Search

Single event upset, or Single Event Effect (SEE) is increasingly important as semiconductor devices are entering into nano-meter scale. The Linear Energy Transfer (LET) concept is commonly used to estimate the rate of SEE. The SEE, however, should be related to energy deposition of each stochastic event, but not LET which is a non-stochastic quantity. Instead, microdosimetry, which uses a lineal calculation of energy lost per step for each specific track, should be used to replace LET to predict microelectronic failure from SEEs. Monte Carlo simulation is used for the demonstration, and there are several parameters needed to optimise for SEE simulation, such as the target size, physical models and scoring techniques. We also show the thickness of the sensitive volume, which also correspond to the size of a device, will change the spectra of lineal energy. With a more comprehensive Monte Carlo simulation performed in this work, we also show and explain the differences in our results and the reported results such as those from Hiemstra et al. which are commonly used in semiconductor industry for the prediction of SEE in devices.

Purpose The aim of this work was to develop a computational scheme for the correction of the LET dependence on the MOSFET response in water phantom dose measurements for a spread-out Bragg peak (SOBP) proton beam. Methods The LET dependence of MOSFET was attributed to the stopping power ratio of SiO2 to H2O and to the fractional hole yield in the SiO2 layer. Using literature values for the stopping powers of the continuous slowing down approximation and measured fractional hole yields vs. electric field and LET, formulas were derived for the computation of a dose-weighted correction factor of a SOBP beam. Results Dose-weighted correction factors were computed for a clinical 190-MeV proton SOBP beam in a high-density polyethylene phantom. By applying correction factors to the SOBP beam, which consisted of weighted monoenergetic Bragg peaks, the MOSFET outputs were predicted and agreed well with the measured MOSFET responses. Conclusion By applying LET dependent correction factors to MOSFET data, quality assurance of dose verification based on MOSFET measurements becomes possible for proton therapy.

Oncogenic N-MYC (MYCN) is widely used as a biomarker in clinics for neuroblastoma (NB) patients; nevertheless, mechanism that underlines MYCN regulation remains elusive. In the present study, we identified calreticulin (CRT) as a novel MYCN suppressor that downregulated MYCN promoter activity and protein expression to modulate neuronal differentiation and stemness. Our data showed that CRT-mediated MYCN suppression led to increased neurite length and commensurate elevation in differentiation marker GAP-43. We examined effect of radiotherapy and discovered that ionizing radiation (IR) was able to augment CRT expression dose-dependently in NB. Interestingly, neuronal differentiation and neurosphere formation (NSF) of NB were not only co-modulated by IR and CRT but were also dependent on Ca2+-buffering domain (C-domain) of CRT. Mutagenesis analysis showed that C-domain was indispensable for CRT-mediated MYCN regulation in NB differentiation and NSF. Of note, IR-induced formation of neural stem-like neurospheres (NS) was significantly impaired in CRT-overexpressed NB cells. The occupancy of CRT on MYCN 5' proximal promoter was confirmed by chromatin immunoprecipitation assays, revealing potential CRT binding sites that coincided with transcription factor E2F1 binding elements. In addition, we identified a physical interaction between CRT and E2F1, and demonstrated that CRT occupancy on MYCN promoter prevented E2F1-mediated MYCN upregulation. In line with in vitro findings, hampered tumor latency and retarded tumor growth in xenograft model corroborated IR and CRT co-mediated neuronal differentiation of NB. Together, our data delineated a novel mechanism of CRT-mediated MYCN regulation and warranted further preclinical investigation towards new therapeutic strategy for NB. CRT suppresses MYCN expression and promotes neuronal differentiation in NB. CRT regulates MYCN via interaction with E2F1 and direct binding to MYCN promoter. Ca2+-buffering domain of CRT is critical in MYCN regulation and NB differentiation. CRT-MYCN axis impacts on NB stemness by modulating neurosphere formation. Xenograft model corroborates in vitro NB differentiation mediated by CRT and IR.

The potential radiosensitizing effects of gold nanoparticles (GNPs) in radiation therapy have been extensively studied. GNPs loaded in the tumor produce a large number of low-energy secondary electrons which contribute to the local dose enhancement and elevate the relative biological effectiveness (RBE). Thus GNP enhanced responses in 192Ir, a widely used brachytherapy source, are the subject of interest. These radiosensitizing effects can be examined through comparisons between microdosimetric spectra of secondary electrons resulting from the 192Ir in the presence and absence of GNPs in cells. Measurements of single-event lineal energy spectra give indications of a relative therapeutic advantage. In the present work, a mini tissue equivalent proportional counter (TEPC) was constructed to measure the microdosimetric spectra in cells containing and without GNPs for 192Ir source. A plug, either contained a 50 nm-thick gold foil coated on its inner surface or without the foil, was inserted into the TEPC wall during measurements. Analyses of the measured un-normalized and normalized dose lineal energy spectra provided information on the enhancements in absorbed dose and RBE. The dose enhancement was 1.27 and 1.37 for 1 and 0.5 μm-diameter sites, respectively. The increase in dose enhancement with decreasing site diameter revealed the sharp fall-off in the local dose enhancement. Applying the biological weighting function method, the RBE enhancement was 1.32 based on the lineal energy spectra of 1 μm site.

Radiation-induced bone diseases were frequently reported in radiotherapy patients. To study the diseases, microdosimeters were constructed with walls of A150-A150, A150-B100, B100-A150 and B100-B100 interfaces. Monte Carlo simulations of these microdosimeters were performed to determine the lineal energy spectra of an interface site at different depths in water for 230 MeV protons. Comparing these spectra with data of ICRU tissue and bone walls, better agreements were found at shallow depths for protons and delta-rays than deep depths for nuclear interactions.

