Narrow Pencil Beam Research Articles

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)].

Combinations of low/high permittivity and/or permeability substrates for highly directive planar metamaterial antennas

An investigation of planar metamaterial antennas consisting of grounded metamaterial substrates with low and/or high values of the electric permittivity and/or the magnetic permeability excited by dipole sources is presented. Their performances are characterised in terms of their capability to radiate high levels of power density in the broadside direction and to produce narrow pencil beams pointing at broadside with high directivity. To achieve a high directivity, a pair of weakly attenuated cylindrical leaky waves is excited along the metamaterial substrate; sufficient conditions are established for the existence of such leaky waves in terms of the values of the substrate permittivity and permeability. Approximate closed-form expressions are derived for the phase and attenuation constants of the leaky waves. Numerical results are given in order to illustrate the radiative features of this class of antennas and to validate the theoretical analysis.

217 Development of a scanning system for intensity modulated radiation therapy with photon pencil beams

The increasing cancer incidence requires more accurate and cost-effective diagnostic as well as therapeutic methods. Intensity modulated radiation therapy is now rapidly being introduced at many radiation therapy departments with a clear trend towards radiobiological treatment planning using 4D-diagnostic imaging and -treatment delivery. Therefore a magnetic system for scanning of narrow photon pencil beams has been developed for fast 4D-IMRT. The magnetic scanning system has been modelled in full 3D taken into account all aspects such as fringing field, magnetic leakage and cooling. The magnetic system consists of two perpendicular scanning magnets, a bending magnet and a purging magnet placed after a thin bremstrahlung target and in the same plane as the last scan magnet. The scanning magnets operate in orthogonal planes and the first scanning magnet (bending plane) is placed so that the image of its scanning centre lies near the effective centre of the second scan magnet (cross plane). In this way a very compact deflection system is obtained, with the position of the effective radiation source coinciding for the two orthogonal planes. The focus of the intrinsic electron beam is at the top of the target placed near the end of the last scanning magnet. The composition and thickness of this target determines the energy of the transmitted electrons and the full width half maximum (FWHM) of the photon beam. Here the target is made of 3 mm beryllium creating a therapeutic beam of 10-15 mm (FWHM) for an incident electron beam of 70-50 MeV. The transmitted electron beam has only lost around 0.5-1.0 MeV in the target and must be removed from the therapeutic beam. Therefore downstream of the target a strong and fast purging magnet integrated with an electron collector has been developed. The purging magnet operates in the same plane as the last scanning magnet and deflects the transmitted high energy electrons to the same area on the electron collector independent of the direction of the scanned photon beam. The overall design of the treatment head is compact and the narrow photon beam will allow a very accurate dose delivery particularly when combined with a thin efficient leaf collimator designed for pencil beam IMRT.

Optimization of beam parameters and treatment planning for intensity modulated proton therapy.

One of the objectives of the ongoing research and development work at the Northeast Proton Therapy Center (NPTC) in Boston is to perform optimized intensity modulated proton therapy (IMPT) treatments. Such treatments may be delivered by magnetically scanning a narrow proton pencil beam across the target volume, while both the scanning speed and the intensity of the beam are modulated. Localization of the proton dose in space allows one to yield dose distributions that are highly conformal to the target volume, thus minimizing the dose delivered to the surrounding healthy tissue. The aim of the current research is to determine technically optimal and clinically relevant specifications for the scanned beam delivery system, which is being developed in collaboration with Ion Beam Applications (IBA); and to create a link between the treatment planning and the beam delivery. IMPT treatment planning is performed for patient cases treated at the NPTC, with KonRad Pro software developed at the German Cancer Research Center (DKFZ). For the IMPT delivery, the proton intensity maps, optimized for discrete pencil beam spots, need to be translated into continuous scanning patterns. At the same time it is necessary to minimize the discrepancy between the planned and delivered doses which may result from such conversion, as well as from the technical limitations of the delivery system. Possibilities have been investigated for improving the proton dose conformity by optimizing the beam and scanning nozzle parameters, and by taking the specifics and limitations of the system into account in the treatment planning stage.

