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Hypoxic cancer cells within solid tumours show radio-resistance, leading to malignant progression in fractionated radiotherapy. When prescribing dose to tumours under heterogeneous oxygen pressure with intensity-modulated radiation fields, intercellular signalling could have an impact on radiosensitivity between in-field and out-of-field (OF) cells. However, the impact of hypoxia on radio-sensitivity under modulated radiation intensity remains to be fully clarified. Here, we investigate the impact of hypoxia on in-field and OF radio-sensitivities using two types of cancer cells, DU145 and H1299. Using a nBIONIX hypoxic culture kit and a shielding technique to irradiate 50% of a cell culture flask, oxygen enhancement ratios for double-strand breaks (DSB) and cell death endpoints were determined. These in vitro measurements indicate that hypoxia impacts OF cells, although the hypoxic impacts on OF cells for cell survival were dose-dependent and smaller compared to those for in-field and uniformly irradiated cells. These decreased radio-sensitivities of OF cells were shown as a consistent tendency for both DSB and cell death endpoints, suggesting that radiation-induced intercellular communication is of importance in advanced radiotherapy dose-distributions such as with intensity-modulated radiotherapy.

Radio-resistance induced under low oxygen pressure plays an important role in malignant progression in fractionated radiotherapy. For the general approach to predict cell killing under hypoxia, cell-killing models (e.g. the Linear-Quadratic model) have to be fitted to in vitro experimental survival data for both normoxia and hypoxia to obtain the oxygen enhancement ratio (OER). In such a case, model parameters for every oxygen condition needs to be considered by model-fitting approaches. This is inefficient for fractionated irradiation planning. Here, we present an efficient model for fractionated radiotherapy the integrated microdosimetric-kinetic model including cell-cycle distribution and the OER at DNA double-strand break endpoint (OERDSB). The cell survival curves described by this model can reproduce the in vitro experimental survival data for both acute and chronic low oxygen concentrations. The OERDSB used for calculating cell survival agrees well with experimental DSB ratio of normoxia to hypoxia. The important parameters of the model are oxygen pressure and cell-cycle distribution, which enables us to predict cell survival probabilities under chronic hypoxia and chronic anoxia. This work provides biological effective dose (BED) under various oxygen conditions including its uncertainty, which can contribute to creating fractionated regimens for multi-fractionated radiotherapy. If the oxygen concentration in a tumor can be quantified by medical imaging, the present model will make it possible to estimate the cell-killing and BED under hypoxia in more realistic intravital situations.

We investigate the electrical conductivity of a TiO2 surface layer on insulators induced by radiation exposure. The surface layer is sufficiently thin to transmit radiation (typically γ-rays) without absorption, but exhibits a current signal indicating the interaction processes of the radiation with substrate insulators. We explain this effect arising from the layered structure and demonstrate a Monte Carlo simulation of photons and electrons to show that electrons generated by irradiation in the substrate travel and cause energy deposition onto the thin layer, envisaging a new type of radiation detector.

The arrival-time spectra (ATS) of electrons in mixtures of nitrogen and carbon dioxide have been measured for reduced electric fields from 100 to 1000 Td at room temperature by using a double-shutter drift tube. The drift velocity of the electrons in and mixtures was evaluated from a previously reported ATS method. In mixtures of and , we found that is larger than the value predicted by a linear combination of the drift velocities of the pure gases based on the mole fraction (partial pressure) of in the mixture, k, in the range of low . We refer to this as the `mixing effect'. In contrast, a linear combination of the drift velocities of the pure gases based on k was found to accurately predict the drift velocities in mixtures in the higher region . In addition to the experimental evaluation, calculations of in these gas mixtures were carried out by a Boltzmann equation analysis, and the results were compared with those from the ATS measurements. The ratio of the longitudinal diffusion coefficient to the electron mobility was also estimated. The value of , called the characteristic energy, also shows a small mixing effect with increasing k in the range of low and increases monotonically in the range of , while the value decreases linearly as k increases in the higher range.

The distribution of arrival time spectra (ATS) of electrons in nitrogen and carbon dioxide has been measured over the range of E/N from 20 to 1000 Td (1 ) at room temperature by a double-shutter drift tube. The drift velocity of electrons in and was evaluated from these distributions by the ATS method. Moreover, the ratio of the longitudinal diffusion coefficient to the electron mobility was measured. The values of and in and were in good agreement with experimental and theoretical values obtained by previous investigators except for a few of the earlier studies.

The electron swarm behaviour in SF6 gas is re-analysed over the E/N range 141-7000 Td by a six-term Boltzmann equation method and by a Monte Carlo simulation considering the latest cross section data, in particular, that of the elastic momentum transfer cross section. The Boltzmann equation analysis shows that the present set of cross sections gives the values of swarm parameters such as ionization and electron attachment coefficients, drift velocity, longitudinal and transverse diffusion coefficients in excellent agreement with the respective measurements for a wide range of E/N. The swarm parameters calculated by the six-term approximation analysis agree well with those by Monte Carlo calculation. Furthermore, the Monte Carlo simulation confirms the results of a previous computer simulation study for correspondence between experimental and theoretical electron drift velocities; that is, the drift velocity deduced from Schlumbohm's experiment (1965) assumes a value close to but slightly larger than Wm and the drift velocity deduced from Frommhold's experiment (1959) assumes a value represented by (Wr+Wm)/2, where Wm and Wr are the mean arrival time and the centre-of-mass electron drift velocities, respectively.

