Development and implementation in the Monte Carlo code PENELOPE of a new virtual source model for radiotherapy photon beams and portal image calculation.

This work aims at developing a generic virtual source model (VSM) preserving all existing correlations between variables stored in a Monte Carlo pre-computed phase space (PS) file, for dose calculation and high-resolution portal image prediction. The reference PS file was calculated using the PENELOPE code, after the flattening filter (FF) of an Elekta Synergy 6 MV photon beam. Each particle was represented in a mobile coordinate system by its radial position (r s ) in the PS plane, its energy (E), and its polar and azimuthal angles (φ d and θ d ), describing the particle deviation compared to its initial direction after bremsstrahlung, and the deviation orientation. Three sub-sources were created by sorting out particles according to their last interaction location (target, primary collimator or FF). For each sub-source, 4D correlated-histograms were built by storing E, r s , φ d and θ d values. Five different adaptive binning schemes were studied to construct 4D histograms of the VSMs, to ensure histogram efficient handling as well as an accurate reproduction of E, r s , φ d and θ d distribution details. The five resulting VSMs were then implemented in PENELOPE. Their accuracy was first assessed in the PS plane, by comparing E, r s , φ d and θ d distributions with those obtained from the reference PS file. Second, dose distributions computed in water, using the VSMs and the reference PS file located below the FF, and also after collimation in both water and heterogeneous phantom, were compared using a 1.5%-0 mm and a 2%-0 mm global gamma index, respectively. Finally, portal images were calculated without and with phantoms in the beam. The model was then evaluated using a 1%-0 mm global gamma index. Performance of a mono-source VSM was also investigated and led, as with the multi-source model, to excellent results when combined with an adaptive binning scheme.

A new single crystal diamond dosimeter for small beam: comparison with different commercial active detectors.

Recent developments of new therapy techniques using small photon beams, such as stereotactic radiotherapy, require suitable detectors to determine the delivered dose with a high accuracy. The dosimeter has to be as close as possible to tissue equivalence and to exhibit a small detection volume compared to the size of the irradiation field, because of the lack of lateral electronic equilibrium in small beam. Characteristics of single crystal diamond (tissue equivalent material Z = 6, high density) make it an ideal candidate to fulfil most of small beam dosimetry requirements. A commercially available Element Six electronic grade synthetic diamond was used to develop a single crystal diamond dosimeter (SCDDo) with a small detection volume (0.165 mm(3)). Long term stability was studied by irradiating the SCDDo in a (60)Co beam over 14 h. A good stability (deviation less than ± 0.1%) was observed. Repeatability, dose linearity, dose rate dependence and energy dependence were studied in a 10 × 10 cm(2) beam produced by a Varian Clinac 2100 C linear accelerator. SCDDo lateral dose profile, depth dose curve and output factor (OF) measurements were performed for small photon beams with a micro multileaf collimator m3 (BrainLab) attached to the linac. This study is focused on the comparison of SCDDo measurements to those obtained with different commercially available active detectors: an unshielded silicon diode (PTW 60017), a shielded silicon diode (Sun Nuclear EDGE), a PinPoint ionization chamber (PTW 31014) and two natural diamond detectors (PTW 60003). SCDDo presents an excellent spatial resolution for dose profile measurements, due to its small detection volume. Low energy dependence (variation of 1.2% between 6 and 18 MV photon beam) and low dose rate dependence of the SCDDo (variation of 1% between 0.53 and 2.64 Gy min(-1)) are obtained, explaining the good agreement between the SCDDo and the efficient unshielded diode (PTW 60017) in depth dose curve measurements. For field sizes ranging from 0.6 × 0.6 to 10 × 10 cm(2), OFs obtained with the SCDDo are between the OFs measured with the PinPoint ionization chamber and the Sun Nuclear EDGE diode that are known to respectively underestimate and overestimate OF values in small beam, due to the large detection volume of the chamber and the non-water equivalence of both detectors.

Assessment of small volume ionization chambers as reference dosimeters in high-energy photon beams.

