Exploring novel radiotherapy techniques with Monte Carlo simulation and measurement

This work is the first comprehensive investigation of potential changes in the radiobiological effectiveness of clinical photon beams caused by a redistribution of electrons in a magnetic field. It is also a fundamental study of both the influence of magnetic fields on the peak-to-valley dose ratio of microbeams and the accuracy of theoretical modelling for dose planning in Microbeam Radiation Therapy (MRT). The application of a strong transverse magnetic field to a volume undergoing irradiation by a photon beam can produce localised regions of dose enhancement and dose reduction. Results from Monte Carlo PENELOPE simulation show regions of enhancement and reduction of as much as 111% and 77% respectively for magnetic fields of 1 to 100 T applied to Co60, 6, 10, 15, and 24 MV photon beams. The dose redistribution is shown to occur predominantly through an alteration in the lower energy electron population, which may correspond to a change in the relative biological effectiveness. In MRT, an experimental and theoretical investigation of the influence of transverse and longitudinal magnetic fields on the lateral dose profile and peak-to-valley dose ratio (PVDR) of microbeams is presented. Results show that longitudinal magnetic fields greater than 10 T are needed to produce an effect. Strong transverse magnetic fields, on the other hand, have no influence on microbeam profiles. The radiation response of the edge-on MOSFET and its ability to measure dose profiles of monoenergetic and polyenergetic microbeams are also investigated. Simulations investigating the dependence of microbeam dose profiles on the accuracy of beamline modelling (i.e. synchrotron source, multislit collimator, and beam divergence) are also presented. Results show the asymmetric collimator construction is responsible for a 10% variation in the full-width at half-maximum of microbeams which affects the PVDR. Modelling the distributed source and beam divergence increases the penumbral dose by almost 30%. The influence of the collimator alignment, interaction medium, and the height of scoring regions on the PVDR are also investigated

Magneto-radiotherapy: making the electrons conform

Magneto-radiotherapy is the application of magnetic fields during radiotherapy procedures. It aims to improve the quality of cancer treatment by using magnetic fields to alter the dose-deposition of secondary electrons in tissue. This work compares the performance of PENELOPE and EGS4 MC codes for magnetic fields applied to conventional photon beams. It also investigates the effect of a magnetic field on the electron spectrum and explores the novel idea of applying magnetic fields to MRT (Microbeam Radiation Therapy) for the treatment infantile brain tumours

Experimental benchmark data for Monte Carlo simulated radiation effects of gold nanoparticles. Part II: Comparison of measured and simulated electron spectra from gold nanofoils

Electron emission spectra of a thin gold foil after photon interaction were measured over the energy range between 50 eV and 9500 eV to provide reference data for Monte Carlo radiation-transport simulations. Experiments were performed with the HAXPES spectrometer at the PETRA III high-brilliance beamline P22 at DESY (Hamburg, Germany) for photon energies just below and above each of the gold L-edges, i.e., at 11.9 keV, 12.0 keV, 13.7 keV, 13.8 keV, 14.3 keV, and 14.4 keV. The data were analyzed to obtain the absolute values of the particle radiance of the emitted electrons per incident photon flux. Simulations of the experiment were performed using the Monte Carlo radiation-transport codes Penelope and Geant4. Comparison of the measured and simulated results shows good qualitative agreement. When simulation results are convolved with curves that take into account the effect of lifetime broadening, line shapes of photoelectron and Auger peaks similar to those observed experimentally are obtained. On an absolute scale, the experiments tend to give higher electron radiance values at the lower photon energies studied as well as at the higher photon energies for electron energies below the energy of the Au L3 photoelectron. This is attributed to the linear polarization of the photon beam in the experiments which is not considered in the simulation codes.Comment: Revised manuscript after peer review, 13 pages, 9 figure

Experimental benchmark data for Monte Carlo simulated radiation effects of gold nanoparticles. Part II: comparison of measured and simulated electron spectra from gold nanofoils