A method for calculating the electron swarm parameters in the transverse direction to the electric field by employing a Fourier-transformed Boltzmann equation is presented. In the method, the typical transverse swarm parameter, the transverse diffusion coefficient DT, is derived from the dispersion relation of the eigenvalue of the equation. The method is applied to methane (CH4) and sulphur hexafluoride (SF6). The results are compared with those from the conventional methods and it is shown that the present method is a reliable means to deduce the transverse coefficient DT. The lateral development of the electron swarms for the model of the steady-state Townsend experiment is also discussed.

A new method to calculate the electron swarm parameters by employing a Fourier transformed Boltzmann equation is presented in which the swarm parameters are derived from the dispersion relation of the equation. The method is applied to krypton and sulphur hexafluoride(SF6) gases. The results are compared with those obtained by conventional methods and the practical features of the present method are discussed. It is shown that the method is a reliable means to deduce the electron swarm parameters.

An accurate and efficient method for solving the Boltzmann equation is presented. Using a multi-term expansion technique, the time-of-flight (TOF) electron swarm parameters (such as the centre-of-mass drift velocity Wr, the longitudinal DL and transverse DT diffusion coefficients) can be evaluated. The method utilizes an amalgamation of the conventional two-term expansion and the Galerkin method. The first two terms of the Legendre polynomial expansion of the electron energy distribution are obtained by the two-term method, while the third-order and higher-order terms are deduced by the Galerkin method. The present technique is applied to determine the electron energy distribution using Fourier components to solve the Boltzmann equation and TOF swarm parameters in methane gas. Good agreement with Monte Carlo simulation is obtained and the method is also applied to a previously described model gas.

A method of analysis for the swarm parameters deduced directly from the arrival time spectra (ATS) of an electron swarm in gases is presented. In this method the electron swarm parameters, defined as the space derivatives of the moments of the arrival time of the electron density distribution, are calculated using the new continuity equation derived from the Boltzmann equation, for the first time in a real gas. The method is applied to deduce the electron energy distributions and swarm parameters in krypton gas at E/p0=50, 100 and 150 V cm-1 Torr-1, utilising the conventional two-term expansion for solution of the Boltzmann equation, and good agreement with the result by Monte Carlo simulation is obtained.

The electron swarm parameters in HSi(OC2H5)3 (triethoxysilane, TRIES) vapor have been investigated for relatively wide ranges of reduced electric field (E/N). Based on the arrival-time spectra (ATS) method for electrons using a double-shutter drift tube, the drift velocity and the longitudinal diffusion coefficient were measured for the E/N=20–5000 Td, and the ionization coefficient was obtained for E/N=300–5000 Td. The results were compared with those for SiH4 and Si(OC2H5)4 (tetraethoxysilane, TEOS), to show characteristics similar to the parameters in TEOS. We also determined the electron collision cross sections for TRIES by means of the Boltzmann equation analysis.

We discuss our experience improving the speed of Monte Carlo dose distribution calculations with GEANT4. First we customized the core geometry library to navigate voxelized CT volumes more efficiently. This resulted in a speed improvement of more than two orders of magnitude in our case. We then tried to run the simulation in parallel. However, we found that existing parallelizations of GEANT4 such as TOP-C (Task Oriented Parallel C/C++) did not scale well. We determined that the poor performance was due to excessive network traffic. We abandoned the master/slave model employed in TOP-C in favour of "static scheduling" parallelism where the event list is divided evenly among the processes. Our implementation used only a few low-level MPI calls. With this method, we achieved a linear speed improvement on our cluster of 23 machines. The simulation executes efficiently because there is almost no network traffic during the simulation. Using our customized GEANT4 navigation library running on our cluster with our parallelization method, we achieved a combined speed improvement factor of more than 3000, reducing our simulation time from several days to a few minutes. Although we did not consider issues such as fault tolerance or load sharing, our approach is well suited for many kinds of Monte Carlo simulations on a homogenous cluster of local PCs.

A one-dimensional nonuniform electric field model between parallel plane electrodes is used to study the behavior of electron swarms in spatially distorted electric fields such as that in the dc-sheath region near the anode. We investigate electron kinetics in the nonuniform field, taking into account the α-parameter associated with the ionization and the drift processes of electrons. The general expression of the α-parameter in the Boltzmann equation analysis using the arrival-time spectra (ATS) method is presented. Then, a demonstration of the swarm behavior is carried out using a Monte Carlo simulation to examine the validity of the parameter description.

Titanium nitride (TiN) films have been deposited on silicon substrates at low substrate temperature (400–550°C) by 50 Hz plasma-enhanced chemical vapor deposition (CVD) using a TiCl4+N2+H2 gas mixture. A substrate bias circuit with two diodes was used to enhance the deposition through ion processes. The Vickers hardness values of the deposited TiN films at T sub ranging from 450 to 550°C were above 2200 Hv, and resistivity was below 100 µΩ·cm at T sub≧500°C. The composition of the deposited films was investigated by the Rutherford backscattering spectroscopy (RBS) method to determine that the at.% value of the Cl content [=Cl/(Ti+N+O+C+Cl)] in the TiN films is about 1% at T sub=550°C. X-ray diffraction (XRD) spectra revealed that the film has the preferred orientation of δ-TiN with (200) and (220) under the deposition condition of T sub≧500°C.

A set of electron gas-atom collision cross sections of krypton, giving good agreement between calculated and experimental values of drift velocity and Townsend first ionisation coefficient is obtained for the electron energy range 1< epsilon