LNE-LNHB is involved in a European project aiming at establishing absorbed dose-to-water standards for photon-radiation fields down to 2 × 2 cm². This requires the calibration of reference ionization chambers of small volume. Twenty-four ionization chambers of eight different types with volume ranging from 0.007 to 0.057 cm³ were tested in a 6°Co beam. For each chamber, two major characteristics were investigated: (1) the stability of the measured current as a function of the irradiation time under continuous irradiation. At LNE-LNHB, the variation of the current should be less than ±0.1% in comparison with its first value (over a 16 h irradiation time); (2) the variation of the ionization current with the applied polarizing voltage and polarity. Leakage currents were also measured. Results show that (1) every tested PTW (31015, 31016 and 31014) and Exradin A1SL chambers demonstrate a satisfying stability under irradiation. Other types of chambers have a stability complying with the stability criterion for some or none of them. (2) IBA CC01, IBA CC04 and Exradin A1SL show a proper response as a function of applied voltage for both polarities. PTW, Exradin A14SL and Exradin A16 do not. Only three types of chambers were deemed suitable as reference chambers according to LNE-LNHB requirements and specifications from McEwen (2010 Med. Phys. 37 2179-93): Exradin A1SL chambers (3/3), IBA CC04 (2/3) and IBA CC01 (1/3). The Exradin A1SL type with an applied polarizing voltage of 150 V was chosen as an LNE-LNHB reference chamber type in 2 × 2 cm² radiation fields.

Analytical and Monte Carlo assessment of activity and local dose after a wound contamination by activation products.

The activity and local dose following a right index finger wound contamination by activation products are assessed. Measurements with a high purity germanium detector and a four positions measurement protocol enabled a better localization of the contaminant source. From the source location and detector calibration, the remaining wound activity and local absorbed dose were deduced. An analytical model, based on a two dimensional simplification of the problem, is presented. It is shown to provide a fast and quite accurate activity assessment when the contaminants are described as a point source. The contaminants' location and activity were then more accurately assessed using Monte Carlo calculations based on the OEDIPE software and a voxelized phantom of the index finger. Describing the contaminant mixture as a point source resulted in an agreement of experimental and computed data around 6% for most of the radionuclides. The total activity, due to 11 radionuclides, was estimated to be (9.5 +/- 0.4) kBq at measurement day. Since the point source is found to be less than 1 mm under the skin, the equivalent skin dose is calculated and found to be around 680 mSv in the first year after the contamination, and this value decreases to 250 mSv in the second year. The relevance of equivalent skin dose as an estimate of the sanitary risk is discussed, and it is concluded that for this case it gives the upper end estimate of the risk.

SESAME: a software tool for the numerical dosimetric reconstruction of radiological accidents involving external sources and its application to the accident in Chile in December 2005.

Estimating the dose distribution in a victim's body is a relevant indicator in assessing biological damage from exposure in the event of a radiological accident caused by an external source. This dose distribution can be assessed by physical dosimetric reconstruction methods. Physical dosimetric reconstruction can be achieved using experimental or numerical techniques. This article presents the laboratory-developed SESAME--Simulation of External Source Accident with MEdical images--tool specific to dosimetric reconstruction of radiological accidents through numerical simulations which combine voxel geometry and the radiation-material interaction MCNP(X) Monte Carlo computer code. The experimental validation of the tool using a photon field and its application to a radiological accident in Chile in December 2005 are also described.

Multichannel dosemeter and Al2O3:C optically stimulated luminescence fibre sensors for use in radiation therapy: evaluation with electron beams.

This article proposes an innovative multichannel optically stimulated luminescence (OSL) dosemeter for on-line in vivo dose verification in radiation therapy. OSL fibre sensors incorporating small Al(2)O(3):C fibre crystals (TLD(500)) have been tested with an X-ray generator. A reproducible readout procedure should reduce the fading-induced uncertainty ( approximately - 1% per decade). OSL readouts are temperature-dependent [ approximately 0.3% K(-1) when OSL stimulation is performed at the same temperature as irradiation; approximately 0.16% K(-1) after thermalisation (20 degrees C)]. Sensor calibration and depth-dose measurements with electron beams have been performed with a Saturne 43 linear accelerator in reference conditions at CEA-LNHB (ionising radiation reference laboratory in France). Predosed OSL sensors show a good repeatability in multichannel operation and independence versus electron energy in the range (9, 18 MeV). The difference between absorbed doses measured by OSL and an ionisation chamber were within +/-0.9% (for a dose of about 1 Gy) despite a sublinear calibration curve.

Analysis of the CONRAD computational problems expressing only stochastic uncertainties: neutrons and protons.

