Comparison of Microdosimetric Simulations Using PENELOPE and PITS for a 25 keV Electron Microbeam in Water

Calculations of dose point kernels of 64 Cu in different media with PENELOPE Monte Carlo code

PURPOSE

The unique decay properties of copper-64 (⁶⁴ Cu) has made it a radionuclide of interest in theragnostic applications of nuclear medicine. This study aims to calculate the dose point kernels (DPKs) of ⁶⁴ Cu in various media with PENELOPE Monte Carlo code.

METHODS

Monte Carlo simulations were performed using PENELOPE code (version 2014). To calculate DPKs, the simulation comprised an isotropic point radiation source positioned at the origin of a spherical object of radius 50 cm. The absorbed dose along the radial direction outwards from the point source were scored with a resolution of 20 μm. Validations were firstly performed by calculating the DPKs of monoenergetic electrons and photons in water and the results were compared against the literature values. The continuous energy spectra of the beta minus and positron emissions from ⁶⁴ Cu were numerically modeled and used as inputs to the simulation. DPKs of ⁶⁴ Cu were calculated in water, soft tissue, lung tissue, and cortical bone, including all emissions types.

RESULTS

The simulations have been successfully validated against literature values. The largest deviations have been observed with 10 keV monoenergetic electrons with the average and maximum dose difference of -1.01% and -10.56%. The modeled energy spectra closely compared with the average energies from Brookhaven Laboratory National Nuclear Data Centre and the combined spectral shapes from the RAdiation Dose Assessment Resource (RADAR). The DPKs of ⁶⁴ Cu demonstrated different radial dose deposition in different media owing to the different physical density and effective atomic number.

CONCLUSIONS

The DPKs of ⁶⁴ Cu have been calculated with Monte Carlo simulations in four different media. They will be useful to study the dosimetric properties of ⁶⁴ Cu-labeled radiopharmaceuticals and perform therapeutic dose planning.

Lauriston S. Taylor Lecture on radiation protection and measurements: what makes particle radiation so effective?

The scientific basis for the physical and biological effectiveness of particle radiations has emerged from many decades of meticulous basic research. A diverse array of biologically relevant consequences at the molecular, cellular, tissue, and organism level have been reported, but what are the key processes and mechanisms that make particle radiation so effective, and what competing processes define dose dependences? Recent studies have shown that individual genotypes control radiation-regulated genes and pathways in response to radiations of varying ionization density. The fact that densely ionizing radiations can affect different gene families than sparsely ionizing radiations, and that the effects are dose- and time-dependent, has opened up new areas of future research. The complex microenvironment of the stroma and the significant contributions of the immune response have added to our understanding of tissue-specific differences across the linear energy transfer (LET) spectrum. The importance of targeted versus nontargeted effects remains a thorny but elusive and important contributor to chronic low dose radiation effects of variable LET that still needs further research. The induction of cancer is also LET-dependent, suggesting different mechanisms of action across the gradient of ionization density. The focus of this 35th Lauriston S. Taylor Lecture is to chronicle the step-by-step acquisition of experimental clues that have refined our understanding of what makes particle radiation so effective, with emphasis on the example of radiation effects on the crystalline lens of the human eye.

Irradiation of gold nanoparticles by x-rays: Monte Carlo simulation of dose enhancements and the spatial properties of the secondary electrons production

PURPOSE

The aim of this study is to understand the characteristics of secondary electrons generated from the interaction of gold nanoparticles (GNPs) with x-rays as a function of nanoparticle size and beam energy and thereby further the understanding of GNP-enhanced radiotherapy.

METHODS

The effective range, deflection angle, dose deposition, energy, and interaction processes of electrons produced from the interaction of x-rays with a GNP were calculated by Monte Carlo simulations. The GEANT4 code was used to simulate and track electrons generated from a 2, 50, and 100 nm diameter GNP when it is irradiated with a 50 kVp, 250 kVp, cobalt-60, and 6 MV photon beam in water.

RESULTS

When a GNP was present, depending on the beam energies used, secondary electron production was increased by 10- to 2000-fold compared to an absence of a GNP. Low-energy photon beams were much more efficient at interacting with the GNP by two to three orders of magnitude compared to MV energies and increased the deflection angle. GNPs with larger diameters also contributed more dose. The majority of the energy deposition was outside the GNP, rather than self-absorbed by the nanoparticle. The mean effective range of electron tracks for the beams tested ranged from approximately 3 microm to 1 mm.

CONCLUSIONS

These simulated results yield important insights concerning the spatial distributions and elevated dose in GNP-enhanced radiotherapy. The authors conclude that the irradiation of GNP at lower photon energies will be more efficient for cell killing. This conclusion is consistent with published studies.

Comparison between an event-by-event Monte Carlo code, NOREC, and ETRAN for electron scaled point kernels between 20 keV and 1 MeV

An event-by-event Monte Carlo code called NOREC, a substantially improved version of the Oak Ridge electron transport code (OREC), was released in 2003, after a number of modifications to OREC. In spite of some earlier work, the characteristics of the code have not been clearly shown so far, especially for a wide range of electron energies. Therefore, NOREC was used in this study to generate one of the popular dosimetric quantities, the scaled point kernel, for a number of electron energies between 0.02 and 1.0 MeV. Calculated kernels were compared with the most well-known published kernels based on a condensed history Monte Carlo code, ETRAN, to show not only general agreement between the codes for the electron energy range considered but also possible differences between an event-by-event code and a condensed history code. There was general agreement between the kernels within about 5% up to 0.7 r/r (0) for 100 keV and 1 MeV electrons. Note that r/r (0) denotes the scaled distance, where r is the radial distance from the source to the dose point and r (0) is the continuous slowing down approximation (CSDA) range of a mono-energetic electron. For the same range of scaled distances, the discrepancies for 20 and 500 keV electrons were up to 6 and 12%, respectively. Especially, there was more pronounced disagreement for 500 keV electrons than for 20 keV electrons. The degree of disagreement for 500 keV electrons decreased when NOREC results were compared with published EGS4/PRESTA results, producing similar agreement to other electron energies.