Optimization of 3D conformal electron beam therapy in inhomogeneous media by concomitant fluence and energy modulation

The possibilities of using simultaneous fluence and energy modulation techniques in electron beam therapy to shape the dose distribution and almost eliminate the influences of tissue inhomogeneities have been investigated. By using a radiobiologically based optimization algorithm the radiobiological properties of the tissues can be taken into account when trying to find the best possible dose delivery. First water phantoms with differently shaped surfaces were used to study the effect of surface irregularities. We also studied water phantoms with internal inhomogeneities consisting of air or cortical bone. It was possible to improve substantially the dose distribution by fluence modulation in these cases. In addition to the fluence modulation the most suitable single electron energy in each case was also determined. Finally, the simultaneous use of several preselected electron beam energies was also tested, each with an individually optimized fluence profile. One to six electron energies were used, resulting in a slow improvement in complication-free cure with increasing number of beam energies. To apply these techniques to a more clinically relevant situation a post-operative breast cancer patient was studied. For simplicity this patient was treated with only one anterior beam portal to clearly illustrate the effect of inhomogeneities like bone and lung on the dose distribution. It is shown that by using fluence modulation the influence of dose inhomogeneities can be significantly reduced. When two or more electron beam energies with individually optimized fluence profiles are used the dose conformality to the internal target volume is further increased, particularly for targets with complex shapes.     

On compensator design for photon beam intensity-modulated conformal therapy

Recently the compensator has been shown to be an in expensive and reliable dose delivery device for photon beam intensity-modulated radiation therapy (IMRT). The goal of IMRT compensator design is to produce an optimized primary fluence profile at the patient's surface obtained from the optimization procedure. In this paper some of the problems associated with IMRT compensator design, specifically the beam perturbations caused by the compensator, are discussed. A simple formula is derived to calculate the optimal compensator thickness profile from an optimized primary fluence profile. The change of characteristics of a 6 MV beam caused by the introduction of cerrobend compensators in the beam is investigated using OMEGA Monte Carlo codes. It is found that the compensator significantly changes the energy spectrum and the mean energy of the primary photons at the patient's surface. However, beam hardening does not have as significant an effect on the percent depth dose as it does on the energy spectrum. We conclude that in most situations the beam hardening effect can be ignored during compensator design and dose calculation. The influence of the compensator on the contaminant electron buildup dose is found to be small and independent of the compensator thickness of interest. Therefore, it can be ignored in the compensator design and included as a correction into the final dose distribution. The scattered photons from the compensator are found to have no effect on the surface dose. These photons produce a uniform low fluence distribution at the patient's surface, which is independent of compensator shape. This is also true for very large fields and extremely asymmetric and nonuniform compensator thickness profiles. Compared to the primary photons, the scattered photons have much larger angular spread and similar energy spectrum at the patient's surface. These characteristics allow the compensator thickness profile and the dose distribution to be calculated from the optimized fluence profile of primary photons, without considering the scattered photons.     

Genetics of male infertility

This study was undertaken to analyze the influence of total skin dose and dose-fractionation schedules on the response rate, survival and skin toxicity of patients with mycosis fungoides [MF] treated with total skin electron irradiation [TSEI]. From 1979 to 1992, 40 patients with MF were treated with TSEI using a modified Christie Hospital technique. Mean follow-up time was 48 months [median 20 months]. 37/40 patients completed TSEI; three died due to non-treatment-related conditions during therapy. 34/37 [92%] treated patients achieved complete remission [CR] and 16/40 [40%] are alive with no evidence of disease. Over the years, changes in dose-fractionation schedules were made and correlated with the pattern of CR and skin toxicity. The 5-year actuarial survival [Stanford staging] was 84% in Stages IA-IB [all Stage IA patients are alive] and 59% in Stage II. The probability of survival of Stage III-IV patients was 30% at 30 months. Late skin toxicity was mild to moderate in 60% and severe in 25% of patients. A reduction of the total dose and dose-per-fraction resulted in an acceptable CR rate and a significantly lower toxicity. TSEI is effective in early stage MF. Skin control and late skin toxicity seem to be dose-fractionation-schedule related. For the early stages, the optimal treatment schedule seems to be 24-30 Gy to the whole skin surface in 2.4-3.0 Gy fractions, given twice weekly over a period of four to six weeks. Total doses of 24-30 Gy at 2.4-3.0 Gy per fraction yielded comparable skin control rates with lower skin toxicity.     