Within the scope of CONRAD (A Coordinated Action for Radiation Dosimetry) Work Package 4 on Computational Dosimetry jointly collaborated with the other research actions on internal dosimetry, complex mixed radiation fields at workplaces and medical staff dosimetry. Besides these collaborative actions, WP4 promoted an international comparison on eight problems with their associated experimental data. A first set of three problems, the results of which are herewith summarised, dealt only with the expression of the stochastic uncertainties of the results: the analysis of the response function of a proton recoil telescope detector, the study of a Bonner sphere neutron spectrometer and the analysis of the neutron spectrum and dosimetric quantity H(p)(10) in a thermal neutron facility operated by IRSN Cadarache (the SIGMA facility). A second paper will summarise the results of the other five problems which dealt with the full uncertainty budget estimate. A third paper will present the results of a comparison on in vivo measurements of the (241)Am bone-seeker nuclide distributed in the knee. All the detailed papers will be presented in the WP4 Final Workshop Proceedings.

Analysis of a computational problem involving complex voxel geometries.

This communication briefly summarises the results obtained from the 'International comparison on MC modeling for in vivo measurement of Americium in a knee phantom' organised within the EU Coordination Action CONRAD (Coordinated Network for Radiation Dosimetry) as a joint initiative of EURADOS working groups 6 (computational dosimetry) and 7 (internal dosimetry). Monte Carlo simulations using the knee voxel phantom proved to be a viable approach to provide the calibration factor needed for in vivo measurements.

OEDIPE: a new graphical user interface for fast construction of numerical phantoms and MCNP calculations.

Although great efforts have been made to improve the physical phantoms used to calibrate in vivo measurement systems, these phantoms represent a single average counting geometry and usually contain a uniform distribution of the radionuclide over the tissue substitute. As a matter of fact, significant corrections must be made to phantom-based calibration factors in order to obtain absolute calibration efficiencies applicable to a given individual. The importance of these corrections is particularly crucial when considering in vivo measurements of low energy photons emitted by radionuclides deposited in the lung such as actinides. Thus, it was desirable to develop a method for calibrating in vivo measurement systems that is more sensitive to these types of variability. Previous works have demonstrated the possibility of such a calibration using the Monte Carlo technique. Our research programme extended such investigations to the reconstruction of numerical anthropomorphic phantoms based on personal physiological data obtained by computed tomography. New procedures based on a new graphical user interface (GUI) for development of computational phantoms for Monte Carlo calculations and data analysis are being developed to take advantage of recent progress in image-processing codes. This paper presents the principal features of this new GUI. Results of calculations and comparison with experimental data are also presented and discussed in this work.

Monte Carlo modelling for in vivo measurements of Americium in a knee voxel phantom: general criteria for an international comparison.

The general criteria and the scientific approach adopted for an 'International comparison on Monte Carlo modelling for in vivo measurement of Americium in a knee phantom' that is being organised within the EU Coordination Action CONRAD (Coordinated Network for Radiation Dosimetry) are described her. Detection system and a knee voxel phantom based on a computerised axial tomography of the Spitz anthropometric knee phantom with a homogeneous distribution of 241Am in bone have been considered for the simulation of three specific situations: (a) a single Low Energy Germanium detector for a point 241Am source in air; (b) the calculation of photon fluence spectra in air around the voxel phantom; and (c) the calculation of the energy distribution of pulses and peak detection efficiency in the real detection system geometry.

Coordination of research on internal dosimetry in Europe: the CONRAD project.

The EUropean RAdiation DOSimetry Group (EURADOS) initiated in 2005 the CONRAD Project, a Coordinated Network for Radiation Dosimetry funded by the European Commission (EC), within the 6th Framework Programme (FP). The main purpose of CONRAD is to generate a European Network in the field of Radiation Dosimetry and to promote both research activities and dissemination of knowledge. The objective of CONRAD Work Package 5 (WP5) is the coordination of research on assessment and evaluation of internal exposures. Nineteen institutes from 14 countries participate in this action. Some of the activities to be developed are continuations of former European projects supported by the EC in the 5th FP (OMINEX and IDEAS). Other tasks are linked with ICRP activities, and there are new actions never considered before. A collaboration is established with CONRAD Work Package 4, dealing with Computational Dosimetry, to organise an intercomparison on Monte Carlo modelling for in vivo measurements of (241)Am deposited in a knee phantom. Preliminary results associated with CONRAD WP5 tasks are presented here.