Beam delivery sequencing for intensity modulated proton therapy

Methods of beam fluence sequencing for intensity modulated proton therapy (IMPT) using the beam scanning technique are presented. Proton beam weight maps optimized by the treatment planning system (TPS) for a discrete set of regularly spaced narrow pencil beams were interpolated, using convolution with various kernel functions, to simulate continuous beam scanning on a raster pattern. Expected dose distributions at the proton Bragg peak range were then calculated and compared to those planned by the TPS, to evaluate the discrepancy due to the differences between the treatment planning and delivery approaches. An iteratively optimized adjustment was applied to the simulated continuous beam fluence profiles to reduce such discrepancy. Calculation showed that by accounting for the specifics of the scanning method, the planned dose distribution on the target may be reproduced to within 0.5% of the maximum target dose, given the pencil beam spacing smaller or equal to the beam σ is used for treatment planning. For the beam weight maps generated using a spot spacing larger than σ, a substantial reduction in the calculated dose discrepancy may be attained by applying an iterative adjustment of fluence profiles obtained by interpolating over artificially expanded set of beam spots.

Photon scatter kernels for intensity modulating radiation therapy filters

The most important beam property while optimizing photon therapy is the ability to modulate the intensity of the beam. The use of photon absorbers for intensity modulation of beam profiles requires special attention to be paid to the alteration of beam properties due to scatter and spectral changes, in addition to the desired intensity modulation. In this study the influence of photon scatter in high-density filters irradiated with very narrow photon pencil beams was investigated. A simple analytical relation is developed to quantify the contribution by scattered photons. A scatter kernel was derived by convolving the first Compton scatter distribution with an approximate expression for the second-order scattered photons. The calculations were validated experimentally with film dosimetry and also by using Monte Carlo simulations. Results show that the difference in photon scatter estimation by different methods is relatively small when higher order scattering is accounted for. At 6 MV x-rays the agreement is slightly better than that for 18 MV x-rays results. The simple relation presented in this paper can be used to account for the scattered photon contribution in filter optimization codes to deliver biologically or physically optimized intensity modulated treatments.

X-ray telescope onboard Astro-E II Ground-based x-ray characterization

X-ray characterization measurements of the x-ray telescope (XRT) onboard the Astro-E satellite were carried out at the Institute of Space and Astronautical Science (Japan) x-ray beam facility by means of a raster scan with a narrow x-ray pencil beam. The on-axis half-power diameter (HPD) was evaluated to be 1.8?-2.2?, irrespective of the x-ray energy. The on-axis effective areas of the XRTs for x-ray imaging spectrometers (XISs) were approximately 440, 320, 240, and 170 cm(2) at energies of 1.49, 4.51, 8.04, and 9.44 keV, respectively. Those of the x-ray spectrometer (XRS) were larger by 5-10%. The replication method introduced for reflector production significantly improved the imaging capability of the Advanced Satellite for Cosmology and Astrophyics (ASCA) XRT, whose HPD is ~3.6?. The increase in the effective area by a factor of 1.5-2.5, depending upon the x-ray energy, compared with that of the ASCA, was brought about by mechanical scale up and longer focal lengths. The off-axis HPDs were almost the same as those obtained on the optical axis. The field of view is defined as the off-axis angle at which the effective area becomes half of the on-axis value. The diameter of the field of view was ~19? at 1.49 keV, decreasing with increasing x-ray energy, and became ~13? at 9.44 keV. The intensity of stray light and the distribution of this kind of light on the focal plane were measured at the large off-axis angles 30? and 60?. In the entire XIS field of view (25.4 mm x 25.4 mm), the intensity of the stray light caused by a pointlike x-ray source became at most 1% of the same pointlike source that was on the optical axis.

Composite energy deposition kernels for focused point monodirectional photon beams

A 3D volume overlap algorithm has been developed for converting energy deposition kernels between arbitrary 2D and 3D irradiation geometries. The kernels can be used as convolution kernels in inverse radiation therapy planning and as accurate descriptions of the dose distributions for clinically important beam geometries. The new method of dose calculation combining Monte Carlo and analytical methods has introduced an improved accuracy in dose calculation on the fractional per cent level. The comparisons are also made for a wide range of photon spectra and irradiation geometries from narrow point monodirectional pencil beams to finite uniform beams, 4 steradians isotropically converging beams and divergent beams from isotropic point sources. It is seen that the photon scatter penumbra is highest at low photon energies whereas the secondary electron penumbra is widest at high photon energies, making low energy beams more interesting for small targets and high energy beams most useful for large deep-seated targets.