Purpose: One of the promising radiosensitizers is the ultrasmall gold nanoparticle (GNP) with a hydrodynamic diameter vβ3) in the murine ALTS1C1 glioma cell line. Materials and methods: We investigated the uptake, toxicity and radiosensitivity of GNP–PEG–cRGDfKs in ALTS1C1 cells exposed to protons, kilovoltage photons and megavoltage photons. The in vitro uptake and toxicity of GNPs in the hepatocytes and Kupffer cells were assessed for murine AML12 hepatocyte and RAW 264.7 macrophage cell lines. The in vivo biodistribution of GNPs in the ALTS1C1 tumor model was tested using the inductively coupled plasma mass spectrometry. Results: Results indicated GNPs accumulated in the cytoplasm with negligible toxicity for a moderate concentration of GNPs. Observed sensitizer enhancement ratios and dose enhancement factors are 1.21–1.66 and 1.14–1.33, respectively, for all radiations. Conclusion: Ultrasmall GNP–PEG–cRGD can be considered as a radiosensitizer. For radiotherapy applications, the delivery method should be developed to increase the GNP uptake in the tumor and decrease the uptakes in undesirable organs.

The purpose of this study is to investigate the dose perturbation caused by the metal ovoid structures of a Henschke applicator using Monte Carlo simulation in a realistic phantom. The Henschke applicator has been widely used for gynecologic patients treated by brachytherapy in Taiwan. However, the commercial brachytherapy planning system (BPS) did not properly evaluate the dose perturbation caused by its metal ovoid structures. In this study, Monte Carlo N-Particle Transport Code eXtended (MCNPX) was used to evaluate the brachytherapy dose distribution of a Henschke applicator embedded in a Plastic water phantom and a heterogeneous patient computed tomography (CT) phantom. The dose comparison between the MC simulations and film measurements for a Plastic water phantom with Henschke applicator were in good agreement. However, MC dose with the Henschke applicator showed significant deviation (−80.6%±7.5%) from those without Henschke applicator. Furthermore, the dose discrepancy in the heterogeneous patient CT phantom and Plastic water phantom CT geometries with Henschke applicator showed 0 to −26.7% dose discrepancy (−8.9%±13.8%). This study demonstrates that the metal ovoid structures of Henschke applicator cannot be disregard in brachytherapy dose calculation.

Neutron production is a concern in proton therapy, particularly in scattering proton beam delivery systems. Despite this fact, little is known about the effects of secondary neutron exposure around wobbling scattered proton treatment nozzles. The objective of this study was to estimate the neutron dose level resulting from the use of a wobbling scattered proton treatment unit. We applied the Monte Carlo method for predict the ambient neutron dose equivalent, H*(10), per absorbed dose at the treatment isocenter, D, in the proton therapy center of Chang Gung Memorial Hospital, Linkou, Taiwan. For a 190-MeV proton beam, H * ( 10 ) / D values typically decreased with the distance from the isocenter, being 1.106 mSv/Gy at the isocenter versus 0.112 mSv/Gy at a distance of 150 cm from the isocenter. The H * ( 10 ) / D values generally decreased as the neutron receptors moved away from the isocenter, and increased when the angle from the initial beam axis increased. The ambient neutron dose equivalents were observed to be slightly lower in the direction of multileaf collimator movement. For radiation protection, the central axis of a proton-treated patient is suggested to be at the 0° angle of the beam. If the beam direction at the 90° angle is necessary, the patient axis is suggested to be along with the direction of MLC movement. Our study provides the neutron dose level and neutron energy fluence for the first wobbling proton system at the proton therapy center of Chang Gung Memorial Hospital.

Intensity-modulated radiation therapy is an effective treatment modality for the nasopharyngeal carcinoma. One important aspect of this cancer treatment is the need to have an accurate dose algorithm dealing with the complex air/bone/tissue interface in the head-neck region to achieve the cure without radiation-induced toxicities. The Acuros XB algorithm explicitly solves the linear Boltzmann transport equation in voxelized volumes to account for the tissue heterogeneities such as lungs, bone, air, and soft tissues in the treatment field receiving radiotherapy. With the single beam setup in phantoms, this algorithm has already been demonstrated to achieve the comparable accuracy with Monte Carlo simulations. In the present study, five nasopharyngeal carcinoma patients treated with the intensity-modulated radiation therapy were examined for their dose distributions calculated using the Acuros XB in the planning target volume and the organ-at-risk. Corresponding results of Monte Carlo simulations were computed from the electronic portal image data and the BEAMnrc/DOSXYZnrc code. Analysis of dose distributions in terms of the clinical indices indicated that the Acuros XB was in comparable accuracy with Monte Carlo simulations and better than the anisotropic analytical algorithm for dose calculations in real patients.

The purpose of this study is to compare the angular and spatial distributions of therapeutic-energy proton beams obtained from the FLUKA, GEANT4 and MCNP6 Monte Carlo codes. The Monte Carlo simulations of proton beams passing through two thin targets and a water phantom were investigated to compare the primary and secondary proton fluence distributions and dosimetric differences among these codes. The angular fluence distributions, central axis depth-dose profiles, and lateral distributions of the Bragg peak cross-field were calculated to compare the proton angular and spatial distributions and energy deposition. Benchmark verifications from three different Monte Carlo simulations could be used to evaluate the residual proton fluence for the mean range and to estimate the depth and lateral dose distributions and the characteristic depths and lengths along the central axis as the physical indices corresponding to the evaluation of treatment effectiveness. The results showed a general agreement among codes, except that some deviations were found in the penumbra region. These calculated results are also particularly helpful for understanding primary and secondary proton components for stray radiation calculation and reference proton standard determination, as well as for determining lateral dose distribution performance in proton small-field dosimetry. By demonstrating these calculations, this work could serve as a guide to the recent field of Monte Carlo methods for therapeutic-energy protons.

Nuclear reactions of high-energy protons with treatment equipment, air, and patient tissue during proton therapy generate residual radioactivity and secondary particles including protons, deuterons, alphas, and neutrons. The most up-to-date versions of INCL++ (v5.2.9), TALYS (v1.8), EMPIRE (v3.2.2 Malta), and ALICE/ASH were used in this study to calculate the excitation functions of proton-induced reactions with carbon, nitrogen, oxygen, aluminum, calcium, iron, nickel, copper, zinc, tin, tungsten, and lead nuclei. The cross sections of different nuclear reaction mechanisms, gamma particles, and residual radionuclides were calculated. The obtained results were compared with available experimental data and the ENDF/B-VII.1.

