The variation in output of symmetric, asymmetric and irregularly shaped wedged radiotherapy fields

A model for calculating the variation in output of symmetric, asymmetric and irregularly shaped wedged radiotherapy fields is presented. The variation in output from the treatment head when a wedge is used is calculated by dividing the output into a primary component and one due to scattered radiation. The scatter component is then further subdivided into contributions from elements which have a 1 cm x 1 cm cross-sectional area at the isocentre. The scatter from each element is determined as the contribution from the head scatter component modified by the presence of the wedge and a contribution due to additional scattered radiation from the wedge. The relative intensity of the scattered radiation from the wedge is modelled using a simple first scatter approximation. In this approximation the magnitude of the scatter is given by a t exp(-mu t) function where t is the thickness of the wedge for the selected element. The magnitude of the primary component and the relative intensity of scatter from each element are then obtained by an iterative fit to measured data. The technique has been applied to two different internally mounted wedge designs, for a standard treatment head, two asymmetric treatment heads and two similar multileaf collimators, over a range of energies between 4 and 20 MV. Calculations agree with measured values over a range of field sizes and shapes to within 1.5%.     

带钢热连轧非对称因素对板形的影响

为研究某2250 mm热连轧生产中非对称因素对轧件非对称板形(如楔形和单边浪)的影响,利用基于影响函数法的辊系变形模型、张应力模型和简化的轧制压力横向分布模型相结合的方法,建立了集轧机和轧件为一体的非对称板形计算模型.研究结果表明:来料楔形对轧件楔形的影响明显超过其对轧件平坦度的影响;上游机架和下游机架刚度非对称分别主要影响轧件楔形和平坦度;40℃以内的轧件温度不对称分布对轧件平坦度影响较小,对出口楔形的影响可以忽略;轧件跑偏对楔形和平坦度均有显著影响.根据板形良好条件确定了各非对称参数的允许范围.      

Isoxaben soil biodegradation in pear tree orchard after repeated high dose application

Dose uniformity within the treatment volume for tangent breast irradiation can be significantly improved through dynamic compensating collimation using all four independent jaws to define a superposition of sequentially reduced Enhanced dynamic wedge fields. The enhanced dynamic wedge angle is determined which optimizes dose uniformity in the central axis transverse plane, then the sequential reduction of the superior and inferior jaws is determined to optimize the sagittal dose uniformity. This technique could be applied under computer control through a record and verify system.     

Dosimetry of wedged fields with asymmetric collimation

Shaping of wedged fields with asymmetric collimators (independent jaws) produces dosimetric effects which need to be taken into account when calculating isodose distributions and accelerator monitor units. These effects were studied using ion chamber dosimetry in a water phantom. Wedged fields of 30 degrees to 60 degrees wedge angles were shaped by an asymmetric collimator. A system of dose calculation was developed to take into account the changes in dose in the open portion of the field. The formalism utilizes regular symmetric field parameters (e.g., Sc, Sp, and TPR). Calculated point doses and profiles agreed well with the measured data.     

Monitor unit calculation in 6 MV irregularly shaped beams--accuracy in clinical practice

The results of an investigation of the accuracy of monitor unit (MU) calculation in clinical shaped beams are presented. Measured doses at the reference depth on the beam central axis (isocentre) or on a beam axis representative of the irradiated area (when the isocentre lies under a block or near the edges of the block's shadow) were compared with the expected doses when calculating MUs, by applying different methods normally used in clinical practice. Empirical (areas weighted, Wrede) and scatter summation (Clarkson) methods as well as a pencil-beam based algorithm were applied. 40 irregular fields (6 MV X-rays, CLinac, Varian 6/100), divided into six categories, were considered. Dose measurements were performed with a NE2571 ionization chamber in an acrylic 30 x 30 x 30 cm3 phantom. The depths in acrylic were converted into water-equivalent depths through a correction factor derived from TMR measurements. The method of dose measurements in acrylic was found to be sufficiently accurate for the purpose of this study by comparing expected and measured doses in open square and rectangular fields (mean deviation +0.2%, SD = 0.5%). Results show that all the considered methods are sufficiently reliable in calculating MUs in clinical situations. Mean deviations between measured and expected dose values are around 0 for all the methods; standard deviations range from 1% for the Wrede method to 0.75% for the pencil-beam method. The differences between expected and measured doses were within 1% for about 3/4 of the fields when calculating MUs with all the considered methods. Maximum deviations range from 1.6% (pencil-beam) to 3% (Wrede). Slight differences among the methods of MU calculation were revealed within the different categories of blocked fields analysed. The surprising agreement between measured and expected dose values obtained by using empirical methods (area weighted and Wrede) is probably due to the fact that the reference points were positioned in a "central" region of the unblocked areas.     