Beam characteristics and clinical possibilities of a new compact treatment unit design combining narrow pencil beam scanning and segmental multileaf collimation

Not until the last decade has flexible intensity modulated three-dimensional dose delivery techniques with photon beams become a clinical reality, first in the form of heavy metal transmission blocks and other beam compensators, then in dynamic and segmented multileaf collimation, and most recently by scanning high-energy narrow electron and photon beams. The merits of various treatment unit and bremsstrahlung target designs for high-energy photon therapy are investigated theoretically for two clinically relevant target sites, a cervix and a larynx cancer both in late stages. With an optimized bremsstrahlung target it is possible to generate photon beams with a half-width of about 3 cm at a source to axis distance (SAD) of 100 cm and an initial electron energy of 50 MeV. By making a more compact treatment head and shortening the SAD, it is possible to reduce the half-width even further to about 2 cm at a SAD of 70 cm and still have sufficient clearance between the collimator head and the patient. One advantage of a reduced SAD is that the divergence of the beam for a given field size on the patient is increased, and thus the exit dose is lowered by as much as 1%/cm of the patient cross section. A second advantage of a reduced SAD is that the electron beam on the patient surface will be only about 8 mm wide and very suitable for precision spot beam scanning. It may also be possible to reduce the beamwidth further by increasing the electron energy up to about 60 MeV to get a photon beam of around 15 mm half-width and an electron beam as narrow as 5 mm. The compact machine will be more efficient and easy to work with, due to the small gantry and the reduced isocentric height. For a given target volume and optimally selected static multileaf collimator, it is no surprise that the narrowest possible scanned elementary bremsstrahlung beam generates the best possible treatment outcome. In fact, by delivering a few static field segments with individually optimized scan patterns, it is possible to combine the advantage of being able to fine tune the fluence distribution by the scanning system with the steeper dose gradients that can be delivered by a few static multileaf collimator segments. It is demonstrated that in most cases a few collimator segments are sufficient and often a single segment per beam portal may suffice when narrow scanned photon beams are employed, and they can be delivered sequentially with a negligible time delay. A further advantage is the increase of therapeutically useful photons and improved patient protection, since the pencil beam is only scanned where the leaf collimator is open. Consequently, some of the problems associated with dynamic multileaf collimation such as the tongue and groove and edge leakage effects are significantly reduced. Fast scanning beam techniques combined with good treatment verification systems allow interesting future possibilities to counteract patient and internal organ motions in real time.

Design of a compact high energy treatment unit combining narrow pencil beam scanning and multileaf collimation

The thesis describes the design of scanned narrow photon beam in combination with a few optimally selected collimator settings delivered by a compact treatment machine to effectively produce a truly conformal treatment. The racetrack microtron generates moving photon beams by scanning the electron beam over a thin transmission target of low Z material, such as 3 mm beryllium. The transmitted electrons with an energy of about 49 MeV are removed by a strong purging magnet that will safely bend the beam to a dedicated electron stopper. The photon beam has a full width at half‐maximum of about 20 mm at a 70 cm source to the isocenter distance and the intensity in the forward direction is almost the same as from a full range target. The narrow photon beams are only scanned where the leaf collimator is open so the photons are effectively and safely used during the treatment. Only a few beam directions are needed to achieve almost optimal results. By using three fixed heads, it is possible to simultaneously scan three photon beams synchronously over the target volume. The increased instantaneous dose rate in the tumor enhances the biological effect. The mechanical construction will be very robust and of high mechanical stability since there are few moving parts. The compact size of the treatment heads and the whole gantry will make it an ergonomic and easy to handle unit. When biologically based optimized plans are delivered by such a compact machine, the highest possible complication free tumor control is nearly reached according to radiobiological models for treatment evaluation of intensity modulated beams. At the same time the treatment time will be short and the probability of tumor induction outside the target low. [References to the authors' papers on the thesis topic: Phys. Med. Biol. 39, 37–61 (1994); Med. Phys. 22, 1141–1156 (1995); Phys. Med. Biol. 41, 1353–1397 (1996); Phys. Med. Biol. 43, 1091–1112 (1998).]

