Treatment head design for multileaf collimated high-energy electrons.

This paper describes how a conventional treatment head can be modified for use of multileaf collimated electron beams. Automatic and dynamic beam delivery are possible for both electrons and photons by using the computer controlled multileaf collimator (MLC) for both photon and electron beams. Thereby, the electron beams can be mixed more freely into the treatment to take advantage of the specific depth modulation characteristics of electrons. The investigation was based on Monte Carlo calculations using the software package BEAM. The physical parameters used in this optimization were the beam penumbra and the virtual/effective point source position. These parameters are essential for shaping beams, beam matching and for dosimetry calculations. The optimization was carried out by modifying a number of parameters: replacing the air atmosphere in the treatment head with helium, adding a helium bag below the MLC, changing the position of the scattering foils, modifying the monitor chamber, and adjusting the position of the MLC. The beam characteristics for some of these designs were found to fulfil our criteria for clinically useful beams down to at least 9 MeV.     

Multileaf collimation of electrons--clinical effects on electron energy modulation and mixed beam therapy depending on treatment head design

The aim of this study was to explore the possibilities of using multileaf-collimated electron beams for advanced radiation therapy with conventional scattering foil flattened beams. Monte Carlo simulations were performed with the aim to improve electron beam characteristics and enable isocentric multileaf collimation. The scattering foil positions, monitor chamber thickness, the MLC location and the amount of He in the treatment head were optimized for three common commercial accelerators. The performance of the three optimized treatment head designs was compared for different SSDs in air, at treatment depth in water and for some clinical cases. The effects of electron/photon beam matching including generalized random and static errors using Gaussian one-dimensional (1 D) error distributions, and also electron energy modulation, were studied at treatment depth in water. The modification of the treatment heads improved the electron beam characteristics and enabled the use of multileaf collimation in isocentric delivery of both electron and photon beams in a mixed beam IMRT procedure.     

Mixing intensity modulated electron and photon beams: combining a steep dose fall-off at depth with sharp and depth-independent penumbras and flat beam profiles.

For application in radiotherapy, intensity modulated high-energy electron and photon beams were mixed to create dose distributions that feature: (a) a steep dose fall-off at larger depths, similar to pure electron beams, (b) flat beam profiles and sharp and depth-independent beam penumbras, as in photon beams, and (c) a selectable skin dose that is lower than for pure electron beams. To determine the required electron and photon beam fluence profiles, an inverse treatment planning algorithm was used. Mixed beams were realized at a MM50 racetrack microtron (Scanditronix Medical AB, Sweden), and evaluated by the dose distributions measured in a water phantom. The multileaf collimator of the MM50 was used in a static mode to shape overlapping electron beam segments, and the dynamic multileaf collimation mode was used to realize the intensity modulated photon beam profiles. Examples of mixed beams were generated at electron energies of up to 40 MeV. The intensity modulated electron beam component consists of two overlapping concentric fields with optimized field sizes, yielding broad, fairly depth-independent overall beam penumbras. The matched intensity modulated photon beam component has high fluence peaks at the field edges to sharpen this penumbra. The combination of the electron and the photon beams yields dose distributions with the characteristics (a)-(c) mentioned above.     

Design of a fast multileaf collimator for radiobiological optimized IMRT with scanned beams of photons, electrons, and light ions

