose rate (>10 Gy min(-1)) and submillimeter target localization. The software enables computer-controlled image acquisition, stage motion and target localization providing simple and precise automated target localization. The imaging panel was characterized in terms of signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and spatial resolution. Overall localization accuracy and precision were assessed. SNR, CNR and spatial resolution are 24 dB, 21 dB and 2.8 lp mm(-1), respectively, and localization accuracy is approximately 65 μm with 6 μm precision. With the aid of image guidance, system performance was subsequently used to evaluate radiation response in a rat orthotopic lung tumor effectively sparing normal tissues and in a mouse normal lung. The capabilities of 3D treatment and cone-beam computed tomography are presented for 3D localization and delivery as a work in progress.     

网格精度对肝癌立体定向放射治疗计划的剂量学影响

目的:研究不同网格精度对肝癌立体定向放射治疗（SBRT）计划的剂量学影响,为临床肝癌SBRT计划的设计提供 合适的网格精度。方法:回顾性分析湖北省肿瘤医院2017~2020年间采用VMAT-SBRT治疗方式的肝癌患者10例,在相 同函数配置和参数设置条件下,分别采用1.0、2.0、3.0、4.0、5.0 mm网格精度设计治疗计划。比较靶区D95%、Dmean、适形度指 数、梯度指数,危及器官中肝脏Dmean、正常肝脏的V10、V20、V30,脊髓的Dmax以及计划的计算时间,分析在3 %/3 mm和2%/2 mm标准下Gamma通过率。结果:采用Eclipse系统AAA算法下对网格大小进行更改,结果显示靶区D95%随着网格值增 大呈下降趋势,GI随着网格值增大呈增长趋势;在Dmean和适形度指数上,网格值对其产生的影响未见显著差异;在危及器 官保护方面,1.0 mm网格表现出更优异的计算结果;在计算时间方面,随着网格大小减小,剂量计算时间呈大幅度增长;在 计划验证通过率方面,随着网格大小增大,两种标准（2%/2 mm和3%/3 mm）下的通过率不断减小。结论:随着计算网格 大小的改变,靶区和危及器官的剂量受量以及计算时间和Gamma通过率都有不同程度的改变。综合计划剂量优化结果 以及计算时间和验证通过率,建议在肝癌SBRT计划设计中使用2.0 mm大小的计算网格。     

Commissioning and evaluation of a new commercial small rodent x-ray irradiator

An appropriate radiation source is essential in studies of tissue response in animal models. This paper reports on the evaluation and commissioning of a new irradiator suitable for studies using small animals or cell culture. The Faxitron is a 160-kVp x-ray machine that was adapted from an x-ray imaging unit through modifications to facilitate experimental irradiation. The x-ray unit is housed in a shielded cabinet, and is configured to allow multiple irradiation positions and a range of field sizes and dose rates. Use of this machine for animal irradiation requires characterisation of relevant dosimetry, and development of methodology for secondary beam collimation and animal immobilisation. In addition, due to the limitation of the irradiator, the optimal selection of three characteristics of the x-ray beam is important. These three characteristics, namely, the dose rate, the beam uniformity, and the field size are inter-dependent and the selection of a combination of these parameters is often a compromise and is dependent on the application. Two different types of experiments are selected to illustrate the applicability of the Faxitron. The Faxitron could be useful for experimental animal irradiation if the experimental design is carried out carefully to ensure that accurate and uniform radiation is delivered.     

Performance evaluation of a CyberKnife G4 image-guided robotic stereotactic radiosurgery system