Monte Carlo method to study the proton fluence for treatment planning

The proton beam at the Hahn Meitner Institute (HMI) in Berlin will be used for proton therapy of eye melanoma in the near future. As part of the pre-therapeutic studies, Monte Carlo calculations have been performed to investigate the primary fluence distribution of the proton beam including the influence of scattering foils, range shifters, modulator wheels, and collimators. Any material in the beam path will modify the therapeutic beam because of energy loss, multiple scattering, range straggling, and nuclear reactions. The primary fluence information is a pre-requisite for most pencil-beam treatment planning algorithms. The measured beam penumbra has been used as one of the parameters to characterize a proton beam for further calculations in a treatment planning algorithm. However, this phenomenological quantity represents only indirect information about the properties of the proton beam. In this work, an alternative parameterization of the beam exiting the vacuum window of the accelerator, as well as the beam right in front of the patient collimator, is introduced. A beam is fully characterized if one knows (for instance from Monte Carlo simulations) the particle distribution in energy, position, and angle, i.e., the phase space distribution. Therefore, parameters derived from this distribution can provide an alternative input in treatment planning algorithms. In addition, the method of calculation is introduced as a tool to investigate the influence of modifications in the beam delivery system on the behavior of the therapeutic proton beam.     

An adaptive control algorithm for optimization of intensity modulated radiotherapy considering uncertainties in beam profiles, patient set-up and internal organ motion

A new general beam optimization algorithm for inverse treatment planning is presented. It utilizes a new formulation of the probability to achieve complication-free tumour control. The new formulation explicitly describes the dependence of the treatment outcome on the incident fluence distribution, the patient geometry, the radiobiological properties of the patient and the fractionation schedule. In order to account for both measured and non-measured positioning uncertainties, the algorithm is based on a combination of dynamic and stochastic optimization techniques. Because of the difficulty in measuring all aspects of the intra- and interfractional variations in the patient geometry, such as internal organ displacements and deformations, these uncertainties are primarily accounted for in the treatment planning process by intensity modulation using stochastic optimization. The information about the deviations from the nominal fluence profiles and the nominal position of the patient relative to the beam that is obtained by portal imaging during treatment delivery, is used in a feedback loop to automatically adjust the profiles and the location of the patient for all subsequent treatments. Based on the treatment delivered in previous fractions, the algorithm furnishes optimal corrections for the remaining dose delivery both with regard to the fluence profile and its position relative to the patient. By dynamically refining the beam configuration from fraction to fraction, the algorithm generates an optimal sequence of treatments that very effectively reduces the influence of systematic and random set-up uncertainties to minimize and almost eliminate their overall effect on the treatment. Computer simulations have shown that the present algorithm leads to a significant increase in the probability of uncomplicated tumour control compared with the simple classical approach of adding fixed set-up margins to the internal target volume.     

Intensity modulation with electrons: calculations, measurements and clinical applications

Intensity modulation of electron beams is one step towards truly conformal therapy. This can be realized with the MM50 racetrack microtron that utilizes a scanning beam technique. By adjusting the scan pattern it is possible to obtain arbitrary fluence distributions. Since the monitor chambers in the treatment head are segmented in both x- and y-directions it is possible to verify the fluence distribution to the patient at any time during the treatment. Intensity modulated electron beams have been measured with film and a plane parallel chamber and compared with calculations. The calculations were based on a pencil beam method. An intensity distribution at the multileaf collimator (MLC) level was calculated by superposition of measured pencil beams over scan patterns. By convolving this distribution with a Gaussian pencil beam, which has propagated from the MLC to the isocentre, a fluence distribution at isocentre level was obtained. The agreement between calculations and measurements was within 2% in dose or 1 mm in distance in the penumbra zones. A standard set of intensity modulated electron beams has been developed. These beams have been implemented in a treatment planning system and are used for manual optimization. A clinical example (prostate) of such an application is presented and compared with a standard irradiation technique.     