Intensity modulated radiation therapy is rapidly becoming the treatment of choice for most tumors with respect to minimizing damage to the normal tissues and maximizing tumor control. Today, intensity modulated beams are most commonly delivered using segmental multileaf collimation, although an increasing number of radiation therapy departments are employing dynamic multileaf collimation. The irradiation time using dynamic multileaf collimation depends strongly on the nature of the desired dose distribution, and it is difficult to reduce this time to less than the sum of the irradiation times for all individual peak heights using dynamic leaf collimation [Svensson et al., Phys. Med. Biol. 39, 37-61 (1994)]. Therefore, the intensity modulation will considerably increase the total treatment time. A more cost-effective procedure for rapid intensity modulation is using narrow scanned photon, electron, and light ion beams in combination with fast multileaf collimator penumbra trimming. With this approach, the irradiation time is largely independent of the complexity of the desired intensity distribution and, in the case of photon beams, may even be shorter than with uniform beams. The intensity modulation is achieved primarily by scanning of a narrow elementary photon pencil beam generated by directing a narrow well focused high energy electron beam onto a thin bremsstrahlung target. In the present study, the design of a fast low-weight multileaf collimator that is capable of further sharpening the penumbra at the edge of the elementary scanned beam has been simulated, in order to minimize the dose or radiation response of healthy tissues. In the case of photon beams, such a multileaf collimator can be placed relatively close to the bremsstrahlung target to minimize its size. It can also be flat and thin, i.e., only 15-25 mm thick in the direction of the beam with edges made of tungsten or preferably osmium to optimize the sharpening of the penumbra. The low height of the collimator will minimize edge scatter from glancing incidence. The major portions of the collimator leafs can then be made of steel or even aluminum, so that the total weight of the multileaf collimator will be as low as 10 kg, which may even allow high-speed collimation in real time in synchrony with organ movements. To demonstrate the efficiency of this collimator design in combination with pencil beam scanning, optimal radiobiological treatments of an advanced cervix cancer were simulated. Different geometrical collimator designs were tested for bremsstrahlung, electron, and light ion beams. With a 10 mm half-width elementary scanned photon beam and a steel collimator with tungsten edges, it was possible to make as effective treatments as obtained with intensity modulated beams of full resolution, i.e., here 5 mm resolution in the fluence map. In combination with narrow pencil beam scanning, such a collimator may provide ideal delivery of photons, electrons, or light ions for radiation therapy synchronized to breathing and other organ motions. These high-energy photon and light ion beams may allow three-dimensional in vivo verification of delivery and thereby clinical implementation of the BioArt approach using Biologically Optimized three-dimensional in vivo predictive Assay based adaptive Radiation Therapy [Brahme, Acta Oncol. 42, 123-126 (2003)].     

Testing of the stability of intensity modulated beams generated with dynamic multileaf collimation, applied to the MM50 racetrack microtron

Recently, we have published a method for the calculation of required leaf trajectories to generate optimized intensity modulated x-ray beams by means of dynamic multileaf collimation [Phys. Med. Biol. 43, 1171-1184 (1998)]. For the MM50 Racetrack Microtron it has been demonstrated that the dosimetric accuracy of this method, in combination with the dose calculation algorithm of the Cadplan 3D treatment planning system, is adequate for a clinical application (within 2% or 0.2 cm). Prior to initiating patient treatment with dynamic multileaf collimation (DMLC), tests have been performed to investigate the stability of DMLC fields generated at the MM50, (i) in time, (ii) subject to gantry rotation and (iii) in case of treatment interrupts, e.g., caused by an error detected by the treatment machine. The stability of relative dose profiles, normalized to a reference point in a relatively flat part of the modulated beam profile, was assessed from measurements with an electronic portal imaging device (EPID), with a linear diode array attached to the collimator and with film. The dose in the reference point was monitored using an ionization chamber. Tests were performed for several intensity modulated fields using 10 and 25 MV photon beams. Based on film measurements for sweeping 0.1 cm leaf gaps it was concluded that in an 80 days period the variation in leaf positioning was within 0.05 cm, without requiring any recalibration. For a uniform 10x10 cm2 field, realized dynamically by a scanning 0.4x10 cm2 slit beam, a maximum variation in slit width of 0.01 cm was derived from ionization chamber measurements, both in time and for gantry rotation. For a clinical example, the dose in the reference point reproduced within 0.2% (1 SD) over a period of 100 days. Apart from regions with very large dose gradients, variations in the relative beam profiles measured with the EPID were generally less than 1% (1 SD). For different gantry angles the dose profiles also reproduced within 1%, showing that gravity has a negligible influence. No significant deviations between uninterrupted and interrupted treatments could be observed, indicating that the effects of acceleration and deceleration of the leaves are negligible and that a DMLC treatment can be finished correctly after a treatment interrupt. Our previous and present studies have demonstrated that the dosimetric accuracy and stability of intensity modulated beams, generated at the MM50 by means of dynamic multileaf collimation, are adequate for clinical use. Patient treatment using dynamic multileaf collimation has been started in our clinic.     