板坯连铸结晶器内非对称流动对渣钢界面的影响

结晶器渣 钢界面的控制是板坯连铸操作中的重要环节。为研究渣 钢界面的流体动力学行为,建立了“空气 保护渣 钢液”三相流模型,运用雷诺应力模型和非均相混合模型对结晶器三相流场进行模拟研究,采用等体积分数法确定渣 钢界面的形状及位置。浸入式水口对中不良时会导致结晶器内产生非对称流场,研究发现该流场的渣 钢界面波动比相同操作参数下对称流场的界面波动要剧烈得多,且非对称流场的渣 钢界面会有偏流甚至旋涡出现,对结晶器操作危害很大。     

Clinical implementation of a two-component x-ray source model for calculation of head-scatter factors

A calculation method is described, which presumes that linac output has two components. One component is direct x rays from the target and the remaining component is an extra-focal, distributed source of radiation that is scattered from the flattening filter, primary collimator, and the jaws of the moveable collimator. This calculation method gives values for output factors that differ from measured values by no more than 0.5% for field widths from 4 to 40 cm, rectangular fields with long to short axis ratios as great as 10, symmetric and asymmetric fields, and source-to-axis distances from 65 to 360 cm. While this calculation method has great accuracy and flexibility, a minimal amount of input data is required: (1) measured output factors at a source-to-axis distance of 100 cm for square fields; (2) positions of the collimator with respect to the x-ray source target; and (3) output factors measured at various source-to-axis distances for a 10 cm x 10 cm field.     

Intracerebroventricular administration of neuropeptide Y to normal rats increases obese gene expression in white adipose tissue

Dose planning programs originally intended for use with symmetric fields have been adapted for use with asymmetric fields. An accurate representation of the change in primary beam quality with off-axis distance and depth is essential for accurate dose calculation and is usually represented in the computer as a primary radiation profile or primary off-center ratio (POCR). The original field edge correction (FEC) method described by Cadman [Med. Phys. 22, 457 (1995)] to determine POCRs has been extended to allow accurate POCR values to be obtained to an off-axis distance defined by the corners of the largest field, typically at an off-axis distance of 28.3 cm. This technique requires only routine symmetric field measurements including beam profiles, TMRs, and collimator and phantom scatter factors. The POCRs obtained using the FEC technique were used to generate off-center ratios (OCRs) using the boundary factor technique of Chui et al. [Med. Phys. 15, 92 (1988)]. Excellent agreement with measured values was obtained for cross-beam OCRs using a 10 x 10-cm2 field defined by a single set of asymmetric jaws with a field center offset of 15 cm and for diagonal OCRs using a 20 x 20-cm2 field with each pair of jaws in a half-blocked configuration.     

Dosimetric aspects of physical and dynamic wedge of Clinac 2100C linear accelerator

AIM: To investigate variation of wedge factors on field size and depth for physical and dynamic wedges of identical wedge angles for Clinac 2100C linear accelerator and its clinical implementation. MATERIAL AND METHODS: A computer controlled water phantom dosimetric system is used to generate profile data for physical wedges, whereas a 0.6 cm3 ion chamber is used for generation of profiles for dynamic wedge and wedge factors for both types of wedges. The method has been discussed to handle the dynamic wedge dosimetry in absence of linear array of detectors or film densitometer. RESULTS: A systematic dependence on wedge factor is observed for physical wedge, with respect to depth and wedge angle but not depending on field size. Whereas dynamic wedge shows strong dependence on field size and is not systematic because the dynamic wedge is controlled by segmented treatment tables depending on field size and energy and no significant variation is observed on depth for various wedge angles. The handling of beam data in a commercially available treatment planning system is discussed and a comparison has been made for iso-doses of both types of wedges. CONCLUSION: The dynamic wedge isodose curves shows rather straight lines than physical wedge but larger hot spots at thin edge which needs careful consideration during planning.     

