Physical characterization of single convergent beam device for teletherapy: theoretical and Monte Carlo approach

Effects on the accelerating electron bunches due to the presence of sulfur hexafluoride or air in the linac waveguide

Sulfur hexafluoride gas (SF6) is used as a dielectric insulator in the acceleration process of certain medical linear accelerator waveguides. Nevertheless, some innovative development and investigation cases require intervention in the linear accelerator or, specifically, on the waveguide, which could affect the sealing of the device. In this regard, vacuum sealing systems can be compromised, affecting the properties of the radiation beams produced. The presence of sulfur hexafluoride or air inside the VARIAN 6/100 waveguide was investigated under different pressure conditions and non-uniform electric fields, adapting Monte Carlo simulation techniques for modeling radiation transport coupled with electric fields. Obtained results indicated the suitability of the proposed approach, while comparisons with theoretical approaches and experimental evidence supported the model's consistency.

Monte Carlo study of a convergent X-ray beam for high resolution X-ray fluorescence imaging

The traditional X-ray tube design is built upon the impact of energetic electrons on high atomic number absorbers producing the X-ray output, consisting of photons due to Bremsstrahlung and fluorescence. Typically, electrons current hits the target within a limited area of a few millimeters square stopping the electrons, which lose their energy and produce the X rays constituting an inherently divergent beam. This geometrical property of traditional X-ray beams is responsible for several undesirable effects when trying to optimize applications requiring high incident fluence spatial concentration, like X-ray fluorescence imaging. This work presents a Monte Carlo study about a novel X-ray tube design, based on a cylindrical target that is capable of producing a convergent X-ray beam aimed at improving overall performance and spatial resolution in certain applications, like X-ray fluorescence imaging. Main design characteristics for relevant parts, like target/anode, filter, and collimator, have been carefully investigated by means of Monte Carlo simulation using two independent codes: FLUKA and PENELOPE. The obtained results suggest the feasibility of the proposed design remarking that high fluence concentration can be achieved, which can be particularly useful for further applications, like tumor targeting by X-ray fluorescence imaging by means of high atomic number nanoparticle infusion, as reported in this work.

Trajectory control of electron beams using high intensity permanent magnests for linac-adaptable convergent beam radiotherapy

Convergent beam radiotherapy, or CBRT, currently under development is based on the adaptation of a linear accelerator (linac) to a device which allows to dynamically curve the original trajectory of the electron beam so that it impacts upon a target. This produces a photon beam via Bremsstrahlung which converges on a predetermined focus point (isocenter). Adaptation of the RTHC device is only possible if it is sufficiently compact, as the device must be placed between the linac head exit and the gurney. This requires that new magnetic deflection devices be developed. This paper describes the theoretical and experimental development of controlled-deflection electron beam systems (at energies in MeV ranges) generated in a dual linear accelerator waveguide. A device which follows RTHC geometry is adapted for the system, using new magnetic deflector designs based on permanent neodymium magnets which reach magnetic field intensities in the order of Tesla. The methodology that was developed includes calculations of the radii of curvature with relativistic considerations for mono- and poly-energetic electrons. Deflection angles were calculated based on this theoretical foundation, using a program developed in MatLab® which shows the trajectory of electrons both under ideal conditions (uniform magnetic field) and real conditions (magnetic field defined through intensity distribution). Monte Carlo simulation subroutines were implemented in order to estimate the spectrum of electrons issuing from the linac as well as to directly determine the electron beam trajectory with magnetic deflectors present. Theoretical and simulated results were compared to experiments performed with a clinical linear accelerator, demonstrating correspondence between different methodologies and confirming the ability to achieve electron beam deflection levels necessary for implementation of convergent beam radiotherapy device.