Application of Fermi scattering theory to a magnetically scanned electron linear accelerator

This paper uses a solution to the Fermi electron transport equation for an isotropic point source to characterize the magnetically scanned broad electron beams from the Sagittaire Therac 40 accelerator in the air space above patients. Thick lead collimation is shown to be adequately modeled by an infinitely thin absorbing plate when used to predict penumbra shape. A relationship between broad beam penumbra width and the value of the root-mean-square spatial Gaussian spread sigma (z) of an elementary pencil beam is derived. This relationship is applicable for any rectangular field size. Measurement of the variation in broad beam penumbra width with source-surface distance (SSD) for a 7-MeV beam locates the isotropic source to be coincident with the exit window of the accelerator and indicates that the scattering effect of the monitor chamber may be considered negligibly small. Using this source location accurate predictions of beam profile shape for any clinically used beam energy, SSD, or field size are made in the presence of lead trimmer collimation. Field penumbra beyond the photon collimation system is formed in each lateral direction by two lead blocks whose faces are aligned along a diverging ray emanating from the source. The photon collimator closest to the source restricts the field size causing a variation of both fluence and the mean square angle spread of the electrons across the plane at the level of the lower collimator. This variation is accounted for by introducing an empirical perturbation factor into the mathematical formalism. An interesting feature of this perturbation factor is that it is field size dependent and its effect on penumbra width may be scaled for both beam energy and SSD to accurately predict beam profile shape.     

The in-air scattering of clinical electron beams as produced by accelerators with scanning beams and diaphragm collimators

The electron distribution F(x, y, z, theta x, theta y) in air has been evaluated for a clinical electron beam emanating from a scanning beam accelerator in which the collimation of the beam is performed by means of diaphragm collimators. The multiple scattering theory of Fermi turns out to be adequate in describing this electron distribution. In this theory, the only parameter to be determined experimentally is the angular variance at the level of the collimator blocks. Generally, this angular variance features the same energy dependence as the angular scattering power and its value at an arbitrary energy can be derived from measuring the penumbra widths of off-axis profiles in air, at various distances beyond the collimator blocks. Then, the angular variance at the level of a secondary diaphragm collimator can be calculated, as well as off-axis profiles in air at arbitrary distances. In this way, the relative electron distribution at the surface of patients can be calculated easily. This in turn serves adequately as input to the calculation of patient dose distributions in radiation therapy planning.     

On the initial angular variances of clinical electron beams

Electron beam radiotherapy treatment planning systems need to be fed with the characteristics of the high-energy electron beams (4-50 MeV) from the specifically applied accelerator. Beams can be characterized by their mean initial energy, effective initial angular variance, virtual source position and the resulting central axis depth dose distribution in water. This information is the only input to pencil beam dose calculation models. Newer calculation models like macro Monte Carlo, voxel Monte Carlo and phase space evolution require as input the full initial phase space or a parametrization of that initial phase space, generally consisting of a primary beam component and one or more scatter components. This primary beam component is often characterized by initial energy, primary beam initial angular variance and virtual source distance. The purpose of the present investigation was to investigate to what extent standard values can be used both for the effective initial angular variance as input to pencil beam models and for the primary beam initial angular variance. Comprehensive benchmark data were obtained on the initial angular variance of various types of accelerator, for various energies and field sizes. The initial angular variance sigma2theta(x) has been derived from penumbra measurements in air by means of film dosimetry at various distances from the lower collimator. For the types of accelerator used in radiotherapy nowadays the measurements show values for sigma2theta(x)/T(E) of around 13 cm where T(E) is the ICRU-35 linear angular scattering power in air. This value can be chosen as standard value for the primary beam initial angular variance, only slightly compromising the dose calculation accuracy. As input to pencil beam models, an effective sigma2theta(x)/T(E) should be used incorporating the scatter from the lower collimator. For the case that the air gaps between lower collimator and patient are small (5-10 cm) an effective sigma2theata(x)/T(E) of 20 cm has been found and is recommended as the standard input for pencil beam models. Of the accelerators investigated, a different value was found only for the Elekta SL15, i.e. 50% higher for the effective sigma2theta(x)/T(E).     

