Search

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 rangedmore » from approximately 3 {mu}m 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.« less

Purpose: The impact of photon beam energy and tissue heterogeneities on dose distributions and dosimetric characteristics such as point dose, mean dose, and maximum dose was investigated in the context of small-animal irradiation using Monte Carlo simulations based on the EGSnrc code. Methods: Three Monte Carlo mouse phantoms, namely, heterogeneous, homogeneous, and bone homogeneous were generated based on the same mouse computed tomography image set. These phantoms were generated by overriding the tissue type of none of the voxels (heterogeneous), all voxels (homogeneous), and only the bone voxels (bone homogeneous) to that of soft tissue. Phase space files of the 100 and 225 kVp photon beams based on a small-animal irradiator (XRad225Cx, Precision X-Ray Inc., North Branford, CT) were generated using BEAMnrc. A 360 deg. photon arc was simulated and three-dimensional (3D) dose calculations were carried out using the DOSXYZnrc code through DOSCTP in the above three phantoms. For comparison, the 3D dose distributions, dose profiles, mean, maximum, and point doses at different locations such as the isocenter, lung, rib, and spine were determined in the three phantoms. Results: The dose gradient resulting from the 225 kVp arc was found to be steeper than for the 100 kVp arc. Themore » mean dose was found to be 1.29 and 1.14 times higher for the heterogeneous phantom when compared to the mean dose in the homogeneous phantom using the 100 and 225 kVp photon arcs, respectively. The bone doses (rib and spine) in the heterogeneous mouse phantom were about five (100 kVp) and three (225 kVp) times higher when compared to the homogeneous phantom. However, the lung dose did not vary significantly between the heterogeneous, homogeneous, and bone homogeneous phantom for the 225 kVp compared to the 100 kVp photon beams. Conclusions: A significant bone dose enhancement was found when the 100 and 225 kVp photon beams were used in small-animal irradiation. This dosimetric effect, due to the presence of the bone heterogeneity, was more significant than that due to the lung heterogeneity. Hence, for kV photon energies of the range used in small-animal irradiation, the increase of the mean and bone dose due to the photoelectric effect could be a dosimetric concern.« less

This work contributed the following new information to the study of inhomogeneity correction algorithm: (1) Evaluation of lung dose calculation methods as a function of lung relative electron density (rhoe,lung) and treatment geometry and (2) comparison of doses calculated using the collapsed cone convolution (CCC) and adaptive convolution (AC) in lung using the Monte Carlo (MC) simulation with the EGSnrc-based code. The variations of rhoe,lung and geometry such as the position and dimension of the lung were studied with different photon beam energies and field sizes. Three groups of inhomogeneous lung phantoms, namely, "slab," "column," and "cube," with different positions, volumes, and shapes of lung in water as well as clinical computed tomography lung images were used. The rhoe,lung in each group of phantoms vary from 0.05 to 0.7. 6 and 18 MV photon beams with small (4 x 4 cm2) and medium (10 x 10 cm2) field sizes produced by a Varian 21 EX linear accelerator were used. This study reveals that doses in the inhomogeneous lung calculated by the CCC match well with those by AC within +/- 1%, indicating that the AC, with an advantage of shorter computing times (three to four times shorter than CCC), is a good substitute for CCC. Comparing the CCC and AC to MC in general, significant dose deviations are found when the rhoe,lung is 2 mm) occur in the column phantoms, with two lung volumes separated by a unit density column along the CAX in the middle using the 18 MV beam with 4 x 4 cm2 field for rhoe,lung < or =0.1. This study provides new dosimetric data to evaluate the impact of the variations of rhoe,lung and geometry on dose calculations in inhomogeneous media using CCC and AC.