In vivo range verification plays an important role in proton therapy to fully utilize the benefits of the Bragg peak (BP) for delivering high radiation dose to tumor, while sparing the normal tissue. For accurately locating the position of BP, camera equipped with collimators (multi-slit and knife-edge collimator) to image prompt gamma (PG) emitted along the proton tracks in the patient have been proposed for range verification. The aim of the work is to compare the performance of multi-slit collimator and knife-edge collimator for non-invasive proton beam range verification. PG imaging was simulated by a validated GATE/GEANT4 Monte Carlo code to model the spot-scanning proton therapy and cylindrical PMMA phantom in detail. For each spot, 10 8 protons were simulated. To investigate the correlation between the acquired PG profile and the proton range, the falloff regions of PG profiles were fitted with a 3-line-segment curve function as the range estimate. Factors including the energy window setting, proton energy, phantom size, and phantom shift that may influence the accuracy of detecting range were studied. Results indicated that both collimator systems achieve reasonable accuracy and good response to the phantom shift. The accuracy of range predicted by multi-slit collimator system is less affected by the proton energy, while knife-edge collimator system can achieve higher detection efficiency that lead to a smaller deviation in predicting range. We conclude that both collimator systems have potentials for accurately range monitoring in proton therapy. It is noted that neutron contamination has a marked impact on range prediction of the two systems, especially in multi-slit system. Therefore, a neutron reduction technique for improving the accuracy of range verification of proton therapy is needed.

Knowledge about the single-event energy deposition in a subcellular biological target, e.g. cell nucleus, is required in order to understand the radiation action and the biological effectiveness of ionizing radiation. This energy deposition can be measured using a tissue equivalent proportional counter (TEPC). Such measurements are particularly useful to find the microdosimetric spectra for mixed radiation fields comprising different radiation types and energies. In the present work, several mixed radiation fields were generated by irradiating a mini TEPC, containing a small plug inserted into the hole on the counter wall, with reactor neutrons. The plug, made of A150, boron, nitrogen, lithium or cadmium mixture, was designed to generate different ionizing particles under the neutron irradiation. The measured spectra separated the contributions to the absorbed dose from electrons (photons), protons (neutrons) and heavy ions. The lineal energy spectra demonstrated that three distinct regions of lineal energy y , were identified, i.e. the region from heavy ions at y >100 keV/µm, the region for recoil protons at 10 y y y region depended on the yields and stopping powers of nuclear reaction products. For instance, lineal energy peaks were identified at y >100 keV/µm due to the 10 B (n, α) 7 Li reaction. If one substitutes the plug material by a radionuclide, microdosimetry spectra could be measured for internal dosimetry applications.

Monte Carlo FLUKA and MCDS codes were used to study 1H, 4He and 12C ion beams with similar ranges in water phantom. The longitudinal and lateral dose profiles and the relative biological effectiveness (RBE) were investigated. Simulations were performed with particular emphasis in the vicinity of the Bragg peak (BP). The impact of individual nuclear fragments created by the primary beams was examined. Comparisons were made for the BP width, lateral dose profile width, and RBE value at different depths in the phantom. RBE values for the induction of DNA double-strand breaks were estimated, particularly in the vicinity of the BP. Results showed that 12C beam had the narrowest BP width, smallest lateral dose profile, and largest RBE value, followed by 4He and 1H beams. However, the differences between 4He and 12C were smaller than those between 1H and 4He. It also showed that the influence of nuclear fragments was significant for 12C beam, moderate for 4H beam, and minor for 1H beam. This influence was important for 12C at depths beyond the BP. Our results suggest that 4He beam is also a promising option for cancer therapy.

The purpose of this study was to investigate the density heterogeneity pattern as a factor affecting Bragg peak degradation, including shifts in Bragg peak depth (ZBP), distal range (R80 and R20), and distal fall-off (R80–R20) using Monte Carlo N-Particles, eXtension (MCNPX). Density heterogeneities of different patterns with increasing complexity were placed downstream of commissioned proton beams at the Proton and Radiation Therapy Centre of Chang Gung Memorial Hospital, including one 150 MeV wobbling broad beam (10×10 cm2) and one 150 MeV proton pencil beam (FWHM of cross-plane=2.449 cm, FWHM of in-plane=2.256 cm). MCNPX 2.7.0 was used to model the transport and interactions of protons and secondary particles in density heterogeneity patterns and water using its repeated structure geometry. Different heterogeneity patterns were inserted into a 21×21×20 cm3 phantom. Mesh tally was used to track the dose distribution when the proton beam passed through the different density heterogeneity patterns. The results show that different heterogeneity patterns do cause different Bragg peak degradations owing to multiple Coulomb scattering (MCS) occurring in the density heterogeneities. A trend of increasing R20 and R80–R20 with increasing geometry complexity was observed. This means that Bragg peak degradation is mainly caused by the changes to the proton spectrum owing to MCS in the density heterogeneities. In contrast, R80 did not change considerably with different heterogeneity patterns, which indicated that the energy spectrum has only minimum effects on R80. Bragg peak degradation can occur both for a broad proton beam and a pencil beam, but is less significant for the broad beam.

The Multiple Coulomb Scattering (MCS) model plays an important role in accurate MC simulation, especially for small field applications. The Rossi model is used in MCNPX 2.7.0, and the Lewis model in Geant4.9.6.p02. These two models may generate very different angular and spatial distributions in small field proton dosimetry. Beside angular and spatial distributions, step size is also an important issue that causes path length effects. The Energy Fraction (EFAC) value can be used in MCNPX 2.7.0 to control step sizes of MCS. In this study, we use MCNPX 2.7.0, Geant4.9.6.p02, and one pencil beam algorithm to evaluate the effect of dose deposition because of different MCS models and different EFAC values in proton disequilibrium situation. Different MCS models agree well with each other under a proton equilibrium situation. Under proton disequilibrium situations, the MCNPX and Geant4 results, however, show a significant deviation (up to 43%). In addition, the path length effects are more significant when EFAC is equal to 0.917 and 0.94 in small field proton dosimetry, and using a 0.97 EFAC value is the best for both accuracy and efficiency