多叶准直器的叶片最优位置的分析

多叶准直器使射线束的截面形状与病灶的轮廓相吻合,可以实现适形放射治疗。由于多叶准直器的叶片宽度的限制,平滑的病灶廓只能由多叶准直器的不同叶片形成的阶梯状轮廓来近似,叶片的位置决定了近似带来的误差,本文给出一种确定叶片最优位置的方法。     

Compact multileaf collimator for conformal and intensity modulated fast neutron therapy: electromechanical design and validation

The electromechanical properties of a 120-leaf, high-resolution, computer-controlled, fast neutron multileaf collimator (MLC) are presented. The MLC replaces an aging, manually operated multirod collimator. The MLC leaves project 5 mm in the isocentric plane perpendicular to the beam axis. A taper is included on the leaves matching beam divergence along one axis. The 5-mm leaf projection width is chosen to give high-resolution conformality across the entire field. The maximum field size provided is 30 x 30 cm2. To reduce the interleaf transmission a 0.254-mm blocking step is included. End-leaf steps totaling 0.762 mm are also provided allowing opposing leaves to close off within the primary radiation beam. The neutron MLC also includes individual 45 degrees and 60 degrees automated universal tungsten wedges. The automated high-resolution neutron collimation provides an increase in patient throughput capacity, enables a new modality, intensity modulated neutron therapy, and limits occupational radiation exposure by providing remote operation from a shielded console area.     

Description and dosimetric verification of the PEREGRINE Monte Carlo dose calculation system for photon beams incident on a water phantom

PEREGRINE is a three-dimensional Monte Carlo dose calculation system written specifically for radiotherapy. This paper describes the implementation and overall dosimetric accuracy of PEREGRINE physics algorithms, beam model, and beam commissioning procedure. Particle-interaction data, tracking geometries, scoring, variance reduction, and statistical analysis are described. The BEAM code system is used to model the treatment-independent accelerator head, resulting in the identification of primary and scattered photon sources and an electron contaminant source. The magnitude of the electron source is increased to improve agreement with measurements in the buildup region in the largest fields. Published measurements provide an estimate of backscatter on monitor chamber response. Commissioning consists of selecting the electron beam energy, determining the scale factor that defines dose per monitor unit, and describing treatment-dependent beam modifiers. We compare calculations with measurements in a water phantom for open fields, wedges, blocks, and a multileaf collimator for 6 and 18 MV Varian Clinac 2100C photon beams. All calculations are reported as dose per monitor unit. Aside from backscatter estimates, no additional, field-specific normalization is included in comparisons with measurements. Maximum discrepancies were less than either 2% of the maximum dose or 1.2 mm in isodose position for all field sizes and beam modifiers.     

Energy- and intensity-modulated electron beams for radiotherapy

This work investigates the feasibility of optimizing energy- and intensity-modulated electron beams for radiation therapy. A multileaf collimator (MLC) specially designed for modulated electron radiotherapy (MERT) was investigated both experimentally and by Monte Carlo simulations. An inverse-planning system based on Monte Carlo dose calculations was developed to optimize electron beam energy and intensity to achieve dose conformity for target volumes near the surface. The results showed that an MLC with 5 mm leaf widths could produce complex field shapes for MERT. Electron intra- and inter-leaf leakage had negligible effects on the dose distributions delivered with the MLC, even at shallow depths. Focused leaf ends reduced the electron scattering contributions to the dose compared with straight leaf ends. As anticipated, moving the MLC position toward the patient surface reduced the penumbra significantly. There were significant differences in the beamlet distributions calculated by an analytic 3-D pencil beam algorithm and the Monte Carlo method. The Monte Carlo calculated beamlet distributions were essential to the accuracy of the MERT dose distribution in cases involving large air gaps, oblique incidence and heterogeneous treatment targets (at the tissue-bone and bone-lung interfaces). To demonstrate the potential of MERT for target dose coverage and normal tissue sparing for treatment of superficial targets, treatment plans for a hypothetical treatment were compared using photon beams and MERT.     