The aim of this study is to investigate the influence of the body of the metal-oxide-semiconductor field effect transistor (MOSFET) dosimeter in measuring the electron backscatter from lead. The electron backscatter factor (EBF), which is defined as the ratio of dose at the tissue-lead interface to the dose at the same point without the presence of backscatter, was calculated by the Monte Carlo simulation using the GEANT4 code. Electron beams with energies of 4, 6, 9, and 12 MeV were used in the simulation. It was found that in the presence of the MOSFET body, the EBFs were underestimated by about 2%-0.9% for electron beam energies of 4-12 MeV, respectively. The trend of the decrease of EBF with an increase of electron energy can be explained by the small MOSFET dosimeter, mainly made of epoxy and silicon, not only attenuated the electron fluence of the electron beam from upstream, but also the electron backscatter generated by the lead underneath the dosimeter. However, this variation of the EBF underestimation is within the same order of the statistical uncertainties as the Monte Carlo simulations, which ranged from 1.3% to 0.8% for the electron energies of 4-12 MeV, due to the small dosimetric volume. Such small EBF deviation is therefore insignificant when the uncertainty of the Monte Carlo simulation is taken into account. Corresponding measurements were carried out and uncertainties compared to Monte Carlo results were within +/- 2%. Spectra of energy deposited by the backscattered electrons in dosimetric volumes with and without the lead and MOSFET were determined by Monte Carlo simulations. It was found that in both cases, when the MOSFET body is either present or absent in the simulation, deviations of electron energy spectra with and without the lead decrease with an increase of the electron beam energy. Moreover, the softer spectrum of the backscattered electron when lead is present can result in a reduction of the MOSFET response due to stronger recombination in the SiO2 gate. It is concluded that the MOSFET dosimeter performed well for measuring the electron backscatter from lead using electron beams. The uncertainty of EBF determined by comparing the results of Monte Carlo simulations and measurements is well within the accuracy of the MOSFET dosimeter (< +/- 4.2%) provided by the manufacturer.

Although there are many works on evaluating dose calculations of the anisotropic analytical algorithm (AAA) using various homogeneous and heterogeneous phantoms, related work concerning dosimetry due to tangential photon beam is lacking. In this study, dosimetry predicted by the AAA and collapsed cone convolution (CCC) algorithm was evaluated using the tangential photon beam and phantom geometry. The photon beams of 6 and 15 MV with field sizes of 4×4 (or 7×7), 10×10 and 20×20 cm2, produced by a Varian 21 EX linear accelerator, were used to test performances of the AAA and CCC using Monte Carlo (MC) simulation (EGSnrc‐based code) as a benchmark. Horizontal dose profiles at different depths, phantom skin profiles (i.e., vertical dose profiles at a distance of 2 mm from the phantom lateral surface), gamma dose distributions, and dose‐volume histograms (DVHs) of skin slab were determined. For dose profiles at different depths, the CCC agreed better with doses in the air‐phantom region, while both the AAA and CCC agreed well with doses in the penumbra region, when compared to the MC. Gamma evaluations between the AAA/CCC and MC showed that deviations of 2D dose distribution occurred in both beam edges in the phantom and air‐phantom interface. Moreover, the gamma dose deviation is less significant in the air‐phantom interface than the penumbra. DVHs of skin slab showed that both the AAA and CCC underestimated the width of the dose drop‐off region for both the 6 and 15MV photon beams. When the gantry angle was 0°, it was found that both the AAA and CCC overestimated doses in the phantom skin profiles compared to the MC, with various photon beam energies and field sizes. The mean dose differences with doses normalized to the prescription point for the AAA and CCC were respectively:7.6%±2.6% and 2.1%±1.3% for a 10×10 cm2 field, 6 MV; 16.3%±2.1% and 6.7%±2.1% for a 20×20 cm2 field, 6 MV; 5.5%±1.2% and 1.7%±1.4% for a 10×10 cm2, 15 MV; 18.0%±1.3% and 8.3%±1.8% for a 20×20 cm2, 15 MV. However, underestimations of doses in the phantom skin profile were found with small fields of 4×4 and 7×7 cm2 for the 6 and 15 MV photon beams, respectively, when the gantry was turned 5° anticlockwise. As surface dose with tangential photon beam geometry is important in some radiation treatment sites such as breast, chest wall and sarcoma, it is found that neither of the treatment planning system algorithms can predict the dose well at depths shallower than 2 mm. The dosimetry data and beam and phantom geometry in this study provide a better knowledge of a dose calculation algorithm in tangential‐like irradiation. PACS numbers: 87.55.‐x, 87.53.Bn, 87.55.K‐, 87.55.kh, 87.56.jf

This study investigated the low-energy electrons (LEEs) produced when a gold nanoparticle (GNP) is irradiated by photon beams. The secondary electrons emitted from a GNP (diameter = 100 nm), interacting with photon beams with energies equal to 35, 73.3 and 600 keV, were simulated using the Geant4 Monte Carlo code. The phase spaces of the secondary electrons were then used to simulate the LEEs in water using the NOREC Monte Carlo code. All secondary electrons emitted by the GNP, and all LEEs produced by each secondary electron were tracked in Monte Carlo simulations. It is found that the energy distributions of the LEEs from the GNP do not vary significantly between different photon beam energies. Moreover, the 660 keV photon beam produced more LEEs travelling to a longer range than photon beams of lower energies (35 and 73.3 keV). This higher energy deposition and longer range LEEs produced by the 660 keV photon beam can enhance the cell kill. Based on our Monte Carlo results, it is concluded that the unexpected close of the radiosensitization enhancement factors of the 35 (1.66) and 660 keV (1.18) photon beams from our previous measurements is because of the cell kill enhancement with the increased LEE yield and range in the 660 keV photon beam.