The purpose of this study was to calculate the magnetic field correction factor k B for cylindrical ionization chambers and to examine the effect of orientations and beam energies on k B under different magnetic field strengths using Monte Carlo code TOPAS. Monte Carlo simulations were performed for NE2571, IBA FC65G, IBA FC65P and PTW31010 chambers in a water phantom using data provided by manufacturers and those reported in published literatures. The transverse magnetic field orthogonal to the photon beam was modeled as being parallel or perpendicular to the long axis of the ionization chamber with the magnetic field strength of 0 T, 0.35 T, 0.6 T, 1.0 T, 1.5 T, respectively. The photon beam spectrum of 7 MV Elekta Unity was considered. To quantify the effect of the beam energy on k B , the IBA FC65G Farmer-type ionization chamber was also experimentally irradiated using cobalt-60 beams. To validate the simulation results, the k B of the IBA FC65G chamber at 1.5 T was compared to data in the literature. The results showed that for the parallel configuration, all magnetic field correction factors k B differed from unity by less than 2.1%. On the other hand, for the perpendicular configuration, changes up to 5.5% compared to simulations without the magnetic field were observed for 1.0 T. For the PTW31010 chamber, the change was smaller between these two configurations. It was also found that the beam energy had an impact on the k B . When irradiated by the cobalt-60 beams in both configurations, the k B of IBA FC65G varies similarly within 2% with the case irradiated by 7 MV Elekta Unity beams. The comparison of k B values from this study with those from the literatures indicate that satisfactory k B values within 1% and thus can be simulated utilizing TOPAS Monte Carlo simulation methods reported in this study. The magnetic field correction factor k B at 0.6 T and 1.0 T can be considered as a reference for these two machines at the commissioning stage in the future.

Neutron radiation on advanced integrated circuits (ICs) is becoming important for their reliable operation. However, a neutron test on ICs is expensive and time-consuming. In this work, we employ Monte Carlo simulation to examine if a proton test can replace or even accelerate the neutron test, and we found that 200 MeV protons are the closest to resembling neutron radiation with five main differences. This 200 MeV concur with the suggestion from National Aeronautics and Space Administration (NASA, Washington, DC, USA). However, the impacts of the five differences on single event effects (SEEs) require future work for examination.

Exploration of the optimal combination of range modulation materials (RMMs) for proton beam irradiation in lung heterogeneous media using Monte Carlo simulation was the focus of study. Physical and dosimetric properties of each homogenous RMM irradiated with proton beam were investigated. The water equivalent ratio (WER) and full width at half maximum (FWHM) were compared to determine the primary RMM. The optimized trade-off between the effects of thickness needed for range modulation and FWHMs for selecting optimal RMM combinations was found. The optimal 2nd RMM was determined based on figure-of-merit (FOM) analyses in the water-lung-water media irradiated by proton beams. This study created an easier approach and successfully created the spread–out Bragg peak (SOBP) in lung heterogeneous media, using statistically fitted matrix with uniformity of SOBP better than 2%. The weighting factors matrix for creating SOBP may theoretically be applied to any multiple range modulation systems. The methodology for the 150 MeV proton beams' range modulation in the lung heterogeneous media can be similarly applied to other energy of proton as well as for heavy ion therapy.

At the Chang Gung Memorial Hospital, a wobbled beam together with a set of single scatterers produces a uniform proton field at the first and second gantries for proton radiotherapy. Conformal field shaping was accomplished by using only a multileaf collimator (MLC) or a combination of an MLC and a brass aperture (downstream of the MLC). The MLC-only technique typically delivered extra low dose tails in penumbra compared to the dose delivered using both the MLC and brass aperture/block. This study attempted to provide a physical explanation of this phenomenon by Monte Carlo simulation and phase space analysis. The Particle Therapy System Simulation Framework was used as the Monte Carlo tool for beam propagation simulation. This research team investigated four possible causes for low dose tails including, (a) same-side MLC scattering; (b) opposite-side MLC scattering; (c) MLC transmission; and (d) proton angular distribution broadened by the wobbling magnets. The results indicated that the proton angular distribution broadened by the wobbling magnets is the single cause of low dose tails and can be stopped by inserting a downstream brass aperture.

Since the accident of Fukushima Daiichi nuclear power plant, the environmental radiation dose was increasingly concerned as a global issue. In Taiwan, the detection capability of the all calibration laboratories is just about 50 times the naturally occurring environmental radiation for environmental radiation protection detectors. Undoubtedly the use of environmental radiation detector shows relative low performance in accuracy and the public are very sensitive to the amount of environmental radiation. Thus, the National Radiation Standard Laboratory (NRSL) of the Institute of Nuclear Energy Research (INER) designed a multi-source of low air kerma rate calibration system to respond to the measurement and calibration needs in Taiwan. The system was verified to meet the needs of measurement and calibration through a series of evaluations and comparisons based on ISO 4037-1 criteria (ISO 4037, 1996). This environmental-level dose rate measurement and calibration system contains three radioactive sources respectively penetrating by sets of filter of three different thicknesses and then producing radiation fields of three different strengths. A total of nine standard radiation fields are provided with intensity ranges from 1746.6 μGy/h to 0.092 μGy/h, and the expanded uncertainties of the system were from 1.02% to 5.77% (k = 2).

Monte Carlo simulation for radiation therapy needs a computational model of patient geometry. Voxel model has been so far used for this purpose. However, tetrahedral model is more suitable for representing organ’s shape. In this paper, we report on the development of tetrahedral organ model in Geant4 based particle therapy system simulation framework (PTSIM). Several organ geometries have been successfully constructed with the tetrahedral model in PTSIM. The performance in the simulation is discussed by comparing the tetrahedral model with the voxel model.

In this paper, we present the result of TI MCU TMS570LS3171 radiation test with scanning proton beam in the proton and radiation therapy center of Chang Gung Memorial Hospital (CGMH). CGMH is the first proton therapy center in Taiwan. It is also open for proton radiation validations and tests. The latest proton beam delivery technology, pencil beam scanning (PBS), is also available. In this test, the proton beam energy is 200 MeV. The total delivered fluence is 1.425×1011 proton/cm2 with 25 deliveries. No single-event latch up event is observed. There are ten errors observed. Four of them are internal memory errors. Six test program crashes are reported and can be recover by power cycling.