Automatic application of ICRP biokinetic models in voxel phantoms for in vivo counting and internal dose assessment.

As part of the improvement of calibration techniques of in vivo counting, the Laboratory of Internal Dose Assessment of the Institute of Radiological Protection and Nuclear Safety has developed a computer tool, 'OEDIPE', to model internal contamination, to simulate in vivo counting and to calculate internal dose. The first version of this software could model sources located in a single organ. As the distribution of the contamination evolves from the time of intake according to the biokinetics of the radionuclide, a new facility has been added to the software first to allow complex heterogeneous source modelling and then to automatically integrate the distribution of the contamination in the different tissues estimated by biokinetic calculation at any time since the intake. These new developments give the opportunity to study the influence of the biokinetics on the in vivo counting, leading to a better assessment of the calibration factors and the corresponding uncertainties.

Determination of new European biometric equations for the calibration of in vivo lung counting systems using the Livermore phantom.

New biometric equations used for assessing the thickness of the overlay plate to be added to the physical phantom were determined based on the Computed Tomography (CT) chest images of 33 adult males in order to improve the calibration of in vivo lung counting systems using the Livermore phantom. These equations are specific to systems composed of four germanium detectors with the measured subject in supine position. A comparison with the biometric equations used to date as reference in France was carried out and proved the usefulness of equations directly applicable to the Livermore phantom.

Potential of modern technologies for improvement of in vivo calibration.

In the frame of IDEA project, a research programme has been carried out to study the potential of the reconstruction of numerical anthropomorphic phantoms based on personal physiological data obtained by computed tomography (CT) and Magnetic Resonance Imaging (MRI) for calibration in in vivo monitoring. As a result, new procedures have been developed taking advantage of recent progress in image processing codes that allow, after scanning and rapidly reconstructing a realistic voxel phantom, to convert the whole measurement geometry into computer file to be used on line for MCNP (Monte Carlo N-Particule code) calculations. The present paper overviews the major abilities of the OEDIPE software studies made in the frame of the IDEA project, on the examples of calibration for lung monitoring as well as whole body counting of a real patient.

Implementation of bioassay methods to improve assessment of incorporated radionuclides.

The present work which was carried out in the framework of an EU project (IDEA: Internal Dosimetry-Enhancements in Application; Contract Number: FIKR CT2001 00164) shall provide commonly acceptable guidelines for optimum performance of ICP-MS measurements with focus on urinary measurements of uranium, thorium and actinides. From the results of this work it is recommended that, whenever feasible, 24 h urine sampling should be conducted to avoid large uncertainties in the quantitation of daily urinary excretion values. For storage, urine samples should be acidified and kept frozen before analysis. Measurement of total uranium in urine by ICP-MS at physiological levels (<10 ng.l(-1)) requires no sample preparation besides UV photolysis and/or dilution. For the measurement of thorium in urine by ICP-MS, it can be concluded, that salt removal from the urine samples is not recommended. For the measurement of actinides in urine it is shown that ICP-MS is well-suited and a good alternative to alpha-spectrometry for isotopes with T1/2>5x10(4) years. In general, ICP-MS measurements are an easy, fast and cost-saving methodology. New improved measuring techniques (HR-SF-ICP-MS) with detection limits in urine of 150 pg.l(-1) (1.9 microBq.l(-1)) for 238U, 30 pg.l(-1) (2.4 microBq.l(-1)) for 235U and 100 pg.l(-1) (0.4 microBq.l(-1)) for (232)Th, respectively, meet all necessary requirements. This method should therefore become the routine technique for incorporation monitoring of workers and of members of the general public, in particular for uranium contamination.

Practical implications of procedures developed in IDEA project--comparison with traditional methods.

The idea of the IDEA project aimed to improve assessment of incorporated radionuclides through developments of more reliable and possibly faster in vivo and bioassay monitoring techniques and making use of such enhancements for improvements in routine monitoring. In direct in vivo monitoring technique the optimum choice of the detectors to be applied for different monitoring tasks has been investigated in terms of material, size and background in order to improve conditions namely to increase counting efficiency and reduce background. Detailed studies have been performed to investigate the manifold advantageous applications and capabilities of numerical simulation method for the calibration and optimisation of in vivo counting systems. This calibration method can be advantageously applied especially in the measurement of low-energy photon emitting radionuclides, where individual variability is a significant source of uncertainty. In bioassay measurements the use of inductively coupled plasma mass spectrometry (ICP-MS) can improve considerably both the measurement speed and the lower limit of detection currently achievable with alpha spectrometry for long-lived radionuclides. The work carried out in this project provided detailed guidelines for optimum performance of the technique of ICP-MS applied mainly for the determination of uranium and thorium nuclides in the urine including sampling procedure, operational parameters of the instruments and interpretation of the measured data. The paper demonstrates the main advantages of investigated techniques in comparison with the performances of methods commonly applied in routine monitoring practice.