Electron emission spectra of a thin gold foil after photon interaction were measured over the energy range between 50 eV and 9500 eV to provide reference data for Monte Carlo radiation-transport simulations. Experiments were performed with the HAXPES spectrometer at the PETRA III high-brilliance beamline P22 at DESY (Hamburg, Germany) for photon energies just below and above each of the gold L-edges, that is, at 11.9 keV, 12.0 keV, 13.7 keV, 13.8 keV, 14.3 keV, and 14.4 keV. The data were analyzed to obtain the absolute values of the particle radiance of the emitted electrons per incident photon flux. Simulations of the experiment were performed using the Penelope and Geant4 Monte Carlo radiation-transport codes. Comparison of the measured and simulated results shows good qualitative agreement. On an absolute scale, the experiments tend to produce higher electron radiance values at the lower photon energies studied as well as at the higher photon energies for electron energies below the energy of the Au L3 photoelectron. This is attributed to the linear polarization of the photon beam in the experiments, something which is not considered in the simulation codes

Magneto-radiotherapy: effect of magnetic field on dose distribution and RBE

Magneto-radiotherapy is the application of magnetic fields during radiotherapy procedures. It aims to improve the quality of cancer treatment by altering the dose deposition of charged particles in the tissue volume being targeted. Our research has focused on applying the concept of magneto-radiotherapy to conventional linac treatment and MRT (Microbeam Radiation Therapy) oncology modalities

Effect of transverse magnetic fields on dose distribution and RBE of photon beams: comparing PENELOPE and EGS4 Monte Carlo codes

The application of a strong transverse magnetic field to a volume undergoing irradiation by a photon beam can produce localized regions of dose enhancement and dose reduction. This study uses the PENELOPE Monte Carlo code to investigate the effect of a slice of uniform transverse magnetic field on a photon beam using different magnetic field strengths and photon beam energies. The maximum and minimum dose yields obtained in the regions of dose enhancement and dose reduction are compared to those obtained with the EGS4 Monte Carlo code in a study by Li et al (2001), who investigated the effect of a slice of uniform transverse magnetic field (1 to 20 Tesla) applied to high-energy photon beams. PENELOPE simulations yielded maximum dose enhancements and dose reductions as much as 111% and 77%, respectively, where most results were within 6% of the EGS4 result. Further PENELOPE simulations were performed with the Sheikh-Bagheri and Rogers (2002) input spectra for 6, 10 and 15 MV photon beams, yielding results within 4% of those obtained with the Mohan et al (1985) spectra. Small discrepancies between a few of the EGS4 and PENELOPE results prompted an investigation into the influence of the PENELOPE elastic scattering parameters C1 and C2 and lowenergy electron and photon transport cut-offs. Repeating the simulations with smaller scoring bins improved the resolution of the regions of dose enhancement and dose reduction, especially near the magnetic field boundaries where the dose deposition can abruptly increase or decrease. This study also investigates the effect of a magnetic field on the low-energy electron spectrum that may correspond to a change in the radiobiological effectiveness (RBE). Simulations show that the increase in dose is achieved predominantly through the lower energy electron population

Deposition of Gold Nanoparticles on a Self‐Supporting Carbon Foil

Electron emission cross sections of gold nanoparticles (AuNPs) are important for assessing their radiosensitizing effects from ionizing radiation using Monte Carlo simulations. Measurements of these data require samples of sufficiently large area density, homogeneous nanoparticle distribution, and a mechanically stable sample holder to ensure a low background signal. While several methods exist for the deposition of nanoparticles, there is little information regarding the deposition of AuNPs in an aqueous solution onto a self-supporting film. The aim of this is to find suitable preparation techniques for AuNP samples which fulfill the above requirements. AuNP samples are produced using different deposition techniques and a 50 nm-thick carbon foil as the substrate. These samples are characterized with respect to the size and spatial distribution of AuNPs using a scanning electron microscope. The drop-casting technique yields the best results, while those obtained with the spin-coater technique are less reproducible regarding sample stability. The microdrop method is deemed unsuitable due to its tendency to form AuNP clusters. Measurements conducted with a synchrotron radiation source, as well as with protons and electrons, confirm the suitability of these samples for studying electron emission spectra of AuNPs for different radiation types

Doubly differential cross sections for electron-impact ionization of propane in the energy range from 30 eV to 1 keV