In this study, we used the Geant4 toolkit to demonstrate the impacts of the material composition of tissues on proton range variation. Bragg curves of different materials subjected to a 250 MeV mono-energy proton beam were simulated and compared. These simulated materials included adipose, heart, brain, cartilage, cortical bone and water. The results showed that there was significant proton range deviation between Bragg curves, especially for cortical bone. The R50 values for a 250 MeV proton beam were approximately 39.55 cm, 35.52 cm, 37.00 cm, 36.51 cm, 36.72 cm, 22.53 cm, and 38.52 cm in the phantoms that were composed completely of adipose, cartilage, tissue, heart, brain, cortical bone, and water, respectively. Mass density and electron density were used to scale the proton range for each material; electron density provided better range scaling. In addition, a similar comparison was performed by artificially setting all material density to 1.0 g/cm3 to evaluate the range deviation due to chemical components alone. Tissue heterogeneity effects due to density variation were more significant, and less significant for chemical composition variation unless the Z/A was very different.

The purpose of this study is to access proton dose distribution in dual resolution phantoms using MCNPX 2.6.0. The dual resolution phantom uses higher resolution in Bragg peak, area near large dose gradient, or heterogeneous interface and lower resolution in the rest. MCNPX 2.6.0 was installed in Ubuntu 10.04 with MPI for parallel computing. FMesh1 tallies were utilized to record the energy deposition which is a special designed tally for voxel phantoms that converts dose deposition from fluence. 60 and 120 MeV narrow proton beam were incident into Coarse, Dual and Fine resolution phantoms with pure water, water–bone–water and water–air–water setups. The doses in coarse resolution phantoms are underestimated owing to partial volume effect. The dose distributions in dual or high resolution phantoms agreed well with each other and dual resolution phantoms were at least 10 times more efficient than fine resolution one. Because the secondary particle range is much longer in air than in water, the dose of low density region may be under-estimated if the resolution or calculation grid is not small enough.

In the treatment planning of radiation therapy, patients may be administrated with contrast media in CT scanning to assist physicians for accurate delineation of the target or organs. However, contrast media are not used in patients during the treatment delivery. In particular, contrast media contain materials with high atomic numbers and dosimetric variations may occur between scenarios where contrast media are present in treatment planning and absent in treatment delivery. In this study we evaluate the effect of contrast media on the dosimetry and biological consequence. An analytical phantom based on AAPM TG 119 and five sets of CT images from clinical patients are included. Different techniques of treatment planning are considered, including 1-field AP, 2-field AP+PA, 4-field box, 7-field IMRT, and RapidArc. RapidArc is a recent technique of volumetric modulated arc therapy and is used in our study of contrast media in clinical scenarios. The effect of RapidArc on dosimetry and biological consequence for administration of contrast media in radiotherapy is not discussed previously in literature. It is shown that dose difference is reduced as the number of external beams is increased, suggesting RapidArc may be favored to be used in the treatment planning enhanced by contrast media. Linear trend lines are fitted for assessment of percent dose differences in the planning target volume versus concentrations of contrast media between plans where contrast media are present and absent, respectively.

Measurement-based Monte Carlo (MBMC) simulation using a high definition (HD) phantom was used to evaluate the dose distribution in nasopharyngeal cancer (NPC) patients treated with intensity modulated radiation therapy (IMRT). Around nasopharyngeal cavity, there exists many small volume organs-at-risk (OARs) such as the optic nerves, auditory nerves, cochlea, and semicircular canal which necessitate the use of a high definition phantom for accurate and correct dose evaluation. The aim of this research was to study the advantages of using an HD phantom for MBMC simulation in NPC patients treated with IMRT. The MBMC simulation in this study was based on the IMRT treatment plan of three NPC patients generated by the anisotropic analytical algorithm (AAA) of the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA, USA) using a calculation grid of 2 mm2. The NPC tumor was treated to a cumulative dose of 7000 cGy in 35 fractions using the shrinking-field sequential IMRT (SIMRT) method. The BEAMnrc MC Code was used to simulate a Varian EX21 linear accelerator treatment head. The HD phantom contained 0.5 × 0.5 × 1 mm3 voxels for the nasopharyngeal area and 0.5 × 0.5 × 3 mm3 for the rest of the head area. An efficiency map was obtained for the amorphous silicon aS1000 electronic portal imaging device (EPID) to adjust the weighting of each particle in the phase-space file for each IMRT beam. Our analysis revealed that small volume organs such as the eighth cranial nerve, semicircular canal, cochlea and external auditory canal showed an absolute dose difference of ≥200 cGy, while the dose difference for larger organs such as the parotid glands and tumor was negligible for the MBMC simulation using the HD phantom. The HD phantom was found to be suitable for Monte Carlo dose volume analysis of small volume organs.

Purpose: One of the promising radiosensitizers is the ultrasmall gold nanoparticle (GNP) with a hydrodynamic diameter vβ3) in the murine ALTS1C1 glioma cell line. Materials and methods: We investigated the uptake, toxicity and radiosensitivity of GNP-PEG-cRGDfKs in ALTS1C1 cells exposed to protons, kilovoltage photons and megavoltage photons. The in vitro uptake and toxicity of GNPs in the hepatocytes and Kupffer cells were assessed for murine AML12 hepatocyte and RAW 264.7 macrophage cell lines. The in vivo biodistribution of GNPs in the ALTS1C1 tumor model was tested using the inductively coupled plasma mass spectrometry. Results: Results indicated GNPs accumulated in the cytoplasm with negligible toxicity for a moderate concentration of GNPs. Observed sensitizer enhancement ratios and dose enhancement factors are 1.21-1.66 and 1.14-1.33, respectively, for all radiations. Conclusion: Ultrasmall GNP-PEG-cRGD can be considered as a radiosensitizer. For radiotherapy applications, the delivery method should be developed to increase the GNP uptake in the tumor and decrease the uptakes in undesirable organs.

