Absorbed dose distributions in lineal energy for neutrons and gamma rays were measured by using both a tissue-equivalent walled counter (TEPC) and a graphite-walled low pressure proportional counter (GPC) in the Am-Be neutron source facility at UOIT. A series of measurements were performed with the counters filled with propane-based TE gas (55.1% C3H8, 39.5% CO2 and 5.4% N2) at operating gas pressures corresponding to tissue spheres 2.0 , 4.0 and 8.0 μm in diameter. The results of these measurements indicated satisfactory performance of counters to measure microdosimetric spectra extending down to event-sizes that cover the gamma component of a mixed field. The spectra and the related mean values ̅y F and ̅y D are compared with other similar work but with monoenergetic neutrons of different energy range, the agreement between them is good. An assessment of the performance of different size TEPC has been done. An excellent agreement between their event size spectra was found and the proton edge appears at the same position on the lineal energy scale and differences in microdosimetric parameters ̅ and ̅ is not exceeding 3%, which is in the region of counting statistics. In Am-Be neutron field, the efficiency of the TEPCs was measured to have an average value of 250 counts per μSv or equivalently about 4.17 counts per minutes per μSv/hr. This efficiency is reasonable for dose equivalent measurements but needs a long integration period. The measurements showed that the dose equivalent which depends on the measurement of energy deposition by the secondary charged particles was originated mainly from elastic collisions of the incident neutrons with hydrogen atoms. Moreover the number of events in the sensitive gas is dominated by proton recoils. A non- negligible fraction of the dose equivalent resulted from gamma interactions, alpha and recoil nuclei. The energy deposition patterns in these micro-scale targets are strongly dependent on radiation quality, so differences of linear energy transfer (LET) of the components in a mixed radiation field are significant. Accordingly, in a radiation field with an unknown gamma ray energy spectrum, absorbed dose for neutrons can be obtained by the separation of neutron induced events from gamma events using their distribution in lineal energy. To separate neutron dose from gamma dose a simple lineal energy threshold technique has been used in addition to a more sophisticated methods using γ-fitting and the graphite-walled counter measurements. The results of this study will establish the degree of error introduced by using a lineal energy threshold, which is likely to be used in any hand-held neutron monitor based on TEPCs. / UOIT

The thesis aims to improve treatment dose accuracy in brachytherapy using a high dose rate (HDR) Ir-192 stepping source and in external beam therapy using intensity modulated radiation therapy (IMRT). For HDR brachytherapy, this has been achieved by investigating dose errors in the near field and the transit dose of the HDR brachytherapy stepping source. For IMRT, this study investigates the volume effect of detectors in the dosimetry of small fields, and the clinical implementation and dosimetric verification of a 6MV photon beam for IMRT. For the study of dose errors in the near field of an HDR brachytherapy stepping source, the dose rate at point P at 0.25 cm in water from the transverse bisector of a straight catheter was calculated with Monte Carlo code MCNP 4.A. The Monte Carlo (MC) results were used to compare with the results calculated with the Nucletron Brachytherapy Planning System (BPS) formalism. Using the MC calculated radial dose function and anisotropy function with the BPS formalism, 1% dose calculation accuracy can be achieved even in the near field with negligible extra demand on computation time. A video method was used to analyse the entrance, exit and the inter-dwell transit speed of the HDR stepping source for different path lengths and step sizes ranging from 2.5 mm to 995 mm. The transit speeds were found to be ranging from 54 to 467 mm/s. The results also show that the manufacturer has attempted to compensate for the effects of inter-dwell transit dose by reducing the actual dwell time of the source. A well-type chamber was used to determine the transit doses. Most of the measured dose differences between stationary and stationary plus inter-dwell source movement were within 2%. The small-field dosimetry study investigates the effect of detector size in the dosimetry of small fields and steep dose gradients with a particular emphasis on IMRT measurements. Due to the finite size of the detector, local discrepancies of more than 10 % are found between calculated cross profiles of intensity modulated beams and intensity modulated profiles measured with film. A method to correct for the spatial response of finite sized detectors and to obtain the