Theory, simulation and experiments for precise deflection control of radiotherapy electron beams

Conventional radiotherapy is mainly applied by linear accelerators. Although linear accelerators provide dual (electron/photon) radiation beam modalities, both of them are intrinsically produced by a megavoltage electron current. Modern radiotherapy treatment techniques are based on suitable devices inserted or attached to conventional linear accelerators. Thus, precise control of delivered beam becomes a main key issue. This work presents an integral description of electron beam deflection control as required for novel radiotherapy technique based on convergent photon beam production. Theoretical and Monte Carlo approaches were initially used for designing and optimizing device´s components. Then, dedicated instrumentation was developed for experimental verification of electron beam deflection due to the designed magnets. Both Monte Carlo simulations and experimental results support the reliability of electrodynamics models used to predict megavoltage electron beam control.

Laue lens for radiotherapy applications through a focused hard x-ray beam: a feasibility study on requirements and tolerances

Focusing a hard x-ray beam would represent an innovative technique for tumour treatment, since such a beam may deliver a dose to a tumour located at a given depth under the skin, sparing the surrounding healthy cells. A detailed study of a focusing system for hard x-ray aimed at radiotherapy is presented here. Such a focusing system, named Laue lens, exploits x-ray diffraction and consists of a series of crystals disposed as concentric rings capable of concentrating a flux of x-rays towards a focusing point. A feasibility study regarding the positioning tolerances of the crystalline optical elements has been carried out. It is shown that a Laue lens can effectively be used in the context of radiotherapy for tumour treatments provided that the mounting errors are below certain values, which are reachable in the modern micromechanics. An extended survey based on an analytical approach and on simulations is presented for precisely estimating all the contributions of each mounting error, analysing their effect on the focal spot of the Laue lens. Finally, a simulation for evaluating the released dose in a water phantom is shown.

Dosimetric and bremsstrahlung performance of a single convergent beam for teletherapy device

The present work investigates preliminary feasibility and characteristics of a new type of radiation therapy modality based on a single convergent beam of photons. The proposal consists of the design of a device capable of generating convergent X-ray beams useful for radiotherapy. The main goal is to achieve high concentrated dose delivery. The first step is an analytical approach in order to characterize the dosimetric performance of the hypothetical convergent photon beam. Then, the validated FLUKA Monte Carlo main code is used to perform complete radiation transport to account also for scattering effects. The proposed method for producing convergent X-rays is mainly based on the bremsstrahlung effect. Hence the operating principle of the proposed device is described in terms of bremsstrahlung production. The work is mainly devoted characterizing the effect on the bremsstrahlung yield due to accessories present in the device, like anode material and geometry, filtration and collimation systems among others. The results obtained for in-depth dose distributions, by means of analytical and stochastic approaches, confirm the presence of a high dose concentration around the irradiated target, as expected. Moreover, it is shown how this spot of high dose concentration depends upon the relevant physical properties of the produced convergent photon beam. In summary, the proposed design for producing single convergent X-rays attained satisfactory performance for achieving high dose concentration around small targets depending on beam spot size that may be used for some applications in radiotherapy, like radiosurgery.

Laue lens to focus an X-ray beam for radiation therapy

A Laue lens is an optical component composed of a set of crystals that produce a convergent beam exploiting X-ray diffraction in transmission geometry. Employment of a system formed by a properly designed Laue lens coupled with an X-ray unit to selectively irradiate tumours is proposed. A convergent beam leads to a depth dose profile with a pronounced peak at the focal depth, which may result in a high precision of the dose delivery. Using a custom-made Monte Carlo code and the GAMOS code, we carried out a design study to determine the geometry and the optimal features of the crystals composing the lens. As an application, a Laue lens capable of focusing a 80 keV beam 50 cm downstream of the lens has been designed. The lens is composed of an ensemble of Si crystals with curved diffracting planes. The lens produces a focal spot of 2 mm enclosing 7.64 × 106 photons for an electron charge of 1 mC impinging on the surface of the X-ray tube anode. The combination of these important figures of merit makes the proposed system suitable for irradiating both sub-cm and larger tumour masses efficiently. A dose of 2 Gy can be delivered to a small tumour in a few seconds, sparing at the same time the surrounding tissues.