Purpose Beam hardening effects due to photons passing through the multi-leaf collimator (MLC) frequently exist in Intensity Modulated Radiotherapy (IMRT) fields. A fast online dose transmission verification system, MBMC (Measurement Based Monte Carlo), can verify IMRT delivery by using a fluence efficiency map to replace MLC geometry and movement simulation. This system, however, ignores beam hardening effects, and assumes that dose disturbances through the MLC are not significant for IMRT fields. This assumption has to be justified before it can be applied in the clinic. Methods In this study, we simulated several field sizes (0.5 × 0.5, 1 × 1, 3 × 3, 5 × 5, and 10 × 10 cm2) to evaluate the dose influence of beam hardening effects under clinical conditions. In addition, a LATCH technique was used during simulation processes, which can record each particle interaction with specific gantry components, to distinguish between dose contribution from the total beam and MLC mediated beam. Results The MLC indeed caused significant beam hardening effects, but the dose contribution fraction from the MLC was noticeable only for field sizes less than 1 × 1 cm2. Furthermore, in mixed fields containing both the total beam and MLC mediated beam, the maximum dose deviation due to the presence of the MLC is small even for the 0.5 × 0.5 cm2 field size (∼2%). Conclusions The MLC causes noticeable beam hardening effects, but this effect results in only slight dose differences that are only noticeable for small field sizes in IMRT delivery. The use of a fluence efficiency map was feasible in our MBMC system.

In order to improve the scanning efficiency of commercial optical computed tomography (OptCT) scanners which use a conventional scanning laser, a novel optical CT scanner is being developed which employs a laser-based line source for the gel-scanning process. This line-source laser system can generate a uniform and parallel illumination profile in a line pattern, and consists of a Gaussian spot laser, a beam shaping module to convert a Gaussian laser beam into a 90° fan-angle line pattern with flat-top profile, and a set of cylindrical lenses which converts the rays into parallel light. The optical CT scanner consists of a gel cylinder mounted inside the scanning tank, two scanning motors to operate the rotations and translations of the gel cylinder, and a CCD detector to record the image data. We present our analysis of the sources of uncertainty and evaluate the basic performance parameters of the optical CT scanner in conjunction with polymer gel dosimeters. To improve dose accuracy of this prototype, it may also be necessary to decrease the wall thicknesses of both the gel container and the matching liquid tank and more precisely to match the refractive indices of the whole system.

Neutron spectra were measured at different locations around the couch and in the maze of a Varian Clinac 2100 linac for 15 MV X-rays. A slab of PMMA, Al or Pb was also positioned near a measurement point. The Bonner sphere spectrometry using paired thermoluminescence dosemeters, TLD-600 and -700, was used to measure the neutron spectra. Eight polyethylene spheres of diameters between 3 and 10 inches were employed. The unfolding of neutron spectrum was performed with the MAXED package. Using measured neutron spectra, the average neutron energy, absorbed dose, ambient dose equivalent, and effective dose were determined.

In vivo dosimetry, which is commonly practised in Europe and North America, is a purely measurement-based check which can independently verify the entire treatment procedure. In vivo dosimetry can be performed using either one or two detectors (i.e. diodes), and the later one applying two detectors in both the entrance and exit surface can independently verify the patients’ dose. However, this type of two-detector measurement can only be implemented with tumours of symmetric geometry at the midline, and can only be applied to opposite large-field irradiation. Nasopharyngeal cancer (NPC) is the most interesting symmetric cancer to medical physicists in Asia since it preferentially occurs in 40–50 year old males who have familial responsibilities. The tissues/structures around the nasopharynx are more complicated than those around the prostate, and may introduce unwanted bias to in vivo dosimetry. Asymmetry and inhomogeneity are the most important systematic uncertainties with in vivo dosimetry. This study demonstrates the bias of in vivo dosimetry caused by tissue asymmetry and heterogeneity, and provides corrections accordingly. We have demonstrated that asymmetry correction reduced the uncertainty of deviation between in vivo dosimetry and the prescribed dose from 3.12% to 2.39% for in vivo dosimetry of 102 patients. Heterogeneity correction reduced the mean deviation between in vivo dosimetry and the prescribed dose from 0.56% to 0.22%.

This study proposes to implement a realistic X-ray source and a high definition voxelised human model, VIP-Man into a Monte Carlo simulation for chest examination. A detailed simulation of a diagnostic X-ray dose requires three major inputs: operation conditions, X-ray spectral and spatial distributions, and a human model. This study collected commonly-used operation parameters such as kVp, mAs, and SSD for chest X-ray examinations. These parameters were inputted to a Monte Carlo code, BEAMnrc, to build a virtual X-ray machine for calculating X-ray spectral and spatial distributions. After that, a high resolution voxelised human model, VIP-Man, was constructed from a segmented Visible Male Dataset, and was irradiated with our virtual X-ray machine. X-ray spectra simulated using BEAMnrc were almost identical to those listed in the literature, or from a spectra simulator, XCOMP3. Heel effects could be observed in our simulation of X-ray spatial distribution. The simulated organ doses were compared to those published in NRPB-R262. There is some significant dose deviation between our results and NRPB’s, owing to the differences in stature and anatomical features, especially for liver, GI tract, lung, pancreas, and red bone marrow. These deviations are mainly caused by the different volume and position of the organs between the mathematical model and VIP-Man.

A dose simulation system was constructed based on the MCNPX Monte Carlo package to simulate proton dose distribution in homogeneous and CT phantoms. Conversion from Hounsfield unit of a patient CT image set to material information necessary for Monte Carlo simulation is based on Schneider's approach. In order to validate this simulation system, inter-comparison of depth dose distributions among those obtained from the MCNPX, GEANT4 and FLUKA codes for a 160 MeV monoenergetic proton beam incident normally on the surface of a homogeneous water phantom was performed. For dose validation within the CT phantom, direct comparison with measurement is infeasible. Instead, this study took the approach to indirectly compare the 50% ranges (R50%) along the central axis by our system to the NIST CSDA ranges for beams with 160 and 115 MeV energies. Comparison result within the homogeneous phantom shows good agreement. Differences of simulated R50% among the three codes are less than 1 mm. For results within the CT phantom, the MCNPX simulated water equivalent Req,50% are compatible with the CSDA water equivalent ranges from the NIST database with differences of 0.7 and 4.1 mm for 160 and 115 MeV beams, respectively.

