An innovative in vitro device providing continuous low doses of γ-rays mimicking exposure to the space environment: A dosimetric study.

Astronauts are exposed to microgravity and chronic irradiation but experimental conditions combining these two factors are difficult to reproduce on earth. We have created an experimental device able to combine chronic irradiation and altered gravity that may be used for cell cultures or plant models in a ground based facility. Irradiation was provided by thorium nitrate powder, conditioned so as to constitute a sealed source that could be placed in an incubator. Cell plates or plant seedlings could be placed in direct contact with the source or at various distances above it. Moreover, a random positioning machine (RPM) could be positioned on the source to simulate microgravity. The activity of the source was established using the Bateman formula. The spectrum of the source, calculated according to the natural decrease of radioactivity and the gamma spectrometry, showed very good adequacy. The experimental fluence was close to the theoretical fluence evaluation, attesting its uniform distribution. A Monte Carlo model of the irradiation device was processed by GATE code. Dosimetry was performed with radiophotoluminescent dosimeters exposed for one month at different locations (x and y axes) in various cell culture conditions. Using the RPM placed on the source, we reached a mean absorbed dose of gamma rays of (0.33 ± 0.17) mSv per day. In conclusion, we have elaborated an innovative device allowing chronic radiation exposure to be combined with altered gravity. Given the limited access to the International Space Station, this device could be useful to researchers interested in the field of space biology.

Monte Carlo dose calculation in presence of low-density media: Application to lung SBRT treated during DIBH.

PURPOSE: Commercial algorithms used in Radiotherapy include approximations that are generally acceptable. However their limits can be seen when confronted with small fields and low-density media. These conditions exist during the treatment of lung cancers with Stereotactic Body Radiation Therapy (SBRT) achieved with the "Deep Inspiration Breath Hold" (DIBH) technique. A Monte Carlo (MC) model of a linear accelerator was used to assess the performance of two algorithms (Varian Acuros and AAA) in these conditions. This model is validated using phantoms with different densities. Lastly, results for SBRT cases are compared to both Acuros and AAA. METHODS: A Varian TrueBeam linac was modeled using GATE/Geant4 and validated by comparing dose distributions for simple fields to measurements in water and in heterogeneous phantoms composed of PMMA and two types of cork (corresponding to lung densities during free-breathing and DIBH). Experimental measurements are also compared to AAA and Acuros. Finally, results of Acuros/AAA are compared to MC for a clinical case (SBRT during DIBH). RESULTS: Based on 1D gamma index comparisons with measurements in water, the TrueBeam model was validated (>97% of points passed this test). In heterogeneous phantoms, and in particular for small field sizes, very low density (0.12g.cm-3) and at the edge of the field, MC model was still in good agreement with measurements whilst AAA and Acuros showed discrepancies. With the patient CT, similar differences between MC and AAA/Acuros were observed for static fields but disappeared using an SBRT arc field. CONCLUSIONS: Our MC model is validated and limits of commercial algorithms are shown in very low densities.

Implementation of new physics models for low energy electrons in liquid water in Geant4-DNA.

A new alternative set of elastic and inelastic cross sections has been added to the very low energy extension of the Geant4 Monte Carlo simulation toolkit, Geant4-DNA, for the simulation of electron interactions in liquid water. These cross sections have been obtained from the CPA100 Monte Carlo track structure code, which has been a reference in the microdosimetry community for many years. They are compared to the default Geant4-DNA cross sections and show better agreement with published data. In order to verify the correct implementation of the CPA100 cross section models in Geant4-DNA, simulations of the number of interactions and ranges were performed using Geant4-DNA with this new set of models, and the results were compared with corresponding results from the original CPA100 code. Good agreement is observed between the implementations, with relative differences lower than 1% regardless of the incident electron energy. Useful quantities related to the deposited energy at the scale of the cell or the organ of interest for internal dosimetry, like dose point kernels, are also calculated using these new physics models. They are compared with results obtained using the well-known Penelope Monte Carlo code.

Resistor mesh model of a spherical head: part 1: applications to scalp potential interpolation.

A resistor mesh model (RMM) has been implemented to describe the electrical properties of the head and the configuration of the intracerebral current sources by simulation of forward and inverse problems in electroencephalogram/event related potential (EEG/ERP) studies. For this study, the RMM representing the three basic tissues of the human head (brain, skull and scalp) was superimposed on a spherical volume mimicking the head volume: it included 43 102 resistances and 14 123 nodes. The validation was performed with reference to the analytical model by consideration of a set of four dipoles close to the cortex. Using the RMM and the chosen dipoles, four distinct families of interpolation technique (nearest neighbour, polynomial, splines and lead fields) were tested and compared so that the scalp potentials could be recovered from the electrode potentials. The 3D spline interpolation and the inverse forward technique (IFT) gave the best results. The IFT is very easy to use when the lead-field matrix between scalp electrodes and cortex nodes has been calculated. By simple application of the Moore-Penrose pseudo inverse matrix to the electrode cap potentials, a set of current sources on the cortex is obtained. Then, the forward problem using these cortex sources renders all the scalp potentials.

Resistor mesh model of a spherical head: part 2: a review of applications to cortical mapping.

A resistor mesh model (RMM) has been validated with reference to the analytical model by consideration of a set of four dipoles close to the cortex. The application of the RMM to scalp potential interpolation was detailed in Part 1. Using the RMM and the same four dipoles, the different methods of cortical mapping were compared and have shown the potentiality of this RMM for obtaining current and potential cortical distributions. The lead-field matrices are well-adapted tools, but the use of a square matrix of high dimension does not permit the inverse solution to be improved in the presence of noise, as a regularisation technique is necessary with noisy data. With the RMM, the transfer matrix and the cortical imaging technique proved to be easy to implement. Further development of the RMM will include application to more realistic head models with more accurate conductivities.

Clinical results of an EPID-based in-vivo dosimetry method for pelvic cancers treated by intensity-modulated radiation therapy.

The purpose of our work was to investigate the feasibility of using an EPID-based in-vivo dosimetry method initially designed for conformal fields on pelvic dynamic IMRT fields. The method enables a point dose delivered to the patient to be calculated from the transit signal acquired with an electronic portal imaging device (EPID). After defining a set of correction factors allowing EPID pixel values to be converted into absolute doses, several tests on homogeneous water-equivalent phantoms were performed to estimate the validity of the method in reference conditions. The effects of different treatment parameters, such as delivered dose, field size dependence and patient thickness were also studied. The model was first evaluated on a group of 53 patients treated by 3D conformal radiotherapy (3DCRT) and then on 92 patients treated by IMRT, both for pelvic cancers. For each measurement, the dose was reconstructed at the isocenter (DREC) and compared with the dose calculated by our treatment planning system (DTPS). Excellent agreement was found between DREC and DTPS for both techniques. For 3DCRT treatments, the mean deviation between DREC and DTPS for the 211 in-vivo dose verifications was equal to -1.0  ±  2.2% (1SD). Concerning IMRT treatments, the averaged deviation for the 418 fields verified was equal to -0.3 ± 2.6% (1SD) proving that the method is able to reconstruct a dose for dynamic IMRT pelvic fields. Based on these results, tolerance criteria and action levels were established before its implementation in clinical routine.

An optimized calibration method for surface measurements with MOSFETs in shaped-beam radiosurgery.

Nowadays MOSFET dosimeters are widely used for dose verification in radiotherapy procedures. Although their sensitive area satisfies size requirements for small field dosimetry, their use in radiosurgery has rarely been reported. The aim of this study is to propose and optimize a calibration method to perform surface measurements in 6 MV shaped-beam radiosurgery for field sizes down to 18 × 18 mm(2). The effect of different parameters such as recovery time between 2 readings, batch uniformity and build-up cap attenuation was studied. Batch uniformity was found to be within 2% and isocenter dose attenuation due to the build-up cap over the MOSFET was near 2% irrespective of field size. Two sets of sensitivity coefficients (SC) were determined for TN-502RD MOSFET dosimeters using experimental and calculated calibration; the latter being developed using an inverse square law model. Validation measurements were performed on a realistic head phantom in irregular fields. MOSFET dose values obtained by applying either measured or calculated SC were compared. For calibration, optimal results were obtained for an inter-measurement time lapse of 5 min. We also found that fitting the SC values with the inverse square law reduced the number of measurements required for calibration. The study demonstrated that combining inverse square law and Sterling-Worthley formula resulted in an underestimation of up to 4% of the dose measured by MOSFETs for complex beam geometries. With the inverse square law, it is possible to reduce the number of measurements required for calibration for multiple field-SSD combinations. Our results suggested that MOSFETs are suitable sensors for dosimetry when used at the surface in shaped-beam radiosurgery down to 18 × 18 mm(2).

In-vivo dosimetry for field sizes down to 6 × 6 mm2 in shaped beam radiosurgery with microMOSFET.

The aim of this study is to evaluate microMOSFET as in-vivo dosimeter in 6 MV shaped-beam radiosurgery for field sizes down to 6 × 6 mm2. A homemade build-up cap was developed and its use with microMOSFET was evaluated down to 6 × 6 mm2. The study with the homemade build-up cap was performed considering its influence on field size over-cover occurring at surface, achievement of the overall process of electronic equilibrium, dose deposition along beam axis and dose attenuation. An optimized calibration method has been validated using MOSFET in shaped-beam radiosurgery for field sizes from 98 × 98 down to 18 × 18 mm2. The method was detailed in a previous study and validated in irregular field shapes series measurements performed on a head phantom. The optimized calibration method was applied to microMOSFET equipped with homemade build-up cap down to 6 × 6 mm2. Using the same irregular field shapes, dose measurements were performed on head phantom. MicroMOSFET results were compared to previous MOSFET ones. Additional irregular field shapes down to 8.8 × 8.8 mm2 were studied with microMOSFET. Isocenter dose attenuation due to the homemade build-up cap over the microMOSFET was near 2% irrespective of field size. Our results suggested that microMOSFET equipped with homemade build-up cap is suitable for in-vivo dosimetry in shaped-beam radiosurgery for field sizes down to 6 × 6 mm2 and therefore that the required build-up cap dimensions to perform entrance in-vivo dosimetry in small-fields have to ensure only partial charge particle equilibrium.

Anatomy and time course of discrimination and categorization processes in vision: an fMRI study.

Studies investigating the cerebral areas involved in visual processes generally oppose either different tasks or different stimulus types. This work addresses, by fMRI, the interaction between the type of task (discrimination vs. categorization) and the type of stimulus (Latin letters, well-known geometrical figures, and Korean letters). Behavioral data revealed that the two tasks did not differ in term of percentage of errors or correct responses, but a delay of 185 ms was observed for the categorization task in comparison with the discrimination task. All conditions activated a common neural network that includes both striate and extrastriate areas, especially the fusiform gyri, the precunei, the insulae, and the dorsolateral frontal cortex. In addition, interaction analysis revealed that the right insula was sensitive to both tasks and stimuli, and that stimulus type induced several significant signal variations for the categorization task in right frontal cortex, the right middle occipital gyrus, the right cuneus, and the left and right fusiform gyri, whereas for the discrimination task, significant signal variations were observed in the right occipito-parietal junction only. Finally, analyzing the latency of the BOLD signal also revealed a differential neural dynamics according to tasks but not to stimulus type. These temporal differences suggest a parallel hemisphere processing in the discrimination task vs. a cooperative interhemisphere processing in the categorization task that may reflect the observed differences in reaction time.