Application of voxel phantoms in whole-body counting for the validation of calibration phantoms and the assessment of uncertainties.

This article is dedicated to the application of voxel phantoms in whole-body counting calibration. The first study was performed to validate this approach using IGOR, a physical phantom dedicated to fission and activation product (FAP) measurement, and a graphical user interface, developed at the IRSN internal dose assessment laboratory, called OEDIPE (French acronym for the tool for personalised internal dose assessment) associated with the Monte Carlo code MCNP. The method was validated by comparing the results of real measurements and simulations using voxel phantoms obtained from CT scan images of IGOR. To take this application further, two studies were carried out and are presented in this article. First, a comparison was made between the IGOR voxel based phantom (IGOVOX) and a voxel human body (Zubal Phantom) to confirm whether IGOR could be considered as a realistic representation of a human. Second, the errors made when considering sources homogeneously distributed in the body were assessed against real contamination by taking into account the biokinetic behaviour of the radioactive material for two modes of exposure: the ingestion of 137Cs in soluble form and the inhalation of insoluble 60Co several days after acute incorporation.

Current developments at IRSN on computational tools dedicated to assessing doses for both internal and external exposure.

The paper presents the OEDIPE (French acronym that stands for tool for personalised internal dose assessment) and SESAME (for simulation of external source accident with medical images) computational tools, dedicated to internal and external dose assessment, respectively, and currently being developed at the Institute for Radiological Protection and Nuclear Safety. The originality of OEDIPE and SESAME, by using voxel phantoms in association with Monte Carlo codes, lies in their ability to construct personalised voxel phantoms from medical images and automatically generate the Monte Carlo input file and visualise the expected results. OEDIPE simulates in vivo measurements to improve their calibration, and calculates the dose distribution taking both internal contamination and internal radiotherapy cases into account. SESAME enables radiological overexposure doses to be reconstructed, as also victim, source and accident environment modelling. The paper presents the principles on which these tools function and an overview of specificities and results linked to their fields of application.

New method of voxel phantom creation: application for whole-body counting calibration and perspectives in individual internal dose assessment.

The purpose of this work is to present an innovative approach for the creation and application of voxel phantoms associated with the Monte Carlo calculation (MCNP) for the calibration of whole-body counting systems dedicated to the measurement of fission and activation products. The new method is based on a graphical user interface called 'OEDIPE' that allows to simulate a whole measurement process using all measurement parameters, the final goal being to approach a numerical calibration of the facilities. The creation of voxel phantoms and validation of the method are presented in this paper using the IGOR phantom. Finally, the efficiency of the method is discussed, in particular, with the perspective of validating IGOR as a suitable human-equivalent phantom and for the assessment of uncertainties in dose estimation due to the inhomogeneous distribution of activity in the body, correlated to the bio-kinetic behaviour of the radionuclides.

Dosimetric comparison of Monte Carlo codes (EGS4, MCNP, MCNPX) considering external and internal exposures of the Zubal phantom to electron and photon sources.

This paper aims at comparing dosimetric assessments performed with three Monte Carlo codes: EGS4, MCNP4c2 and MCNPX2.5e, using a realistic voxel phantom, namely the Zubal phantom, in two configurations of exposure. The first one deals with an external irradiation corresponding to the example of a radiological accident. The results are obtained using the EGS4 and the MCNP4c2 codes and expressed in terms of the mean absorbed dose (in Gy per source particle) for brain, lungs, liver and spleen. The second one deals with an internal exposure corresponding to the treatment of a medullary thyroid cancer by 131I-labelled radiopharmaceutical. The results are obtained by EGS4 and MCNPX2.5e and compared in terms of S-values (expressed in mGy per kBq and per hour) for liver, kidney, whole body and thyroid. The results of these two studies are presented and differences between the codes are analysed and discussed.