Radioiodine therapy for hyperthyroidism in young patients--perception of risk and use

In the radiation treatment of inclined lesions the longitudinal axis of beam should be always parallel to the longitudinal axis of lesion in the treatment plane. Hence the choice of correct collimator angle is of prime importance in the oblique field off-axis treatment planning of inclined lesions. In this paper a graphical method is discussed to determine the collimator angles from the CT scans taken at different off-axis planes. A phantom was constructed which can simulate conditions like inclined lesion in the patient and CT scans were taken at different levels. The collimator angles estimated from CT scans are found to match with the collimator angles obtained from simulator within +/- 1 degree for various gantry angles. The method is applied to plan a case of cancer of the oesophagus (upper third), which is a typical example of inclined lesion.     

Improvements in the design of linear diffusers for photodynamic therapy

The angular radiance distribution of several linear diffusers used for photodynamic therapy (PDT) was measured. The forward scattering found previously was not observed for these designs. The improved isotropy leads to a better agreement between intended treatment site and actual maximum of the fluence rate profile when the linear diffuser is used in a hollow, cylindrical organ.     

An evaluation of epoxy resin phantom materials for electron dosimetry

The use of epoxy resin 'solid water' (water substitute) phantoms is becoming increasingly common in radiotherapy dosimetry, and depth ionization curves and conversion factors from ionization to dose identical to water have often been assumed. Fluence ratios of water to solid water for WTe (produced by Radiation Physics, St Bartholomew's Hospital, London) and RMI 457 (produced by Radiation Measurements Inc., Middleton, Wisconsin) have therefore been determined and have been found to decrease with energy, which, within measurement uncertainty, can be described with a linear function dependent on mean electron beam energy at the depth of measurement, Ed. The fluence ratios for WTe are very close to unity (i.e. within the measuring uncertainty) for most of the energies examined, the exception being a nominal 20 MeV beam. The results also show that an assumption of unity for the fluence ratios of RMI 457 may introduce a systematic error of the order of 1% in electron beam dosimetry at lower energies. As regards the depth ionization curves measured in the respective solid water materials, these are shown to be in agreement with those measured in water within the limits of the measuring uncertainty.     

Numerical Investigation of Residual Stress in Thick Titanium Alloy Plate Joined with Electron Beam Welding

A finite-element (FE) simulation process integrating three dimensional (3D) with two-dimensional (2D) models is introduced to investigate the residual stress of a thick plate with 50-mm thickness welded by an electron beam. A combined heat source is developed by superimposing a conical volume heat source and a uniform surface heat source to simulate the temperature field of the 2D model with a fine mesh, and then the optimal heat source parameters are employed by the elongated heat source for the 3D simulation without trial simulations. The welding residual stress also is investigated with emphasis on the through-thickness stress for the thick plate. Results show that the agreement between simulation and experiment is good with a reasonable degree of accuracy in respect to the residual stress on the top surface and the weld profile. The through-thickness residual stress of the thick plate induced by electron beam welding is distinctly different from that of the arc welding presented in the references.     

A generalized film technique for the verification of vertex fields used in the treatment of brain tumors