During radiotherapy treatments, quality assurance/control is essential, particularly dose delivery to patients. This study was designed to verify midline doses with diode in vivo dosimetry. Dosimetry was studied for 6-MV bilateral fields in head and neck cancer treatments and 10-MV bilateral and anteroposterior/posteroanterior (AP/PA) fields in pelvic cancer treatments. Calibrations with corrections of diodes were performed using plastic water phantoms; 190 and 100 portals were studied for head and neck and pelvis treatments, respectively. Calculations of midline doses were made using the midline transmission, arithmetic mean, and geometric mean algorithms. These midline doses were compared with the treatment planning system target doses for lateral or AP (PA) portals and paired opposed portals. For head and neck treatments, all 3 algorithms were satisfactory, although the geometric mean algorithm was less accurate and more uncertain. For pelvis treatments, the arithmetic mean algorithm seemed unacceptable, whereas the other algorithms were satisfactory. The random error was reduced by using averaged midline doses of paired opposed portals because the asymmetric effect was averaged out. Considering the simplicity of in vivo dosimetry, the arithmetic mean and geometric mean algorithm should be adopted for head/neck and pelvis treatments, respectively.

The purpose of this work was to study the calculated dose uncertainties induced by the material classification that determined the interaction cross-sections and the water-to-material stopping-power ratios. Calculations were made for a head- and neck-cancer patient treated with five intensity-modulated radiotherapy fields using 6 MV photon beams. The patient's CT images were reconstructed into two voxelized patient phantoms based on different CT-to-material classification schemes. Comparisons of the depth–dose curve of the anterior-to-posterior field and the dose–volume-histogram of the treatment plan were used to evaluate the dose uncertainties from such schemes. The results indicated that any misassignment of tissue materials could lead to a substantial dose difference, which would affect the treatment outcome. To assure an appropriate material assignment, it is desirable to have different conversion tables for various parts of the body. The assignment of stopping-power ratio should be based on the chemical composition and the density of the material.

In recent years the Henschke applicator has been widely used for gynecologic patients treated by brachytherapy in Taiwan. However, the commercial brachytherapy planning system did not properly evaluate the dose perturbation caused by the Henschke applicator. Since the European Society for Therapeutic Radiology and Oncology advised that the effect of source shielding should be incorporated into the brachytherapy planning system, it required calculation and comparison of the dose distribution around the applicator. This study used the Monte Carlo MCNP code to simulate the dose distribution in a water phantom that contained the Henschke applicator with one tandem and two ovoids. Three dwell positions of a high dose rate 192 Ir source were simulated by including and excluding the applicator. The mesh tally option of the MCNP was applied to facilitate the calculation of a large number of tallies in the phantom. The voxel size effect and the charge particle equilibrium were studied by comparing the results calculated with different tally options. The calculated results showed that the brachytherapy planning system overestimated the rectal dose and that the shielding material in the applicator contributed more than 40% to the rectal dose.

A small animal irradiator is critical for providing optimal radiation dose distributions for pre-clinical animal studies. This paper focuses on the evaluation of using 6 or 18 MeV electron beams as small animal irradiators. Compared with all other prototypes which use photons to irradiate small animals, an electron irradiator has many advantages in its shallow dose distribution. Two major approaches including simulation and measurement were used to evaluate the feasibility of applying electron beams in animal irradiation. These simulations and measurements were taken in three different fields (a 6 cm x 6 cm square field, and 4 mm and 30 mm diameter circular fields) and with two different energies (6 MeV and 18 MeV). A PTW Semiflex chamber in a PTW-MP3 water tank, a PTW Markus chamber type 23343, a PTW diamond detector type 60003 and KODAK XV films were used to measure PDDs, lateral beam profiles and output factors for either optimizing parameters of Monte Carlo simulation or to verify Monte Carlo simulation in small fields. Results show good agreement for comparisons of percentage depth doses (

The objectives of this work were to calculate the cellular doses and the lineal energies of low-energy electrons in liquid water for different source–target geometry in a cell. Calculated specific cellular doses and their variations were analyzed for the dependences on electron energy, source–target geometry, elastic interaction, and type of energy depositions, i.e. starter, stopper, insider and crosser. Two approaches, i.e. the probabilistic method and the mixed method, were applied. In the probabilistic method, the Monte Carlo Penelope code was used. In the mixed method, the range-energy relation and the sampling of electron paths were applied. It was found that for N ← Cy elastic interactions led to a change of the specific cellular dose by about 30% for electron energies below 10 keV. Here N ← Cy denotes electrons emitted from the source region, Cy (cytoplasm), to deposit energy in the target region, N (cell nucleus). The variation of specific cellular dose was found greater (more than 10%) for N ← Cy than N ← N, C ← C and C ← CS, where C and CS denote the cell and cell surface, respectively. The lineal energy distribution varied substantially with electron energy, source–target geometry, and target size. The maximum values of the relative dose-mean lineal energy for 1, 5 and 10 keV electrons, relative to 36 keV reference electrons used to define the relative biological effectiveness, occurred at target radii of several tens, hundreds and thousands nanometers, respectively.