Design considerations for a computer controlled multileaf collimator for the Harper Hospital fast neutron therapy facility

The d(48.5) + Be neutron beam from the Harper Hospital superconducting cyclotron is collimated using a unique multirod collimator (MRC). A computer controlled multileaf collimator (MLC) is being designed to improve efficiency and allow for the future development of intensity modulated radiation therapy with neutrons. For the current study the use of focused or unfocused collimator leaves has been studied. Since the engineering effort associated with the leaf design and materials choice impacts significantly on cost, it was desirable to determine the clinical impact of using unfocused leaves in the MLC design. The MRC is a useful tool for studying the effects of using focused versus unfocused beams on beam penumbra. The effects of the penumbra for the different leaf designs on tumor and normal tissue DVHs in two selected sites (prostate and head and neck) was investigated. The increase in the penumbra resulting from using unfocused beams was small (approximately 1.5 mm for a 5 x 5 cm2 field and approximately 7.6 mm for a 25 x 25 cm2 field at 10 cm depth) compared to the contribution of phantom scatter to the penumbra width (5.4 and 20 mm for the small and large fields at 10 cm depth, respectively). Comparison of DVHs for tumor and critical normal tissue in a prostate and head and neck case showed that the dosimetric disadvantages of using an unfocused rather than focused beam were minimal and only significant at shallow depths. For the rare cases, where optimum penumbra conditions are required, a MLC incorporating tapered leaves and, thus, providing focused collimation in one plane is necessary.     

Electron beam collimation with focused and curved leaf end MLCs--experimental verification of Monte Carlo optimized designs

In general, electron beams from conventional accelerators using applicators with lead alloy inserts are not suitable for advanced conformal radiation therapy. However, interesting electron treatments have been demonstrated on a few advanced accelerators. These accelerators have been equipped with helium filled treatment heads and computer controlled MLCs that produce clinically useful energy modulated electron beams or mixed photon electron beams in an automated sequence. This study analyzes the characteristics of different MLC designs, curved and focused leaf ends in helium filled treatment heads, with respect to their effect on electron beams. In addition, this study analyzes the effects that different treatment head designs have on the output factor due to collimator scattering and shielding of secondary sources during treatment. The investigation of the different treatment head designs was performed with the Monte Carlo package BEAM and was verified by experimental methods. The results show that the difference between curved leaf ends and focused ends is negligible in most practical cases. The results also show the importance of scattering foil optimization in the optimization of parameters such as penumbra, virtual source position, and in the reduction of the output variation. In all cases, the experimental data verifies the calculations.     

The flatness of Siemens linear accelerator x-ray fields

The primary definer for Siemens MXE and MDX linear accelerators projects a circular opening with a radius of 25 cm at 100 cm from the target. Our measurements of photon beam profiles, however, indicate that the photon fluence drops to 95% of the central axis value at a radius of 18 cm. The flattening filter for these machines projects a flattened field size that is much smaller than the primary definer would allow. The clinical implications of this mismatch for large rectangular fields and for fields defined by asymmetric jaws are discussed and solutions are considered. A large field flattener was designed for our Siemens MXE 6 MV beam using Monte Carlo simulation of the treatment head and water phantom. The accuracy required of source and geometry details for dose distributions calculation is presented. The key parameters are the mean energy and focal spot size of the electron beam incident on the exit window, the material composition, and thickness profile of the exit window, target, flattener, and primary collimator, and the position of the primary collimator relative to the target. Profiles were more sensitive than central axis depth doses to simulation details. The beam energy and primary collimator position were selected to achieve good agreement between measured and calculated dose distributions. The flattener we designed with Monte Carlo was machined from brass and mounted on our MXE treatment unit. Measurements demonstrate that the large field flattener extends the useful radius of the field out to 22 cm, right into the penumbra cast by the primary collimator.     

Collimated electron beams and their associated penumbra widths

The Fermi-Eyges multiple-scattering theory for electrons is applied to calculate profiles of collimated electron beams. The dose profile below the collimator is a convolution of the intensity distribution of the electrons at the level of the collimator and the distribution arising from the propagation of a Gaussian point source from the collimator to the level of the calculation. The electrons at the level of the collimator possess an angular distribution characteristic of the configuration of the electron beam at the vacuum window. Hence, the dose profile and its associated penumbra width can be expressed in terms of the angular moments of the distribution of the electrons at the collimator. The dependence of the penumbra width on the configuration-dependent angular spread of the electrons at the collimator accounts for differences in the size of the penumbra between two broad-beam configurations. These differences are also seen experimentally. We have also studied the dependence of the angular moments of the electrons upon scattering foils present above the collimator and the position of the beam-broadening device in the accelerator head.     