With the availability of commercial three-dimensional (3D)-treatment planning systems, more and more treatment plans call for the use of noncoplanar conformal beams for the treatment of brain tumors. However, techniques for the verification of many noncoplaner beams, such as vertex fields which involve any combination of gantry, collimator, and table angles, do not exist. The purpose of this work is to report on the results of an algorithm and a technique that have been developed for the verification of noncoplanar vertex fields used in the treatment of brain tumors. This technique is applicable to any geometric orientation of the beam, i.e., a beam orientation that consists of any combination of gantry, table, and collimator rotations. The method consists of superimposing a central plane image of a correctly magnified vertex field on a lateral or oblique field port film. To achieve this, the 3D coordinates of the projection of the isocenter onto the film for lateral (or oblique) as well as the vertex fields are determined and then appropriately matched. Coordinate transformation equations have been developed that enable this matching precisely. A film holder has been designed such that a film cassette can be secured rigidly along the side rails of the treatment table. The technique for taking a patient treatment setup verification film consists of two steps. In the first step, the gantry, table, and collimator angles for the lateral (or oblique) field are set and the usual double exposures are made; the first exposure corresponds to that of the treatment portal with the isocenter clearly identified and the second one a larger radiation field so that the peripheral anatomy is visible on the film. In the next step, the gantry, table, and collimator angles are positioned for the vertex field and the table is moved laterally and vertically and the film longitudinally to a position that will enable precise matching of the isocenter on the film. A third exposure is then taken with the vertex portal. What is seen on the film is a superposition of a central plane image of the vertex field onto the image of the lateral or oblique field. This technique has been used on 60 patients treated with noncoplanar fields for brain tumors. In all of these cases, the coincidence of the projection of the isocenter for the lateral (or oblique) and the vertex fields was found to be within 3 mm.     

Structural evolution of a strip-cast al alloy sheet processed by continuous equal-channel angular pressing

Experiments were conducted on strip-cast 1050 Al alloy sheets using an equal-channel angular pressing (ECAP) process to investigate the feasibility of the technique for producing metal strips. The developed process is capable of introducing shear deformation into metal strips in a continuous mode at a relatively fast forming speed of 10 to 50 m/min. The actual shear-flow patterns as a result of the continuous ECAP were demonstrated and compared with those obtained from numerical calculations. The effects of die geometry on the mechanical properties of the strips were investigated. Observations of the microstructural evolution in the equal-channel angular pressed (ECAPed) samples were conducted using transmission electron microscopy (TEM) as a function of oblique angles. The texture evolution was investigated using orientation distribution function (ODF) analysis. A possible application of this process for producing an Al alloy sheet with high formability and low earing was discussed by calculating the Lankford parameter and the planar anisotropy.     

Two replica blotting methods for fast immunological analysis of common proteins in two-dimensional electrophoresis

Knowledge of the photon spectrum of a radiotherapy beam is often needed for three-dimensional (3-D) dose calculations using Monte Carlo methods and/or algorithms employing energy deposition kernels. Direct measurement of the x-ray energy fluence spectrum is not feasible for the high-energy photon beams used clinically. In this paper, the spectrum is extracted from basic beam data that are readily obtained for a clinical beam. We describe the photon spectrum using just two parameters. One parameter, which determines the high-energy part of the spectrum, is obtained using the measured dose in the buildup region for a small field, where electron contamination of the beam can be neglected. The other parameter is extracted from the photon beam attenuation in water. The results compare favorably to spectra generated from Monte Carlo simulations.     

X-Ray microanalysis of grain boundary segregations in steels using the scanning transmission electron microscope

This paper describes an analysis which allows real composition segregation profiles to planar grain boundaries to be determined quantitatively from STEM X-ray microanalysis on thin foils. This analysis is applied to the measurement of such segregations in ironbased alloys. The influence of electron beam spreading within the foil is evaluated and an analysis is developed which allows solute composition profiles to be obtained from measured X-ray intensity profiles. The influence of various experimental parameters on the measured X-ray intensity profile are examined and discussed. The analysis procedure is applied to experimental measurements of tin segregated to prior austenite grain boundaries within the heat affected zone of a ’/2 PGt CrMoV low alloy steel weldment taken from a boiler steam chest.     

A method for implementing dynamic photon beam intensity modulation using independent jaws and a multileaf collimator