Purpose: This study evaluated the surfacedosimetry predicted by the anisotropic analytical algorithm (AAA) and collapsed cone convolution (CCC) algorithm using the tangential‐like photon beam and phantom geometry. Monte Carlo(MC) simulation (EGSnrc code) was used as a benchmark for comparison. Methods: The 6 and 15 MV photon beams with field sizes of 4×4 (or 7×7), 10×10 and 20×20 cm2, produced by a Varian 21EX linear accelerator were used. Horizontal dose profiles at different depths, phantom skin profiles (i.e. vertical dose profiles at a distance of 2 mm from the phantom lateral surface), gamma dose distributions, and dose‐volume histograms (DVHs) of skin slab were determined. Results: For dose profiles at different depths, the CCC agreed better with doses in the air‐phantom region, while both the AAA and CCC agreed well with doses in the penumbra region, when compared to the MC. Gamma evaluations between the AAA/CCC and MC revealed that deviations of 2D dose distribution occurred in both beam edges in the phantom and air‐phantom interface. DVHs of skin slab showed that both the AAA and CCC underestimated the width of the slope for both the 6 and 15 MV photon beams. The mean dose differences along the phantom skin profiles for the AAA and CCC were respectively: 7.6±2.6% and 2.1±1.3% for a 10×10 cm2 field, 6 MV; 16.3±2.1% and 6.7±2.1% for a 20×20 cm2 field, 6 MV; 5.5±1.2% and 1.7±1.4% for a 10×10 cm2, 15 MV; 18.0±1.3% and 8.3±1.8% for a 20×20 cm2, 15 MV. Conclusions: As surfacedose with tangential‐like photon beam geometry is important in some radiation treatment sites such as breast, chest wall and sarcoma, the dosimetry data and beam and phantom geometry in this study are worthwhile to be considered, when carrying out quality assurance and commissioning for treatment planning systems.

This study evaluated the effect of tissue heterogeneities due to photon beams in the kilo‐voltage energy range in small‐animal irradiation. Monte Carlo(MC) simulation (EGSnrc code) was used. Three MC mouse phantoms were generated from a single mouse CTimage set. These phantoms were generated by overriding the relative electron density of no voxels (heterogeneous), all voxels (homogeneous) and the bone voxels (bone homogeneous) to one. Phase‐space files of the 100 and 225 kVp photon beams produced by a small animal irradiator were generated using BEAMnrc. A 360 deg photon arc was simulated for treatment of the lung, and 3D dose calculations were carried out for the three phantom geometries. The resulting dose profiles for the different phantoms and beam energies were compared. It was found that the 225 kVp photon beams have a better conformai dose distribution than the 100 kVp. The bone doses in the heterogeneous mouse phantom were about 4 – 5 (100 kVp) and 2 (225 kVp) times higher when compared to the homogeneous phantom. However, the lungdose does not vary significantly between the heterogeneous, homogeneous and bone homogeneous phantoms for either the 100 or 225 kVp photon beams. We concluded that bone dose enhancement was found when 100 and 225 kVp photon beams were used in small‐animal irradiation. This dosimetric effect due to the presence of the bone heterogeneity was more significant than the lung heterogeneity, and such bone dose enhancement does not occur in the typical patient's radiotherapy using the MV photon beams.

We demonstrate the potential of a forward-looking Doppler optical coherence tomography (OCT) probe for color flow imaging in several commonly seen narrowed artery morphologies. As a proof of concept, we present imaging results of a surgically exposed thrombotic occlusion model that was imaged superficially to demonstrate that Doppler OCT can identify flow within the recanalization channels of a blocked artery. We present Doppler OCT images in which the flow is nearly antiparallel to the imaging direction. These images are acquired using a flexible 2.2-mm-diam catheter that used electrostatic actuation to scan up to 30 deg ahead of the distal end. Doppler OCT images of physiologically relevant flow phantoms consisting of small channels and tapered entrance geometries are demonstrated.