The aim of this study was to develop a dose simulation system based on portal dosimetry measurements and the BEAMMonte Carlo code for intensity-modulated (IM) radiotherapy dose verification. This measurement-based Monte Carlo (MBMC) system can perform, within one systematic calculation, both pretreatment and on-line transit dose verifications. BEAMnrc and DOSXYZnrc 2006 were used to simulate radiation transport from the treatment head, through the patient, to the plane of the aS500 electronic portal imaging device(EPID). In order to represent the nonuniform fluence distribution of an IM field within the MBMC simulation, an EPID-measured efficiency map was used to redistribute particle weightings of the simulated phase space distribution of an open field at a plane above a patient/phantom. This efficiency map was obtained by dividing the measured energy fluence distribution of an IM field to that of an open field at the EPID plane. The simulated dose distribution at the midplane of a homogeneous polystyrene phantom was compared to the corresponding distribution obtained from the Eclipse treatment planning system (TPS) for pretreatment verification. It also generated a simulated transit dose distribution to serve as the on-line verification reference for comparison to that measured by the EPID. Two head-and-neck (NPC1 and NPC2) and one prostate cancer fields were tested in this study. To validate the accuracy of the MBMC system, film dosimetry was performed and served as the dosimetry reference. Excellent agreement between the film dosimetry and the MBMC simulation was obtained for pretreatment verification. For all three cases tested, gamma evaluation with 3%/3 mm criteria showed a high pass percentage ( > 99.7 % ) within the area in which the dose was greater than 30% of the maximum dose. In contrast to the TPS, the MBMC system was able to preserve multileaf collimatordelivery effects such as the tongue-and-groove effect and interleaf leakage. In the NPC1 field, the TPS showed 16.5% overdose due to the tongue-and-groove effect and 14.6% overdose due to improper leaf stepping. Similarly, in the NPC2 field, the TPS showed 14.1% overdose due to the tongue-and-groove effect and 8.9% overdose due to improper leaf stepping. In the prostate cancer field, the TPS showed 6.8% overdose due to improper leaf stepping. No tongue-and-groove effect was observed for this field. For transit dose verification, agreements among the EPID measurement, the film dosimetry, and the MBMC system were also excellent with a minimum gamma pass percentage of 99.6%.

Interactions of low-energy electrons with liquid water and DNA were investigated. Elastic and inelastic interaction cross sections were calculated using appropriate models for the response of atomic nuclei, valence band and inner shells. These models included an extended Drude dielectric model for valence-band excitations, a sum-rule-constrained binary-encounter model for inner-shell ionizations, and a phase-shift analysis model for elastic interactions. Applying calculated cross sections, an event-by-event Monte Carlo (MC) program was developed to simulate electron transport in liquid water and energy depositions in nanometric volumes. This simulation provided an estimate of strand breaks of DNA due to direct and indirect actions by low-energy electrons in a simplified DNA model. This model consisted of two parallel cylinders of 0.5 nm in diameter, 16 nm in height and 1 nm in separation. Any energy deposition greater than 17.6 eV in the cylinder was assumed to cause a single strand break (ssb) by direct action. An energy deposition of 12.6 eV or greater in liquid water within 0.5 nm of the cylinder surface was assumed to induce an OH radical which had a probability of 0.13 to produce a ssb by indirect action. When two ssbs occurred on opposite strands separated by 10 or fewer base pairs, a double strand break (dsb) was assumed. The number of deposition events for ssbs or dsbs per electron emission was calculated and analyzed for different electron energies and electron emission positions. The simplified model provided useful information on the energy depositions for DNA strand breaks and on the effectiveness of low-energy electrons in various geometric and irradiation conditions.

A study of inelastic interactions of low-energy electrons with biological media is of fundamental importance in the understanding of radiation-induced actions leading to the biological damage. In the present work, electron energy loss properties in liquid water and solid DNA were calculated based on the dielectric response theory for valence band electrons and atomic models for inner-shell electrons. For electron interactions with the valence band, an extended Drude dielectric function was fitted to experimental optical data and then checked by sum rules. The valance band was considered as composed of many subbands with different oscillator strength, binding energy and damping coefficient. The real and imaginary parts of the dielectric function and the energy loss function were all checked. For electron interactions with inner shells, the generalized oscillator strength was calculated using the sum-rule-constrained binary-encounter model and the local-plasma approximation. The differential inverse mean free paths for the valence band and inner shells were calculated using the Born approximation. Exchange effect was estimated using an approach based on the Moller cross-section. Results of the present work were compared with similar calculations by other authors.

Monte Carlo simulations are used to calculate the relative biological effectiveness (RBE) of 300 MeV u(-1) carbon-ion beams at different depths in a cylindrical water phantom of 10 cm radius and 30 cm long. RBE values for the induction of DNA double strand breaks (DSB), a biological endpoint closely related to cell inactivation, are estimated for monoenergetic and energy-modulated carbon ion beams. Individual contributions to the RBE from primary ions and secondary nuclear fragments are simulated separately. These simulations are based on a multi-scale modelling approach by first applying the FLUKA (version 2011.2.17) transport code to estimate the absorbed doses and fluence energy spectra, then using the MCDS (version 3.10A) damage code for DSB yields. The approach is efficient since it separates the non-stochastic dosimetry problem from the stochastic DNA damage problem. The MCDS code predicts the major trends of the DSB yields from detailed track structure simulations. It is found that, as depth is increasing, RBE values increase slowly from the entrance depth to the plateau region and change substantially in the Bragg peak region. RBE values reach their maxima at the distal edge of the Bragg peak. Beyond this edge, contributions to RBE are entirely from nuclear fragments. Maximum RBE values at the distal edges of the Bragg peak and the spread-out Bragg peak are, respectively, 3.0 and 2.8. The present approach has the flexibility to weight RBE contributions from different DSB classes, i.e. DSB0, DSB+ and DSB++.

Purpose To systematically evaluate diffusion anisotropy (DA) using newly defined indices based on the diffusion deviation and mean diffusivity approach. Materials and Methods Measures of amplitude, area, and volume of the DA index (DAI) were measured and compared with regard to their sensitivity to changes in DA, susceptibility to noise in the original diffusion-weighted (DW) images, and contrast-to-noise ratio (CNR) in homogenous regions. Simulations were performed under different levels of noise and DA. Human DTI data were acquired from eight normal volunteers. Results Indices of area and volume measures provided improved resolution for characterizing the DA compared to the eigenvalue ratio. The amplitude measure showed consistent performances with good CNR and less susceptibility to noise in the original data. Conclusion These indices are rotationally invariant without the requirement of eigenvalue sorting. At low anisotropy, all indices have a similar CNR. For larger DA, the first index (the deviation tensor divided by the DT) shows improved sensitivity, contrast-to-noise ratio (CNR), and noise immunity compared to the other indices. J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc.