Search

Gafchromic™ films have become popular due to their ease of use and their near water equivalence. This last property is crucial for stereotactic small beam dosimetry as demonstrated in recent papers. An accurate bi-dimensional dose measurement with Gafchromic™ films is very challenging mainly because of the non-uniformity response of flatbed scanners (used for films digitalization) and their own non-uniformity. The first proposal of this work is to develop bi-dimensional protocol for small beams and evaluate the associated uncertainty. The second proposal is to validate this protocol for the bi-dimensional measurements of treatment plans performed with the CyberKnife ® system. First, the uniformity of an Epson V700 flatbed scanner and a batch of EBT3 Gafchromic™ films has been investigated. A “four films” dosimeter was designed to reduce the errors (statistic and systematic) due to their non-uniformity. Then, the “four films” dosimeter protocol in both a homogeneous (RW3 material) and heterogeneous (RW3, lung-like and bone-like materials) phantoms has been used to measure the bi-dimensional dose distributions of three simple CyberKnife ® treatment plans. Two tumor locations (middle of the lung and near lung/bone interface) were considered for the heterogeneous phantom. These plans were achieved with the 10 mm fixed collimator and modeled with the PENELOPE Monte Carlo code in order to calculate accurate dose distributions. Finally, the “four films” bi-dimensional dose distributions were compared to the PENELOPE Monte Carlo simulations. Regarding the uncertainty associated to the bi-dimensional dose measurement protocol, the relative standard deviation σ D on the dose was 1.2% in the range from 0.5 to 4.0 Gy. Regarding the protocol validation on CyberKnife ® treatment plans, a very good agreement was found with all measurement points passing the {3% - 3 mm} Gamma Index criteria. [ABSTRACT FROM AUTHOR]

Radiochromic film dosimetry is a promising technique, but at this time there are some artefacts, including non-uniformity, energy dependence and scanner artefacts. Accurate dosimetry with radiochromic films requires characterizing the film as well as the scanning and analysis procedures. In this work, the performance of the EBT3 films in combination with the EPSON Dual Lens Perfection V700 scanner for dose distribution measurements in stereotactic radiotherapy has been evaluated. It has been shown that it was necessary to perform 20 blank scans to obtain the stability of the scanner. In order to reduce the uncertainties due to the non-uniformity of the scan field, it was then decided to use the 12 × 12 cm 2 central part of the scanner bed. Regarding EBT3 films, intra-sheet and inter-sheet uniformity of unexposed EBT3 films in terms of pixel value has been found to be respectively 0.27% (1 SD) and 0.15% (1 SD). No significant energy dependence has been observed above 30 keV and no angular dependence has been found. [ABSTRACT FROM AUTHOR]

A new formalism for small field dosimetry has been proposed ( Alfonso et al., 2008 ) with the concept of an additional correction factor ( k Q clin , Q msr f clin , f msr ) which accounts for possible changes in detector response with field size. The aim of this work was to evaluate the response of eight commercially available detectors, then to provide a set of correction factors for a 1000 MU/min CyberKnife ® equipped with fixed collimators and to compare them with those obtained for the 800 MU/min CyberKnife ® version. Measurements were performed on a 1000 MU/min CyberKnife ® with several active detectors designed for small field dosimetry (two chambers (PTW 31014 and 31018), three high resolution diodes (PTW 60016, 60017 and Sun Nuclear EDGE), a natural diamond (PTW 60003)) and two passive dosimeters (Harshaw TLD-700 7 LiF:Mg,Ti thermoluminescent micro-cube and EBT3 radiochromic films). The CyberKnife ® as well as the diode detectors, the PinPoint chamber, the diamond and the LiF micro-cubes were modeled with the PENELOPE Monte Carlo code in order to calculate the output factors in a point-like voxel of water (OF MC,w ). A set of k Q clin , Q msr f clin , f msr correction factors for the active detectors investigated is provided for the 1000 MU/min CyberKnife ® in order to be used with the new formalism. A difference up to 2.4%, 2.0 and 1.7% in the correction factor obtained for the two different CyberKnife ® models is found for the PTW 60003, the PTW 60016 and the PTW 60017 respectively. Although this difference is small, we recommend using specific k Q clin , Q msr f clin , f msr correction factors for the 1000 MU/min CyberKnife ® when they are available. [ABSTRACT FROM AUTHOR]

Purpose: In a previous work, output ratio (ORdet) measurements were performed for the 800 MU/min CyberKnife ® at the Oscar Lambret Center (COL, France) using several commercially available detectors as well as using two passive dosimeters (EBT2 radiochromic film and micro-LiF TLD-700). The primary aim of the present work was to determine by Monte Carlo calculations the output factor in water (OFMC,w) and the kQclin,Qmsrfclin,fmsr correction factors. The secondary aim was to study the detector response in small beams using Monte Carlo simulation. Methods: The LINAC head of the CyberKnife R ® was modeled using the PENELOPE Monte Carlo code system. The primary electron beam was modeled using a monoenergetic source with a radial gaussian distribution. The model was adjusted by comparisons between calculated and measured lateral profiles and tissue-phantom ratios obtained with the largest field. In addition, the PTW 60016 and 60017 diodes, PTW 60003 diamond, and micro-LiF were modeled. Output ratios with modeled detectors (ORMC,det) and OFMC,w were calculated and compared to measurements, in order to validate the model for smallest fields and to calculate kQclin,Qmsrfclin,fmsr correction factors, respectively. For the study of the influence of detector characteristics on their response in small beams; first, the impact of the atomic composition and the mass density of silicon, LiF, and diamond materials were investigated; second, the material, the volume averaging, and the coating effects of detecting material on the detector responses were estimated. Finally, the influence of the size of silicon chip on diode response was investigated. Results: Looking at measurement ratios (uncorrected output factors) compared to the OFMC,w, the PTW 60016, 60017 and Sun Nuclear EDGE diodes systematically over-responded (about +6% for the 5 mm field), whereas the PTW 31014 Pinpoint chamber systematically under-responded (about -12% for the 5 mm field). ORdet measured with the SFD diode and PTW 60003 diamond detectors were in good agreement with OFMC,w except for the 5 mm field size (about -7.5% for the diamond and +3% for the SFD). A good agreement with OFMC,w was obtained with the EBT2 film and micro- LiF dosimeters (deviation less than 1.4% for all fields investigated). kQclin,Qmsrfclin,fmsrcorrection factors for several detectors used in this work have been calculated. The impact of atomic composition on the dosimetric response of detectors was found to be insignificant, unlike the mass density and size of the detecting material. Conclusions: The results obtained with the passive dosimeters showed that they can be used for small beam OF measurements without correction factors. The study of detector response showed that ORdet is depending on the mass density, the volume averaging, and the coating effects of the detecting material. Each effect was quantified for the PTW 60016 and 60017 diodes, the micro- LiF, and the PTW 60003 diamond detectors. None of the active detectors used in this work can be recommended as a reference for small field dosimetry, but an improved diode detector with a smaller silicon chip coated with tissue-equivalent material is anticipated (by simulation) to be a reliable small field dosimetric detector in a nonequilibrium field. [ABSTRACT FROM AUTHOR]

Abstract: The accurate determination of absorbed dose in small photon beams, especially for stereotactic radiation therapy, is a difficult task with commercially available detectors. As these small fields are characterized by high dose gradients, a lack of lateral particle equilibrium and a variation of energy spectra with beam sizes, a dosimeter with high resolution, tissue-equivalence and high precision is required. The new radiochromic film EBT2, which meets these criteria, was fully characterized in Institut de Radioprotection et Sûreté Nucléaire (IRSN) for this application. This type of film was tested with the reading system EPSON Dual Lens Perfection V700 flatbed scanner in transmission mode. Warm-up effects of the scanner were studied as well as the influence of the scanner light. Uniformity of unirradiated and irradiated EBT2 films in terms of pixel value was found to be respectively 0.3% (1 SD) and 0.5% (1 SD). An original, accurate and efficient radiochromic film dosimetry protocol was established. The overall uncertainty for dose measurement with EBT2 films using this protocol was estimated at less than 2% (1 SD). Encouraging measurements of output factors were performed on a Novalis system. [Copyright &y& Elsevier]

Abstract: The dosimetry of the experimental reactor CALIBAN was established for photon and neutron components, and the kerma variations were evaluated according to position (distance from the core, height, angle with the median room axis). Dosimetry was performed with TLD (Al2O3) for the photon component, and with passive silicon diodes, activation detectors (Au, In, Ni, Mg, Cu) and alanine pellets measured by EPR spectrometry for the neutron component. The neutron energy distribution was experimentally evaluated at various distances based on the activities measured at the activated foil using the SNAC2 software. It was then compared with those calculated with the Monte Carlo code MCNPX. [Copyright &y& Elsevier]

Abstract: Following a radiological accident on 11 March 2006 in an industrial irradiation facility involving one victim, a physical dosimetry was reconstructed using on site measurements and numerical simulations in order to estimate as accurately as possible and as quickly as possible the dose received and its distribution throughout the organism so that doctors may fine tune their diagnosis and prescribe the most suitable treatment. The data compiled from the experimental reconstruction resulted in a first estimation of the distribution of the dose in the victim''s organism. The numerical simulations indicated a dose gradient corresponding to a factor of 2 in anteroposterior and between the pelvis and the skull. [Copyright &y& Elsevier]

Abstract: A severe irradiation accident, due to gammagraphy equipment containing an 192Ir radioactive source, involving several victims occurred from the 3 June to 3 August 2006 in two African subsidiaries of the French group Bureau Veritas. This article retraces the circumstances of the accident and the physical dosimetric reconstruction made by IRSN on four patients treated in France. The dose reconstruction was based on numerical simulations and electron paramagnetic resonance (EPR) measurements performed on bone tissue and tooth enamel samples. The data obtained by physical dosimetry, as complementary elements to biological dosimetry and clinical observations, were helpful for the treatment strategy implemented by the medical team. [Copyright &y& Elsevier]

According to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 1993), the dose due to the inhalation of radon decay products represents almost 50% of the total natural radiation dose to the general population. The scientific community is interested in the assessment of the risk induced by domestic radon exposure. The dosimetric models used to estimate the dose are very sensitive to unattached fraction and size distributions, which makes the characterization of the indoor radon decay products aerosol necessary. For this purpose, longterm measurements of unattached fraction (f[sub p]) and equilibrium factor (F) were taken in a dwelling under typical indoor domestic aerosol conditions. An original device consisting of an annular diffusion channel set in parallel with an open filter was developed and calibrated to continuously measure the unattached fraction. Moreover, radon activity concentration and particle concentration were simultaneously monitored. With aged aerosol, particle concentration was found to be very low (between 500 and 5000 cm[sup -3]), radon activity concentration ranged from 240 to 2800 Bq m[sup -3], and the mean values of f[sub p] and F were, respectively, 0.31 (0.08-0.67) and 0.16 (0.04-0.45). With aerosol sources, the high increase in particle concentration led to a negligible unattached fraction and raised the equilibrium factor. A correlation relationship was determined between these two parameters under different aerosol conditions. Finally,our experimental results were compared to results obtained with the PRADDO model; this comparison showed a good agreement between these two different approaches. [ABSTRACT FROM AUTHOR]

An intercomparison exercise is described that examines Monte Carlo modelling of anthropomorphic voxel phantoms in an idealized ground-contamination photon exposure scenario. Thirteen participants calculated and submitted organ and effective dose rates for comparison against a set of verified reference solutions. The effective dose rates are shown to agree with the reference value to within reasonable statistical uncertainties in five of the cases, though in only one of those was similar agreement also demonstrated in the evaluation of all requested organ dose rates. Orders-of-magnitude differences in doses are seen for some of the other participants, both internally within their own dataset and also relative to the reference solutions. Following limited feedback and suggestions from the organizer, up to two sets of revised solutions were resubmitted by some of the participants; these generally exhibited improved agreement, though not always. The overall observations and conclusions from this intercomparison exercise are summarized and discussed. • Presentation of an intercomparison exercise on Monte Carlo modelling of anthropomorphic voxel phantoms. • An idealized ground-contamination photon exposure scenario was proposed. • Thirteen participants submitted organ and effective dose rates for comparison against verified reference solutions. • Effective dose rates appeared to agree with the reference value in five of the cases. • In only one of those cases was similar agreement also demonstrated in the evaluation of all requested organ dose rates. • Orders-of-magnitude differences in doses were seen for some of the participants. • The overall observations and conclusions from the exercise are summarized and discussed. [ABSTRACT FROM AUTHOR]

Purpose: The use of small photon fields is now an established practice in stereotactic radiosurgery and radiotherapy. However, due to a lack of lateral electron equilibrium and high dose gradients, it is difficult to accurately measure the dosimetric quantities required for the commissioning of such systems. Moreover, there is still no metrological dosimetric reference for this kind of beam today. In this context, the first objective of this work was to determine and to compare small fields output factors (OF) measured with different types of active detectors and passive dosimeters for three types of facilities: a CyberKnife® system, a dedicated medical linear accelerator (Novalis) equipped with m3 microMLC and circular cones, and an adaptive medical linear accelerator (Clinac 2100) equipped with an additional m3 microMLC. The second one was to determine the kQclin,Qmsrfclin,fmsr correction factors introduced in a recently proposed small field dosimetry formalism for different active detectors. Methods: Small field sizes were defined either by microMLC down to 6 × 6 mm2 or by circular cones down to 4 mm in diameter. OF measurements were performed with several commercially available active detectors dedicated to measurements in small fields (high resolution diodes: IBA SFD, Sun Nuclear EDGE, PTW 60016, PTW 60017; ionizing chambers: PTW 31014 PinPoint chamber, PTW 31018 microLion liquid chamber, and PTW 60003 natural diamond). Two types of passive dosimeters were used: LiF microcubes and EBT2 radiochromic films. Results: Significant differences between the results obtained by several dosimetric systems were observed, particularly for the smallest field size for which the difference in the measured OF reaches more than 20%. For passive dosimeters, an excellent agreement was observed (better than 2%) between EBT2 and LiF microcubes for all OF measurements. Moreover, it has been shown that these passive dosimeters do not require correction factors and can then be used as reference dosimeters. Correction factors for the active detectors have then been determined from the mean experimental OF measured by the passive dosimeters. Conclusions: Four sets of correction factors needed to apply the new small field dosimetry formalism are provided for several active detectors. A protocol for small photon beams OF determination based on passive dosimeters measurements has been recently proposed to French radiotherapy treatment centers. [ABSTRACT FROM AUTHOR]

This study aimed to measure sensory quality based on objective sensory dimensions (temperature, texture, appearance, flavour) that reflect their relationship with hedonic responses in order to predict food intake. This approach contrasts with institutional assessments based on individual liking self-reports. Sensory evaluation of 3776 food items was performed in a study on the determinants of food intake of hospitalized elderly patients. Sensory evaluation was conducted by trained evaluators using customized sensory profiles consisting in 7–8 sensory parameters judged relevant by experts to the sensory quality of the food item and measured using scales adapted from food manufacturing research and practice or constructed by the research team. Measurements of objective sensory dimensions were then transformed to reflect the hedonic quality of food item. To do so, we captured the relationship between these individual sensory parameters and the hedonic evaluation of the same food item. Sensory dimensions were considered among others as predictors of intake measures for energy and macronutrients. Our regression model showed that temperature could predict energy (r=-1,723, p<0.01), protein (r=-0,089, p<0.001) and lipid intake (r=-0,134, p<0.0001). Texture of food could predict protein (r=-0,701, p<0.0001) and lipid intake (r=-0,396, p<0.05). Finally, flavour could only predict carbohydrate intake (r=2.973, p<0.05). Appearance was not a significant predictor of any intake measure. The benefit of this approach is to provide uniform measures of sensory quality for a variety of food items that can be used to compare the sensory quality of such items or to obtain predictors of food intake based on sensory dimensions of food. [Copyright &y& Elsevier]

The purpose of this work is to assess the feasibility of performing dosimetry with the TruView (ModusQA, London, Ontario, Canada) MTB Fricke gel by MRI reading on a low field MR-Linac (0.35 T). Radiochromic TruView gels were manufactured at IRSN. A 4 echoes time (TE) single-shot fast spin echo sequence (HASTE) was used for 2D images acquisitions. Raw MRI images were denoised (MP-PCA) and transversal relaxation rates (R2) maps were extracted using a homemade Matlab routine. Temperature influences on R2 as well as gel homogeneity were investigated. A dosimetric evaluation of the TruView gel was conducted implying four gels made from the same batch, with three of them dedicated to calibration, and the fourth one received a simple dosimetric plan. Results showed the TruView gel to be sensitive to temperature drift. A protocol was thus developed to ensure thermal stability with the use of a dedicated temperature-controlled water bath. An inhomogeneity that manifests as a R2 increase near the air/gel interface was found to cause a 4% amplitude deviation in R2 along unirradiated gels. Air sealing drastically reduced the inhomogeneity. Dose versus R2 calibration was established. TruView gel sensitivity was found to be low (0.0093 Gy−1 s−1). 1D and 2D comparison to planned dose distribution displayed overall agreement with global gamma index (5%/3 mm) evaluation with an 87 % passing rate. Main discrepancies in the low and gradient dose regions were attributable to the calibration range and diffusion effects. This study demonstrated the feasibility of using the TruView gel for dosimetric evaluation on a low field MR-Linac. The protocol developed needs to be extended to a full 3D evaluation to enable patient QA and may require modifications to the gel formulation to increase its overall sensitivity and ensure limited dependence on temperature drift. • Dosimetry using the TruView Fricke gel, MRI reading on a low-field MR-Linac (0.35 T). • Use of fast MRI sequence (HASTE) for dosimetric evaluations. • Temperature drift and inhomogeneity in the gels, low R2-Dose sensitivity. • Stabilization protocol for the TruView gel to ensure reliable dosimetric evaluations. • TruView gel suitable for dosimetric evaluation, full 3D validation is needed. [ABSTRACT FROM AUTHOR]

Recent use of very small photon beams (down to 4 mm) in stereotactic radiotherapy requires new detectors to accurately determine the delivered dose. Diamond detectors have been presented in the literature as an attractive candidate for this application, due to their small detection volume and the diamond atomic number (Z=6) which is close to water effective atomic number (Zeff∼7.42). However, diamond exhibits a density 3.51 times greater than that of water and recent studies using Monte Carlo simulations have demonstrated the drawback of a high-density detector on small beam output factors. The current study focuses on geometrical parameters of diamond detector, namely, the diamond dimensions and the electrode geometry, in order to solve the dosimetric issues still observed in small photon beams with diamond detectors. To give better insights to these open questions, we have used both computational method and experimental analysis. This study highlighted that reducing diamond dimensions is crucial for small beam output factor measurements and to limit the influence of its high density. Furthermore, electrodes covering the whole diamond surface were essential for a dose rate independence of the diamond detector. The optimal dosimeter derived from this work presented small diamond dimensions of approximately 1-1-0.15mm3, with diamond-like-carbon electrodes covering the whole diamond surface. A dose rate independence of this diamond detector (better than 0.5% over a wide range of dose rates available on a stereotactic dedicated facility) was obtained due to the electrode geometry. Concerning the output factor measurements, a good agreement (better than 1.1%) was observed between this carbon material detector and two types of passive dosimeters (LiF microcubes and EBT2 radiochromic films) for all beam sizes except the smallest field of 0.6-0.6 cm2 with a deviation of 2.6%. This new study showed the high performance of this diamond detector in small photon beams, in comparison with various commercially available passive and active dosimeters. [ABSTRACT FROM AUTHOR]

The structural transformations induced in Gd2O3 and Eu2O3 irradiated with high-energy ions (94-MeV Pb and 1042-MeV Xe), where ionization processes (electronic stopping) dominate the ion slowing-down, have been studied via the combination of XRD and Raman experiments. Irradiation induces a cubic-to-monoclinic phase transformation; the radiation-induced monoclinic phase shows strong preferential orientation. This structural modification is accompanied by changes in the photoluminescence properties of sesquioxides. The results show that swift heavy ion irradiation is a powerful tool to control the properties of oxides at the nanoscale. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the optimal use of XR-RV3 GafChromic® films to assess patient skin dose in interventional radiology while addressing the means to reduce uncertainties in dose assessment. Methods: XR-Type R GafChromic films have been shown to represent the most efficient and suitable solution to determine patient skin dose in interventional procedures. As film dosimetry can be associated with high uncertainty, this paper presents the EURADOS WG 12 initiative to carry out a comprehensive study of film characteristics with a multisite approach. The considered sources of uncertainties include scanner, film, and fitting-related errors. The work focused on studying film behavior with clinical high-dose-rate pulsed beams (previously unavailable in the literature) together with reference standard laboratory beams. Results: First, the performance analysis of six different scanner models has shown that scan uniformity perpendicular to the lamp motion axis and that long term stability are the main sources of scanner-related uncertainties. These could induce errors of up to 7% on the film readings unless regularly checked and corrected. Typically, scan uniformity correction matrices and reading normalization to the scanner-specific and daily background reading should be done. In addition, the analysis on multiple film batches has shown that XR-RV3 films have generally good uniformity within one batch (<1.5%), require 24 h to stabilize after the irradiation and their response is roughly independent of dose rate (<5%). However, XR-RV3 films showed large variations (up to 15%) with radiation quality both in standard laboratory and in clinical conditions. As such, and prior to conducting patient skin dose measurements, it is mandatory to choose the appropriate calibration beam quality depending on the characteristics of the x-ray systems that will be used clinically. In addition, yellow side film irradiations should be preferentially used since they showed a lower dependence on beam parameters compared to white side film irradiations. Finally, among the six different fit equations tested in this work, typically used third order polynomials and more rational and simplistic equations, of the form dose inversely proportional to pixel value, were both found to provide satisfactory results. Fitting-related uncertainty was clearly identified as a major contributor to the overall film dosimetry uncertainty with up to 40% error on the dose estimate. Conclusions: The overall uncertainty associated with the use of XR-RV3 films to determine skin dose in the interventional environment can realistically be estimated to be around 20% (k = 1). This uncertainty can be reduced to within 5% if carefully monitoring scanner, film, and fitting-related errors or it can easily increase to over 40% if minimal care is not taken. This work demonstrates the importance of appropriate calibration, reading, fitting, and other film-related and scan-related processes, which will help improve the accuracy of skin dose measurements in interventional procedures. [ABSTRACT FROM AUTHOR]

Several passive solid state dosemeters, such as Gafchromic™ films and thermoluminescence (TL) detectors, are used to estimate and monitor patient skin doses in interventional radiology. To determine the suitability of XR-TypeR Gafchromic™ films and of detectors based on TL materials: pellets, chips and foils to measure skin dose, an intercomparison exercise has been organized within European Dosimetry Radiation Group – Working Group 12 “European Medical ALARA Network” (EURADOS WG12). To test response detectors were exposed to X-ray beams of energies and qualities applied clinically. A blind test was also performed to investigate the accuracy of the dose estimate by detectors exposed to unknown doses. We found the response of films to be strongly dependent on beam quality and filtration (increasing by up to 80% with respect to reference beam quality). The response of TL detectors was found to be less dependent on beam quality (less than 25% variation), with TL foils showing less than 10% variation with respect to reference beam quality. To accurately estimate patient skin doses in interventional radiology it is important to choose the quality of the calibration beam to be as close as possible to the quality of beams actually applied in clinical work. [ABSTRACT FROM AUTHOR]

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 mm3). Long term stability was studied by irradiating the SCDDo in a 60Co 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 cm2 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 cm2, 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. [ABSTRACT FROM AUTHOR]

Abstract: Recent developments of new therapy techniques using small photon beams, such as stereotactic radiotherapy, require new detectors to precisely determine the delivered dose. The dosimeter has to be as close as possible to tissue equivalence and 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 fulfill most of small beam dosimetry requirements. A commercially Element Six electronic grade synthetic diamond was selected to develop a single crystal diamond dosimeter (SCDDo) with a small detection volume (0.4mm3). The Monte-Carlo code PENELOPE (parallelized version) was used to optimize both geometry and packaging materials of the diamond detector in order to respect the tissue-equivalent requirement. SCDDo dose profile and output factor (OF) measurements, were performed for small photon beams with a micro multileaf collimator m3 (BrainLab) attached to a Varian Clinac 2100 C linear accelerator. These measurements were compared to different detectors, both active and passive: silicon diode (PTW 60017), EBT2 radiochromic films, PinPoint ionization chamber (PTW 31014). SCDDo presents an excellent spatial resolution for dose profile measurements, due its small detection volume. OFs obtained with SCDDo are very satisfactory from 0.6×0.6cm2 to 10×10cm2 field sizes, compared to PinPoint ionization chamber which underestimates OF values in small beam. [Copyright &y& Elsevier]

Evaluates on the heat-dissociated p24 antigen assay as an alternative low-cost tool for diagnosis of HIV-infection and quantification of HIV-1 RNA levels in African adults. Quantitation of p24 antigen antigenemia; Level of correlation between the assays; Two major concerns expressed for treatment monitoring.

Starting in 1996, nonlinear approximation algorithms used for the determination of size distributions of ''unattached'' radon progeny from measurements made with diffusional size classification instruments were compared. Seven participants (one American and six European) took part using various techniques (Simplex, Expectation Maximization, Extreme Value Estimation, Twomey, and Random Walk) which are widely used for this type of work. Simulated input data was supplied to the participants. Their output data indicated that the quality of the results varied according to the algorithm used. This was particularly the case when retrieving parameters from bimodal distributions where large inaccuracies were shown. Variations were found between participants using basically identical algorithms. The influence of measurement uncertainties was investigated and indicated a serious reduction of retrieval accuracy, especially for algorithms with better than average performance for precise data. This work suggests that when high precision results are required, a random walk algorithm should be used and attention should be paid to reducing errors in the input penetration data. [ABSTRACT FROM AUTHOR]