Purpose: Through Monte Carlo simulations, we investigate the spatial and energy characteristics of electrons formed from interactions of keV photon beams irradiating a goldnanoparticle to understand their role in enhancing cell kill. Methods and Materials: The GEANT4 toolkit was used for Monte Carlo simulation. A goldnanoparticle sphere with a 100 nm diameter was irradiated by x‐rays with energies of 35 to 6000 keV inside a tracking volume of water. For each electron emitted from the irradiatedgoldnanoparticle, the following parameters were tracked: i) the physics process that created the electron, ii) energy distribution of the electron, iii) range, and iv) deflection angle. The same simulations were performed by replacing the goldnanoparticle with water. Results: The energy distribution of the secondary electrons when the goldnanoparticle is irradiated by a monoenergetic photon beam with various energies is calculated. The number of interactions for 35 keV photons is about 157 times more than that for 660 keV, and 683 times more than the 6000 keV beam. When the goldnanoparticle is absent, the probability of creating an electron becomes much lower. Specifically, the ratio of total interactions with and without gold is 812, 137, 10, 7, and 2 for energies 35, 73.3, 660, 1200 and 6000 keV respectively. Furthermore, irradiating the goldnanoparticle at a higher photon energy increases the range over which the election can travel, and decreases the deflection angle. This represents a change in volume over which the electrons can deposit dose. Conclusions: We conclude that for a cell of typical size, a low energy (35 keV) photon beam generates a large number of secondary electrons when a goldnanoparticle is present compared to without, and will have sufficient range to cause damage in the cell in which the nanoparticle is uptaken.

Purpose: This study investigated dosimetric characteristics of goldnanoparticle under a photon beam. A single goldnanoparticle (microscopic) and a mixture of gold and water (macroscopic) were considered using Monte Carlo simulations based on the Geant4‐ and EGSnrc‐based code, respectively. Methods and Materials: A single goldnanoparticle (diameter of 100 nm) was irradiated by photon beams with energies of 35.5keV, 73.3keV, 660keV, 1.2MeV and 6MeV in water. 250 million histories were used in Monte Carlo simulation to record different numbers of interactions (e.g. photoelectric and Compton) with and without the goldnanoparticle in water. Moreover, a mixture of gold and water was irradiated with photon beams. The dose enhancement ratios (dose of gold and water mixture/dose of water) were determined with different photon beam energies and concentrations of gold.Results: With a single goldnanoparticle, the number of photoelectric interaction was about 47 times larger than that of Compton for the 35.5keV photon beams. This was opposite to the 6MeV photon beams, where the number of Compton interaction was about 46.5 times larger than that of the photoelectric. Although the values of ratio were similar, the total number of interactions for the 35.5keV photon beams was in fact 348 times larger than that of the 6MeV. A larger energy deposition was therefore found when the photon beam energy was decreased from the MeV to keV range. This result agreed with that from a gold and water mixture. Moreover, increasing the concentration of gold increased the dose enhancement in water.Conclusions: Both microscopic and macroscopic study on goldnanoparticle agree that more energy deposition is found when the photon beam energy is decreased from MeV to keV range, due to the increase of photoelectric interaction. A higher concentration of gold can increase the dose enhancement in water.

This study provides new information on the evaluation of the lung dose calculation algorithms as a function of the relative electron density of lung, ρe,lung . Doses calculated using the collapsed cone convolution (CCC) and adaptive convolution (AC) algorithm in lung with the Pinnacle3 system were compared to those calculated using the Monte Carlo (MC) simulation (EGSnrc-based code). Three groups of lung phantoms, namely, "Slab", "Column" and "Cube" with different ρe,lung (0.05-0.7), positions, volumes and shapes of lung in water were used. 6 and 18MV photon beams with 4×4 and 10×10cm2 field sizes produced by a Varian 21EX Linac were used in the MC dose calculations. Results show that the CCC algorithm agrees well with AC to within ±1% for doses calculated in the lung phantoms, indicating that the AC, with 3-4 times less computing time required than CCC, is a good substitute for the CCC method. Comparing the CCC and AC with MC, dose deviations are found when ρe,lung are ⩽0.1-0.3. The degree of deviation depends on the photon beam energy and field size, and is relatively large when high-energy photon beams with small field are used. For the penumbra widths (20%-80%), the CCC and AC agree well with MC for the "Slab" and "Cube" phantoms with the lung volumes at the central beam axis (CAX). However, deviations >2mm occur in the "Column" phantoms, with two lung volumes separated by a water column along the CAX, using the 18MV (4×4cm2 ) photon beams with ρe,lung ⩽0.1.