The aim of the current work was to present the performance evaluation procedures implemented at our department for the commissioning of a G4 CyberKnife system. This system consists of a robotic manipulator, a target-locating system and a lightweight 6-MV linac. Individual quality assurance procedures were performed for each of the CyberKnife subsystems. The system was checked for the mechanical accuracy of its robotic manipulator. The performance of the target-locating system was evaluated in terms of mechanical accuracy of both cameras' alignment and quality assurance tests of the x-ray generators and the flat-panel detectors. The traditional linac 6-MV beam characteristics and beam output parameters were also measured. Results revealed a manipulator mechanical mean accuracy of approximately 0.1 mm, with individual maximum position uncertainties less than 0.25 mm. The target-locating system mechanical accuracy was found within the acceptance limits. For the most clinically used parameters in the CyberKnife practice, e.g. 100-120 kV and 50-200 ms, kV and exposure time accuracy error were measured as less than 2%, while the precision error of the kV was determined as less than 1%. The acquired images of the ETR grid pattern revealed no geometrical distortion while the critical frequency f(50) values for cameras A and B were calculated as 1.5 lp mm(-1) and 1.4 lp mm(-1), respectively. Dose placement measurements were performed in a head and neck phantom. Results revealed sub-millimeter beam delivery precision whereas the total clinical accuracy of the system was measured equal to 0.44 +/- 0.12 mm, 0.29 +/- 0.10 mm and 0.53 +/- 0.16 mm for the skull, fiducial and Xsight spine tracking methods, respectively. The results of this work certify the G4 CyberKnife SRS system capable of delivering high dose distributions with sub-millimeter accuracy and precision to intracranial and extracranial lesions. Moreover, total clinical accuracy of the investigated G4 system was found to be improved for the skull and fiducial tracking methods and was comparable for Xsight spine tracking method compared with the earlier generation of the instrument.     

Dosimetric characterization of a new miniature multileaf collimator

The dosimetrical characteristics of a new miniature multileaf collimator (ModuLeaf MLC, MRC Systems GmbH, Heidelberg, Germany) attached to the accessory holder of a Siemens accelerator with 6 MV x-rays (PRIMUS, Siemens OCS, Concord, California, USA) have been investigated. In particular, those parameters which are important for the accuracy of the treatment such as output factors, penumbra, field edge precision and transmission/leakage were determined. These data can now be used to implement specific dose calculation procedures for this miniature multileaf collimator in treatment planning systems.     

Dosimetric characterization of Ir-192 LDR elongated sources

Ir-192 wires have been used in low-dose-rate brachytherapy for many years. Commercially available treatment planning systems approximate the dose rate distribution of the straight or curved wires applying the superposition principle using one of the following methods: (i) The wire is modeled as a set of point sources, (ii) the wire is modeled as a set of small straight segment wires, (iii) the values of the parameters and functions of the American Association of Physicists in Medicine (AAPM) Task Group 43 protocol are obtained for wire lengths between 3 and 7 cm assuming some simplifications. The dose rate distributions obtained using these methods for linear wires of different lengths and U-shaped wires present significant deviations compared to those obtained by Monte Carlo. In the present study we propose a new method to model 192Ir wires of any length and shape, named the Two Lengths based Segmented method. This method uses the formalism stated in the AAPM Task Group 43 protocol for two straight wires only, 0.5 and 1 cm, to obtain the dose rate distribution around wires of any length (down to 0.3 cm and up to 10 cm) improving on the results of the aforementioned ones. This method can easily be applied to dose calculations around other wires, such as Pd-103 ones.     

Dosimetric evaluation of a dedicated stereotactic linear accelerator using measurement and Monte Carlo simulation

Dosimetric parameters of a dedicated stereotactic linear accelerator have been investigated using measurements and Monte Carlo simulations. This linac has a unique built in multileaf collimation (MLC) system with the maximum opening of 16 x 21 cm2 and 4 mm leaf width at the isocenter and has successfully been modeled for the first time using the Monte Carlo simulation. The high resolution MLC, combined with its relatively large maximum field size, opens up a new opportunity for expanding stereotactic radiation treatment techniques from traditionally treating smaller targets to larger ones for both cranial and extracranial lesions. Dosimetric parameters of this linac such as accuracy of leaf positioning and field shaping, leakage and transmission, percentage depth doses, off-axes dose profiles, and dose penumbras were measured and calculated for different field sizes, depths, and source to surface distances. In addition, the ability of the linac in accurate dose delivery of several treatment plans, including intensity modulated radiation therapy (IMRT), performed on phantom and patients was determined. Ionization chamber, photon diode detector, films, several solid water phantoms, and a water tank were used for the measurements. The MLC leaf positioning to any particular point in the maximum aperture was accurate with a standard deviation of 0.29 mm. Maximum and average leakages were 1.7% and 1.1% for the reference field of 10.4 x 9.6 cm2. Measured penumbra widths (80%-20%) for this field at source axis distance (SAD) of 100 cm at a depth of 1.5 cm (dmax) were 3.2 and 4 mm for the leaf-sides and leaf-ends, respectively. The corresponding results at 10 cm depth and SAD =100 cm were 5.4 and 6.3 mm. Monte Carlo results generally agreed with the measurements to within 1% and or 1 mm, with respective uncertainties of 0.5% and 0.2 mm. The linac accuracy in delivering non-IMRT treatment plans was better than 1%. Ionization chamber dosimetry results for a phantom IMRT plan in the high dose and low dose regions were -0.5% and +3.6%, respectively. Dosimetry results at isocenter for three patients' IMRT plans were measured to be within 3% of their corresponding treatment plans. Film dosimetry was also used to compare dose distributions of IMRT treatment plans and delivered cumulative doses at different cross sectional planes.     