A fast algorithm for the dose calculation of irregularly shaped fields in stereotactic convergent beam irradiation

A fast, irregular field dose calculation algorithm is described for use in stereotactic convergent beam irradiation (CBI) with high-energy photons. The dose distribution of an irregular field is approximated by a convolution of narrow pencil beams. Decomposition of pencil beams and a `pre-selection' algorithm is used to avoid redundant calculations. A typical dose calculation for CBI with nine couch angles and nine different irregularly shaped fields takes less than 90 s on a DEC VAXstation 3100, Fortran. The difference between the measured and the calculated dose values increases with increasing distance from the isocentre. The error in 10% isodose line is less than 3% of the isocentre dose.

Characteristics of a silicon detector for industrial X-ray computed tomography

We have evaluated a silicon solid state detector (Si-SSD) as a detector for Industrial X-ray Computed Tomography (IXCT) from the viewpoints of sensitivity and radiation damage. The detector which we used in this work, is a diffused junction type Si-SSD (43 × 3.8 × 0.5 mm) fabricated on high purity n-type silicon. We measured pulse height distributions for narrow pencil beam 60Co gamma rays parallelly incident to the detector. The expected deposit energy was 0.146 ± 0.001 MeV (11.7% of the incident photon energy). The calculated pulse height distribution using the general purpose EGS4 code was confirmed to be in good agreement with the measured one. The radiation damage was evaluated using the 60Co gamma ray source. Exposure of up to 88 kGy(Si) was achieved over a period of 20 hours, with an exposure rate of 4.4 kGy(Si)/h (36 mC/kg s). It was confirmed that the Si-SSDs could be used at least up to 88 kGy(Si) with X-rays equivalent to 60Co gamma rays.

Simulation of deep one- and two-dimensional redshift surveys

It is shown that slice or pencil-beam redshift surveys of galaxies can be simulated in a box with nonequal sides. This method saves a lot of computer time and memory while providing essentially the same results as from whole-cube simulations. A 2457.6/h Mpc-long rod (out to a redshift z = 0.58 in two opposite directions) is simulated using the standard biased cold dark matter model as an example to mimic the recent deep pencil-beam surveys by Broadhurst et al. (1990). The structures (spikes) seen in these simulated samples occur when the narrow pencil-beam pierces walls, filaments, and clusters appearing randomly along the line-of-sight. A statistical test for goodness of fit to a periodic lattice has been applied to the observations and the simulations. It is found that the statistical significance level (P = 15.4 percent) is not strong enough to reject the null hypothesis that the observations and the simulations were drawn at random from the same set.

A metropolitan area radio system using scanning pencil beams

A metropolitan-area radio system that, from a centrally located base station, provides continuously 360 degrees coverage over a large service region is proposed. The base station blankets the service region with a raster of very narrow pencil beams that can be rapidly scanned to any position in synchronism with the switching sequences of a time-division-multiple-access (TDMA) assignment. By deploying multiple scanning spot beams, the allocated spectrum can be reused many times. A centralized network controller, executing an efficient TDMA assignment algorithm, dynamically allocates the resources of a small transceiver pool among the far larger number of beam positions in response to real-time requests for service. By varying the dwell time at each beam position in response to the traffic intensity of the position, highly efficient resource utilization is provided. The high antenna gain of the base station antenna provides adequate arain fade margin to permit operation in the uncongested portion of the ratio spectrum above 20 GHz. The system is particularly well suited to emerging, direct-to-end-user wideband digital service offerings.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An underwater acoustic imaging method using cross-fan beam scanning

A real-time underwater acoustic imaging method named cross-fan beam scanning has been developed by OKI Electric Co., Ltd. In this method, acoustic projector elements are arranged vertically, and hydrophone elements are arranged horizontally to form a cross-linear array. That transmitting array generates thin horizontal fan beams, and the receiving array generates multi-thin-fan preformed vertical beams. Total beam patterns are obtained as the product of the transmission and reception beam patterns, so that narrow pencil beams are generated at the intersection of the two beams. Transmission and reception beams are electrically scanned, respectively, and the resulting pencil beams can be pointed at in any direction in a moment. Using this method, several types of imaging systems were completed. These are installed in submersibles, surveying ships, and work vessels such as the “Shinkai-6500” deep-sea manned submersible of the Japan Marine Science and Technology Center and the “Marcas-2500” deep-sea ROV of KDD, etc. In this paper, the imaging principle, the sonar system design, and the characteristics of typical systems are described and the imaging results are shown.