Multiple machine implementation of enhanced dynamic wedge

PURPOSE: After acquiring 4 years of experience with Dynamic Wedge, a software-driven one-dimensional (1D) compensation system, we implemented a new software version called Enhanced Dynamic Wedge (EDW). The EDW allows larger (30 cm) and asymmetric field sizes and additional angles for wedged fields. We implemented this software on four similar dual-energy accelerators that also possess upper and lower physical wedge sets. Our goal was to implement EDW with one common wedge factor (WF) table and one set of treatment-planning files. METHODS AND MATERIALS: We measured WFs with an ionization chamber and isodose profiles with both film and a diode array. We used a calculation scheme that requires only entry of the wedge angle and fixed jaw value. Filters for computerized treatment planning were configured for each wedge angle. We also examined to what degree the multileaf collimation (MLC) orientation, which is orthogonal to the EDW direction, was compromised for specific treatment sites. As a comparative test, we examined the dosimetric consistency for the 8 sets of physical wedges on the four machines. Finally, we updated our DW quality assurance program for EDW. RESULTS: The measured EDW WF was common for all four machines to within +/- 1.5% and the calculation scheme held to within 1.5%. The EDW isodoses were consistent among the machines as measured by film and diode array. The treatment-planning filters provided computed isodose profiles that were nearly identical to measured profiles. Regarding MLC orientation, we found that the collimator angle needed for EDW did not compromise isodose distributions, as apparent in measured isodoses and calculated dose-volume histograms. The consistency of the physical wedges did not fare as well. Two of the lower wedge sets had Wfs and profiles different (> 3%) from the other wedge sets. CONCLUSIONS: We have successfully implemented EDW on four machines using only one WF table and one set of treatment-planning filters. The EDW provides for improved treatment techniques for particular sites due to the large field sizes and additional angles available. Daily treatment efficiency has increased because of the remote capability provided by EDW.     

Relapse after video-assisted thoracoscopic surgery for spontaneous pneumothorax

Many cochlear prostheses employ charge-balanced biphasic current pulses. These pulses have little energy at low frequencies resulting in limited stimulation of low frequency hearing by mechanical responses to the electrical stimulus. However, if electro-mechanical transduction within the cochlea is nonlinear, electrical stimulation with asymmetric, charge-balanced current pulses may result in a mechanical response with significantly more low frequency energy. We estimated the mechanical response at low frequencies to pulsatile electrical stimulation of the cochlea. The auditory nerve compound action potential evoked by low frequency tones was forward-masked by a train of symmetric or asymmetric current pulses. Masking by asymmetric current pulses was not significantly different from masking by symmetric pulses matched for pulse duration and charge. In conclusion, there appears to be no advantage to using asymmetric current pulses for the mechanical stimulation of residual low frequency hearing by electrical stimulation of the cochlea.     

Quality assurance of the dose delivered by small radiation segments

The use of intensity modulation with multiple static fields has been suggested by many authors as a way to achieve highly conformal fields in radiotherapy. However, quality assurance of linear accelerators is generally done only for beam segments of 100 MU or higher, and by measuring beam profiles once the beam has stabilized. We propose a set of measurements to check the stability of dose delivery in small segments, and present measured data from three radiotherapy centres. The dose delivered per monitor unit, MU, was measured for various numbers of MU segments. The field flatness and symmetry were measured using either photographic films that are subsequently scanned by a densitometer, or by using a diode array. We performed the set of measurements at the three radiotherapy centres on a set of five different Philips SL accelerators with energies of 6 MV, 8 MV, 10 MV and 18 MV. The dose per monitor unit over the range of 1 to 100 MU was found to be accurate to within +/-5% of the nominal dose per monitor unit as defined for the delivery of 100 MU for all the energies. For four out of the five accelerators the dose per monitor unit over the same range was even found to be accurate to within +/-2%. The flatness and symmetry were in some cases found to be larger for small segments by a maximum of 9% of the flatness/symmetry for large segments. The result of this study provides the dosimetric evidence that the delivery of small segment doses as top-up fields for beam intensity modulation is feasible. However, it should be stressed that linear accelerators have different characteristics for the delivery of small segments, hence this type of measurement should be performed for each machine before the delivery of small dose segments is approved. In some cases it may be advisable to use a low pulse repetition frequency (PRF) to obtain more accurate dose delivery of small segments.     