RapidArc与Novalis Knife在多发脑转移瘤SRS中的剂量学比较

目的:比较基于多叶准直器的RapidArc与基于圆形限光筒的Novalis Knife在多发脑转移瘤立体定向放射外科（SRS）中的剂量学差异。方法:选取10例已进行Novalis Knife治疗的多发脑转移瘤患者（转移瘤3~5个/人）,共37个转移瘤。重新设计单中心、非共面4弧的RapidArc SRS计划,靶区处方剂量统一为16 Gy。比较Novalis Knife计划和RapidArc SRS计划中靶区的剂量适形度指数和均匀性指数、靶区周围剂量梯度指数、正常脑组织等剂量线体积（V16、V12、V9、V6、V3）以及治疗所需的机器跳数。结果:RapidArc SRS计划相较于Novalis Knife计划,适形度指数更接近于1（0.79[±]0.10 vs 0.50[±]0.22, P=0.000）,均匀性指数更接近于0（0.07[±]0.01 vs 0.15[±]0.07, P=0.000）,但靶区周围剂量梯度指数较大（15.92[±]12.43 vs 5.05[±]3.53, P=0.000）。对于正常脑组织等剂量线体积,RapidArc SRS计划中V16明显小于Novalis Knife计划（P=0.005）;两计划的V12、V9无明显差异（P=0.445, 0.059）;而RapidArc SRS计划中V6、V3明显大于Novalis Knife计划（P=0.005, 0.005）。RapidArc SRS计划的机器跳数明显少于Novalis Knife计划（P=0.005）。结论:RapidArc SRS计划通过设置单一治疗等中心可实现颅内多个转移瘤的同步放射外科治疗,具有更高的靶区剂量适形度及均匀性;但靶区周边剂量跌落陡度不及Novalis Knife计划,正常脑组织低剂量受照范围较大。治疗依从性较差的多发脑转移患者可考虑选择治疗效率更高的RapidArc技术,并根据临床实际情况调整剂量分割模式,以减少放射损伤风险。     

Dosimetric characterization of a large area pixel-segmented ionization chamber

A pixel-segmented ionization chamber has been designed and built by Torino University and INFN. The detector features a 24 x 24 cm2 active area divided in 1024 independent cylindrical ionization chambers and can be read out in 500 micros without introducing dead time; the digital charge quantum can be adjusted between 100 fC and 800 fC. The sensitive volume of each single ionization chamber is 0.07 cm3. The purpose of the detector is to ease the two-dimensional (2D) verifications of fields with complex shapes and large gradients. The detector was characterized in a PMMA phantom using 60Co and 6 MV x-ray photon beams. It has shown good signal linearity with respect to dose and dose rate to water. The average sensitivity of a single ionization chamber was 2.1 nC/Gy, constant within 0.5% over one month of daily measurements. Charge collection efficiency was 0.985 at the operating polarization voltage of 400 V and 3.5 Gy/min dose rate. Tissue maximum ratio and output factor have been compared with a Farmer ionization chamber and were found in good agreement. The dose profiles have been compared with the ones obtained with an ionization chamber in water phantom for the field sizes supplied by a 3D-Line dynamic multileaf collimator. These results show that this detector can be used for 2D dosimetry of x-ray photon beams, supplying a good spatial resolution and sensibly reducing the time spent in dosimetric verification of complex radiation fields.