Electron contamination from different materials in high energy photon beams

Relative lateral electron surface dose distributions from filters and air in high energy photon beams were determined using the Fermi-Eyges theory of multiple scattering. The model includes transmission and angular scattering in materials and in air. Backscatter from the phantom was also estimated. The variation of surface dose with different parameters such as atomic number, thickness and position of material, field size and photon energy was investigated. The calculated data show good agreement with experiment. For 60Co gamma rays, electron filters of medium atomic number give the lowest surface dose, whereas for higher energies a low to medium atomic number material should be used, especially for short material-phantom distances and large field sizes. The contribution to surface dose can be reduced by over 30% by placing a thin foil of high atomic number after a low to medium atomic number filter, thus scattering some electrons from the beam. For 60Co gamma rays the air is often the dominant source of contaminating electrons because of high multiple scatter loss at this energy, while for higher photon energies the beam flattening filter is the main electron source because of the small emission angle of the electrons and the small scattering power at high energies.     

Dosimetric properties of magnetically collimated electron beams for radiation therapy

A method of generating magnetically collimated electron beams is developed and the dosimetric properties of magnetically collimated electrons are investigated. An in-air magnetic collimator device was designed and constructed for the study. The magnetic collimator was placed above the exit port of a 14 x 14 cm2 electron cone. Axial magnetic field of approximately 0.6 Tesla is generated inside the collimator via an array of permanent magnets. Fixed and rotational magnetically collimated electron beams were delivered and measured in phantoms. We found that magnetically collimated electron beams significantly lower the surface dose as compared with conventional electron beams. A magnetically collimated arc beam further reduces the surface dose to less than 20% of the maximum dose inside the target. The dose per monitor unit at d(max) for the magnetically collimated electron beams was significantly (approximately 40%) higher than that of the conventional electron beams. The use of magnetic collimation may lead to improved delivery techniques for breast and head and neck cancer treatments.     

Dose distributions of x-ray fields as shaped with multileaf collimators.

Multileaf collimators (MLC) with various blade widths were simulated using standard cerrobend blocks, and three-dimensional dose computations were carried out to study the resultant radiation field edges. Film measurements made with 6 and 18 MV x-ray beams were compared with calculations that employed a three-dimensional Fourier convolution. A spatial accuracy of better than 3 mm was found in the 50% isodose line of the penumbral region with a calculation voxel size of 5 mm x 5 mm x 5 mm. The computer simulation was used to study the deviation of the calculated 50% isodose line from the desired geometric field edge using various MLC blade positions. The study suggests that multileaf collimation to the outside of the desired field edge will lead to overdose outside the field, whereas multileaf collimation to the inside of the desired field edge will lead to underdose inside the field. When the direction of travel of the leaves with respect to the field edge is near 45 degrees, the 50% isodose of a multileaf-collimated beam will fall close to the desired edge with no underdose when the leaf corners are allowed to insert into the desired field edge by 1.2 mm for 6 MV x-rays and 1.4 mm for 18 MV x-rays using a 1 cm wide leaf. These blade offsets account for the scattering of photons and electrons in the medium within the penumbral region.     

Depth dose enhancement of electron beams subject to external uniform longitudinal magnetic fields: a Monte Carlo study

We studied the dose distributions from electron beams subjected to a longitudinal magnetic field applied to them before they reach the phantom. We found that specific combinations of the length and intensity of the magnetic field produced enhancement of the peaks of the central-axis depth-dose distributions. The EGS4 Monte Carlo system was used in this study. In the simulations, a uniform axial magnetic field parallel to the electron beam direction was applied to the air gap between the collimation and the phantom. We extensively studied the simplified case of an 18 MeV electron beam point source. Dose deposition was calculated for various magnetic field strengths, distances through which the magnetic field was applied, collimation sizes, and source to collimation distances. The magnetic field strengths varied from 0 to 3 T, the source-to-collimation distances studied were 50 and 95 cm, the collimation sizes studied were 10 x 10 and 20 x 20 cm2, and the distance through which the field was applied ranged from 10 to 20 cm. Specific combinations of these variables resulted in as much as a 70% enhancement of the peak dose relative to the surface dose. Finally, to determine how the geometry of a real accelerator affects the resulting dose distribution, we performed a full simulation of an Elekta SL20 linear accelerator and compared the results with the ideal case.