A mathematical model is derived for digitally controlled linear accelerators to deliver a desired photon intensity distribution by combining collimator motion and machine dose rate variations. It shows that, at any instant, the quotient of the machine dose rate and the speed of collimator motion is proportional to the gradient of the desired in-air photon fluence distribution. The model is applicable for both independently controlled collimator jaws and multileaf collimators and can be implemented by controlling different parameters to accommodate linear accelerators from different manufactures. For independent jaws, each pair of jaws creates photon fluence variations along the direction of the jaw movement. For multileaf collimators, where each leaf is independently controlled, any two-dimensional (2D) photon fluence distribution can be delivered. The model has been implemented for wedged isodose distributions using independent jaws, and 2D intensity modulation using a multileaf collimator. One-dimensional (1D) wedged isodose distributions are created by moving an independent jaw at constant speed while varying machine dose rate. 2D intensity modulation has been implemented using a 'dynamic stepping' scheme, which controls the leaf progression during irradiation at constant machine dose rate. With this automated delivery scheme, the beam delivery time for dynamic intensity modulation, which depends on the complexity of the desired intensity distribution, approaches that of conventional beam modifiers. This paper shows the derivation of the model, its application, and our delivery scheme. Examples of 1D dynamic wedges and 2D intensity modulations will be given to illustrate the versatility of the model, the simplicity of its application, and the efficiency of beam delivery. These features make this approach practical for delivering conformal therapy treatments.     

Drawing of continuous sections with a regular polygon profile

When designing the drawing process of sections with a regular polygon profile, it is convenient to use analytical expressions that make it possible to operatively evaluate the effect of different manufacturing factors. A calculation procedure for stress in this process is proposed. The relationships are derived for the calculation of the drawing stress, the optimal angle for the drawing-tool channel, the limiting deformation rate with the use of a standard tool, and the stress-state characteristic in the axial part of the deformation zone. The application of a drawing tool with optimum angles decreases the accumulation rate for defects of the metal structure and makes it possible to decrease the number of intermediate thermal treatments during multiple drawing.     

A model for roughness-induced fatigue crack closure

A model for predicting the crack closing stress intensity factor for roughness-induced closure of fatigue cracks is developed based on a two-dimensional approach considering crack opening and closure of an idealized crack path. The model highlights the contribution of irreversible cyclic planar slip at the crack tip, and is extended to real cases describing roughness-induced crack closure as a function of fracture surface roughness parameters at low ΔK levels where planar slip prevails. The resulting equation indicates that roughness-induced crack closure depends on the maximum stress intensity factor, the standard deviation of heights as well as the standard deviation of angles of the crack profile elements, and the yield stress of the material. Comparison between the prediction of the model and experimental data of K _cl for lamellar microstructures of Ti-2.5 Cu as well as TIMETAL 1100 shows good agreement.     

Global and Local Buckling of a Sandwich Beam

A two-dimensional mechanical model is developed to predict the global and local buckling of a sandwich beam, using classical elasticity. The face sheet and the core are assumed as linear elastic isotropic continua in a state of planar deformation. The core is assumed to have two deformation modes: antisymmetrical and symmetrical with respect to the core geometric midplane. Characteristics of the two deformation modes and the corresponding buckling behavior are shown and it appears that they are identical when the buckling wavelength is short. The present analysis is compared with various previous analytical studies and corresponding experimental results. On the basis of the model developed here, validation and accuracy of several previous theories are discussed for different geometric and material properties of a sandwich beam. The results presented in this paper, verified through finite-element analysis and experiment, are an accurate prediction of the overall buckling behavior of a sandwich beam, for a wide range of material and geometric parameters.     

Dosimetric evaluation of the Siemens Virtual Wedge

Recently, Siemens has introduced its Virtual Wedge (VW). On a Mevatron accelerator, this option generates a wedge-like dose profile by moving a collimator jaw at constant speed while varying the dose rate. In this paper the formalism is given that is used to deliver a wedge profile and from that the expressions for possible combinations of wedge angle, field size and delivered MUs are derived. Also the time needed to deliver a VW field is calculated. An effective attenuation coefficient mu is used in the implementation. For three beam energies, values of mu are determined in order to get VW angles that are as close as possible to the hard wedge angles, over a wide range of field sizes and wedge angles. Linearity with number of MUs and gantry angle dependence of the generated dose profiles were checked. These factors did not have a significant influence on the VW dose profiles. Wedge factors should be close to unity in the VW implementation. We have measured a number of wedge factors and found that they start to deviate from 1 with more than 1% for large wedge angles and field sizes, up to 3.5% for a 19 x 19 cm2, 60 degrees VW field. The Virtual Wedge turned out to be a reliable tool that can be used clinically, provided that it can be handled by the treatment planning system. It provides extra flexibility and usually results in shorter beam on times.