Twenty years of international satellite communication

This paper reviews the development of international satellite communications over the past two decades. To date over 100 satellites have been launched to provide both domestic and overseas commercial (i.e., non‐military) communications services and this paper largely confines itself to describing the technical developments involved in creating the INTELSAT system which are representative of the field as a whole. INTELSAT, is an international organization established in 1964 to provide overseas communications. It has developed a global system designed to provide (1) a high degree of connectivity and (2) a large communications capacity. Connectivity has been achieved using relatively broad (albeit “shaped”) satellite antenna patterns permitting earth stations in many countries that are illuminated by the same beam to be interconnected. This desirable attribute has, however, required the use of large earth stations and one future technical direction is likely to entail using satellites that generate narrow pencil beams permitting the use of smaller earth stations; on‐board switching will then be required to restore connectivity between beams. A large capacity has been achieved through the use of “frequency reuse,” i.e., techniques to utilize the C‐band frequencies assigned for satellite communications several‐fold. This has been accomplished through satellite designs that employ isolated antenna beams to illuminate different parts of the earth and/or beams that view overlapping parts but employ opposite senses of (circular) polarization. Associated with this approach have been efforts to use the assigned bands as efficiently as possible through “channelization,” i.e., subdividing them into frequency intervals, each of which is served by a transponder amplifier designed to yield minimum interference with its neighbors. The paper concludes with a brief discussion of the likely trends in the design of satellite systems as these evolve in competition with fiber‐optic cables.

Interferometry of wave fronts reflected off conical surfaces

Interferometry of wave fronts reflected off conical surfaces requires new interferometric schemes. The pencil beam interferometer is proposed as a metrology tool for conical as well as for spherical optical surfaces. The instrument employs two narrow pencil beams which scan the optical surface to be measured. An electronic fringe position readout system, which measures the location of the fringes that develop through interference of the pencil beams, provides high accuracy surface information. Metrology on a waxicon indicates a measurement precision of ±6 nm. Due to the differential nature of the instrument, alignment requirements are relatively modest.

A geometrical analysis of the gamma-ray light curves of pulsars

In the light curves of the COS-B gamma-ray pulsars, a few common features are clearly recognizable: two main peaks always occur, and the intervening phase lag has not the natural value 0.5. An interpretation is proposed which does not rely upon specific assumptions about the gamma-ray generating processes, and only involves a simplified geometrical description of the emission pattern, namely two opposite hollow cones with a first-order asymmetry with respect to their common axis. In this way it is possible to explain any observed phase difference in terms of the cone angle, and the angles from the line of sight and the magnetic axis to the spin axis. All the available evidence is found to favour wide fan beams, rather than narrow pencil beams, and this should have obvious consequences for future pulsar models.

Some problems of treatment for pion beams

Unlike photon beams, pion beams cannot be represented as being emitted from a small source with a rather well known distribution in angle. Instead one must deal with an extended effective source. Since some beam shaping will be done with collimators, there must be a calculation technique to determine how the beam is affected by collimators located at any chosen distance from the source. Treatment planning techniques must take into account the effects of range straggling and multiple scattering in both the patient and in any energy degrading materials used in the path of the beam. Bones and body cavities will, in general, affect a pion beam more than a photon beam so that shifts of the pion stopping depths due to bones and cavities will have to be accounted for. There must be accurate knowledge of the biological effect along and radially away from the axis of a narrow pencil beam of pions. One approach to treatment planning is to represent the characteristics of the beam with an analytic function. Another possible technique would involve calculating biological effect on a particle-by-particle basis.

The Radio Brightness of the Quiet Sun at 21 cm Wavelength Near Sunspot Maximum

An investigation has been made of the radio emission from the quiet Sun at 21� 2 em wavelength in 1958 (near sunspot maximum). Two different methods have been used, both involving observations with very high angular resolution, to distinguish between the quiet-Sun component and the radiation from localized active regions. In one method, the Sun was scanned with a narrow pencil-beam; in the other, a fan-shaped aerial beam was used to give one-dimensional strip scans. In both cases it was necessary, when analysing the data, to take into account the residual effects of the very intense radiation from the localized sources. The two independent measurements gave results which agree within the limits of error. The apparent disk temperature was found to be approximately 140 000 oK, or twice the value for the same wavelength at sunspot minimum.