Approximate Series Solution for Analysis of FRP Composite Highway Bridges

The design of a deck-and-stringer bridge system is usually reduced to the analysis of a T-beam section, loaded by concentrated loads corresponding to an equivalent fraction of the applied truck load. This equivalent load is defined by wheel load–distribution factors, which approximate the overall behavior of the bridge superstructure. In this paper, a one-term approximation of a macroflexibility series solution including deformations for fiber-reinforced polymer (FRP) deck-and-stringer orthotropic bridge systems, is used to develop explicit expressions for symmetric and asymmetric load distribution factors. It is significant that the equations presented herein include important parameters that represent, as accurately as possible, the response characteristics of the super structure, such as the geometry and material properties of the FRP deck and stringers, bridge aspect ratio, and number and spacing of stringers. As an illustration in actual design applications, the formulation presented in this paper is used to develop an analytical method for FRP deck-and-stringer bridge systems, and the method is verified by predicting the response of an all FRP model bridge in the lab and an FRP deck on steel stringers in the field. The results of the present formulation compare well with experimental lab and field results. The simplified analysis presented in this paper can be used with sufficient accuracy for the design of composite FRP deck on stringers bridges.     

Venous angiography is needed to interpret inferior petrosal sinus and cavernous sinus sampling data for lateralizing adrenocorticotropin-secreting adenomas

Bilateral simultaneous venous sampling of ACTH from the inferior petrosal sinus is a reliable test for diagnosing Cushing's disease, but is not reliable for lateralizing ACTH-secreting pituitary adenomas. We reviewed 23 consecutive patients with Cushing's disease who underwent venous angiography of the cavernous and inferior petrosal sinuses followed by bilateral simultaneous venous sampling of ACTH in the inferior petrosal and cavernous sinuses. Venous drainage was bilaterally symmetric in 14 patients (61%) and asymmetric in 9 (39%). The most common asymmetric pattern (6 patients) was for blood from both cavernous sinuses to drain into the right inferior petrosal sinus, with no significant drainage into the left. Cavernous sinus sampling in 21 patients correctly lateralized the tumor in 12 cases of symmetric venous drainage, but in only 3 cases of asymmetric drainage. Inferior petrosal sinus sampling in all 23 patients correctly lateralized the tumor in 12 cases of symmetric drainage, but in only four cases of asymmetric drainage. Overall, venous sampling correctly lateralized 70% of the tumors. Incorrect lateralization in cases of asymmetric venous drainage is probably attributable to shunting of blood toward the side of dominant venous drainage. Our findings illustrate the need for venography in all patients undergoing venous sampling of ACTH because an understanding of the venous drainage patterns is essential to correctly interpret venous sampling data and warn physicians that the lateralization data may be incorrect or unreliable.     

A calculation strategy for the structure determination of symmetric dimers by 1H NMR

The structure determination of symmetric dimers by NMR is impeded by the ambiguity of inter- and intramonomer NOE crosspeaks. In this paper, a calculation strategy is presented that allows the calculation of dimer structures without resolving the ambiguity by additional experiments (like asymmetric labeling). The strategy employs a molecular dynamics-based simulated annealing approach to minimize a target function. The experimental part of the target function contains distance restraints that correctly describe the ambiguity of the NOE peaks, and a novel term that restrains the symmetry of the dimer without requiring the knowledge of the symmetry axis. The use of the method is illustrated by three examples, using experimentally obtained data and model data derived from a known structure. For the purpose of testing the method, it is assumed that every NOE crosspeak is ambiguous in all three cases. It is shown that the method is useful both in situations where the structure of a homologous protein is known and in ab initio structure determination. The method can be extended to higher order symmetric multimers.     