Doubly differential electron-impact ionization cross sections of propane were comprehensively measured for electron energies between 30 eV and 1 keV as a function of secondary electron energies and emission angles. The measurements were carried out for secondary electron energies from 3 eV to about half of the primary energy and for emission angles between 10° and 135°. To facilitate practical application and implementation of the data into numerical codes used for radiation transport calculations, a semi-empirical formula was constructed on the basis of existing models. The semi-empirical formula is capable of reproducing the measured data well over a wide energy and angular range. Singly differential ionization cross sections were obtained by the integration of the experimental data over the emission angles and total ionization cross sections (TICSs) were determined by the integration of the data both over the emission angles and secondary electron energies. They were compared to the theoretical results calculated using the binary-encounter-Bethe (BEB) model. The calculated TICSs mostly agree with the data published by other groups within the experimental uncertainties

Ionization cross section data of nitrogen, methane, and propane for light ions and electrons and their suitability for use in track structure simulations

Track structure Monte Carlo simulations are frequently applied in micro- and nanodosimetry to calculate the radiation transport in detail. The use of a well-validated set of cross section data in such simulation codes ensures accurate calculations of transport parameters, such as ionization yields. These cross section data are, however, scarce and often discrepant when measured by different groups. This work surveys literature data on ionization and charge-transfer cross sections of nitrogen, methane, and propane for electrons, protons, and helium particles, focusing on the energy range between 100 keV and 20 MeV. Based on the evaluated data, different models for the parametrization of the cross section data are implemented in the code PTRA, developed for simulating proton and alpha particle transport in an ion-counting nanodosimeter. The suitability of the cross section data is investigated by comparing the calculated mean ionization cluster size and energy loss with experimental results in either nitrogen or propane. For protons, generally good agreement between measured and simulated data is found when the Rudd model is used in PTRA. For alpha particles, however, a considerable influence of different parametrizations of cross sections for ionization and charge transfer is observed. The PTRA code using the charge-transfer data is, nevertheless, successfully benchmarked by the experimental data for the calculation of nanodosimetric quantities, but remaining discrepancies still have to be further investigated (up to 13% lower energy loss and 19% lower mean ionization cluster size than in the experiment). A continuation of this work should investigate data for the energy loss per interaction as well as differential cross section data of nitrogen and propane. Interpolation models for ionization and charge-transfer data are proposed. The Barkas model, frequently used for a determination of the effective charge in the ionization cross section, significantly underestimates both the energy loss (by up to 19%) and the mean ionization cluster size (up to 65%) for alpha particles. It is, therefore, not recommended for particle-track simulations

Microbeam radiation therapy: A Monte Carlo study of the influence of the source, multislit collimator, and beam divergence on microbeams

Microbeam radiation therapy (MRT) is a new oncology method currently under development for the treatment of inoperable pediatric brain tumors. Monte Carlo simulation, or the computational study of radiation transport in matter, is often used in radiotherapy to theoretically estimate the dose required for treatment. However, its potential use in MRT dose planning systems is currently hindered by the significant discrepancies that have been observed between measured and theoretical dose and the PVDR (peak to valley dose ratio). The need to resolve these discrepancies is driven by the desirability of making MRT available to humans in the next few years. This article aims to resolve some of the discrepancies by examining the simplifications adopted in previous MRT Monte Carlo studies, such as the common practice of commencing microbeam transport on the surface of the target which neglects the influence of the distributed synchrotron source, multislit collimator, and the beam divergence between them. This article uses PENELOPE Monte Carlo simulation to investigate the influence of these beamline components upstream of the target on the lateral dose profiles and PVDRs of an array of 25 microbeams. It also compares the dose profiles and PVDRs of a microbeam array produced from a single simulation (full array) to those produced from the superposition of a single microbeam profile (sup array). The effect of modeling the distributed source and the beam divergence was an increase in the absorbed dose in the penumbral and valley regions of the microbeam profiles. Inclusion of the multislit collimator resulted in differences of up to 5 μm in the FWHM of microbeam profiles across the array, which led to minor variations in the corresponding PVDR yields