Verification of the omni wedge technique

The optimal field shape achieved using a multileaf collimator (MLC) often requires collimator rotation to minimize the adverse effects of the scalloped dose distribution the leaf steps produce. However, treatment machines are designed to deliver wedged fields parallel or perpendicular to the direction of the leaves. An analysis of cases from our clinic showed that for 25% of the wedged fields used to treat brain and lung tumors, the wedge direction and optimal MLC orientation differed by 20 degrees or more. The recently published omni wedge technique provides the capability of producing a wedged field with orientation independent of the orientation of the collimator. This paper presents a comparison of the three-dimensional (3D) dose distributions of the omni wedged field with distributions of wedged fields produced using both the universal and dynamic wedge techniques. All measurements were performed using film dosimetry techniques. The omni wedge generated fields closely matched the conventional wedged fields. Throughout 95% of the irradiated volume (excluding the penubra), the dose distribution of the omni wedged field ranged from +5.5 to -3.5 +/- 1.5% of that of the conventionally wedged fields. Calculation of the omni wedged field is as accurate as conventional wedged field calculation when using a 3D treatment planning systems. For two-dimensional treatment planning systems, where one must assume that the omni wedged field is identical to a conventional field, the calculated field and the delivered field differs by a small amount.     

Submerged Hydraulic Jumps below Abrupt Expansions

In submerged flows below an expanding outlet, both symmetric and asymmetric flows are observed. The hydraulic conditions required to form a symmetric submerged jump are necessary for design purposes. In this study, the flow conditions are presented for the submerged flows at an abrupt-symmetrical expansion. Also, the hydraulic conditions for the formation of a symmetric submerged jump are clarified, and an expression is developed for the transition between symmetric and asymmetric flows. Further, an equation on the length of submerged jumps is derived.     

Relationship between structure and neutralizing activity of rabbit tetanus antibodies elicited by acellular and whole-cell pertussis DTP vaccines

Symmetric and asymmetric IgGs having different neutralizing capacity are synthesized in variable proportions by the same clones during the course of immune response. The neutralizing activity of tetanus antibodies was studied in rabbits vaccinated with acellular (DTPa) or whole-cell pertussis (DTPw) vaccines. Symmetric and asymmetric F(ab)'2 fragments from the IgG fraction of the peak serum pools from each group of rabbits were purified by concanavalin A chromatography and measured by ELISA. After the third vaccine dose the asymmetric antibody percentage for DTPw (40%) was twice that for DTPa (20%). The neutralizing activity of asymmetric antibodies was roughly sixfold lower than symmetric ones. When antibody values titrated by ELISA approach minimal protective level, the proportion of symmetric antibodies with high toxin neutralizing activity acquires crucial importance.     

粗轧中间坯走偏对镰刀弯的影响分析和控制

 针对某常规热连轧粗轧中间坯的镰刀弯问题,首先使用ABAQUS/Standard建立轧辊-轧件静力学耦合模型,定量计算了不同入口厚度、宽度、压下量、两侧温度差和工作辊初始辊形下的入口中间坯走偏因素对出口中间坯楔形的影响,同时,得到变形后的工作辊辊形曲线;再使用ABAQUS/Explicit建立动态分析模型,并采用此辊形曲线作为动态模型的工作辊辊形,在与静力学分析相同的走偏工况下,模拟中间坯镰刀弯现象,进一步计算了不同入口厚度、宽度、压下量、两侧温度差和工作辊初始辊形下的入口中间坯走偏因素对出口中间坯镰刀弯的影响。基于上述计算结果,提出了优化粗轧侧导板的开口度裕量和工作辊初始辊形的方案,提高了轧制过程对中性,从而改善了中间坯镰刀弯现象,使中间坯中心线偏移量未达标率从6.51%下降到2.93%,取得了显著的效果,同时成品楔形命中率达到89%以上,极大提高了产品质量。