A dosimetria de campos mistos de nêutrons e gamas é uma área de pesquisa que apresenta grande oportunidade de estudos devido ao aumento da utilização de procedimentos médicos como protonterapia e Terapia de Captura de Nêutrons (NCT Neutron Capture Therapy), além da importância para cálculo de doses ocupacionais e dos campos de irradiação em reatores nucleares. Para a dosimetria de campos mistos de nêutrons e gamas, a Comissão Internacional de Unidades de Medidas (ICRU) recomenda o uso de dosímetros com sensibilidades distintas para cada componente do campo. Os dosímetros termoluminescentes (TLDs) apresentam-se como uma alternativa para a realização da dosimetria de campos mistos de nêutrons e gamas em particular à utilização do par de TLDs de LiF TLD 100 e TLD 700, que apresentam respostas distintas às componentes de campo em virtude da diferença na quantidade do isótopo 6Li em suas composições. Porém, esta escolha apresenta algumas dificuldades pois a característica da resposta dos TLDs para cada componente de campo ainda não é totalmente compreendida. Este trabalho apresenta primeiramente um estudo de um sistema para moderação de uma fonte de AmBe para realizar os estudos de sensibilidade e linearidade dos TLDs quando irradiados em um campo misto de nêutrons e gamas. O sistema de moderação se faz necessário pois a fonte de AmBe emite preferencialmente nêutrons com alta energia, e a sensibilidade dos TLDs de LiF é preferencialmente para nêutrons de baixa energia. Entretanto, um dos objetivos do Grupo de Pesquisa em Física Médica do CEN/IPEN é a realização da dosimetria de campos mistos de alta intensidade, como por exemplo, o campo proveniente de um reator nuclear. Dessa forma esse trabalho realizou um estudo das respostas dos TLDs 100 e TLDs 700 quando irradiados no interior do núcleo do reator IPEN/MB-01 em duas diferentes configurações: cilíndrica com \"flux trap\" e retangular num arranjo de 26 x 28 varetas combustíveis. Esse trabalho contou com simulações com o código de Monte Carlo, MCNP5, para fornecer os fluxos e doses devido a cada componente de campo ao qual os TLDs estariam expostos. E a partir dos dados obtidos tanto através das simulações, quanto através dos experimentos, foi proposta uma metodologia para a utilização do TLD 100 para a dosimetria de nêutrons em campos com alta fluência de nêutrons, como é o caso do núcleo do reator IPEN/MB-01. / Mixed radiation field dosimetry is a research area that presents a great opportunity for studies due to the increased use on medical procedures such as proton therapy and Neutron Capture Therapy (NCT), as well as the importance of calculating occupational doses and radiation fields in nuclear reactors. For the dosimetry of mixed fields of neutrons and gammas, the International Commission on Radiation Units and Measurements (ICRU) recommends the use of dosimeters with distinct sensitivities for each component of the field. Thermoluminescent dosimeters (TLDs) are presented as an alternative for the dosimetry of neutron and gamma-mixed fields. In particular, the use of LiF TLD pair TLD 100 and TLD 700, which present distinct responses to the field components due to the difference in the amount of the 6Li isotope in its compositions. However, this choice presents some difficulties because the characteristic of the TLD response for each field component is not fully understood. This work presents a study of a system for moderation of an AmBe source to perform the screening and linearity studies of the TLDs when irradiated in a mixed field of neutrons and gammas. The moderation system is necessary since the AmBe source preferably emits neutrons with high energy, and the sensitivity of the LiF TLDs is preferably for low energy neutrons. However, one of the objectives of the Monte Carlo and Dosimetry for Medical Physics Research Group of CEN / IPEN is to perform the dosimetry of high intensity mixed fields, such as the field from a nuclear reactor. Thus, this work carried out a study of the responses of the TLDs 100 and TLDs 700 when irradiated inside the core of the IPEN/MB-01 reactor in two different configurations: cylindrical with flux trap and rectangular in an arrangement of 26 x 28 fuel rods. This work relied on simulations with the Monte Carlo code MCNP5 to provide the fluxes and doses due to each field component in which the TLDs would be exposed. From the data obtained, both through the simulations and through the experiments, a methodology was proposed for the use of the TLD 100 for the neutron dosimetry in fields with high neutron fluence, such as the IPEN/MB-01.

Purpose: To examine the dosimetric characteristics of Gafchromic EBT3 film when measuring small fields of radiation, and compare it against other common radiation detectors. Methods and Materials: EBT3 film was placed in a solid water phantom and irradiated with 6MV photons, field sizes from 10x10cm2 down to 6x6mm2. The films were scanned with a Vidar DosimetryPRO Advantage Red scanner, and analyzed with RIT113 software. The films were also scanned at different orientations and times to quantify the discrepancies associated with scanning orientation and post-exposure darkening. The same fields were measured with a PTW TN30013 farmer chamber, an Exradin T1 cylindrical ion chamber, a PTW parallel plate ion chamber, and a Sun Nuclear Edge Detector (diode). Output factors were calculated for each detector and compared for accuracy. The output factors were measured from a Varian Clinac iX, Clinac 21EX, Trilogy, and TrueBeam; as well as a Novalis Tx. The outputs from different machines at different clinics were compared. Results: The EBT3 film and Edge Detector were the only detectors that succeeded in accurately measuring the output from all field sizes; the ion chambers were too large and failed for field sizes below 4x4cm2 due to volume averaging. The dose measured with the film increased by an average of 8.8% after one week post-irradiation. The dose measured was also reduced by an average of 4.4% by scanning the film in landscape orientation, as opposed to portrait orientation. It was shown that the output factors for the smallest field of 6x6mm2--successfully measured with film and diode--varied between 0.54-0.74 for five different machines at three different clinics. Conclusions: The feasibility of using Gafchromic EBT3 film to measure very small fields of radiation is confirmed. Of the other 4 detectors used, only the diode was shown to be capable of accurately measuring small fields of radiation. The need to optimize the film dosimetry process--including the time films are scanned post-irradiation, the consistency of the scanning orientation of the calibration and subsequent films, and the measurement procedure on the computer software--is highlighted.

Chemical Vapour Deposition (CVD) diamond detectors were modelled for dosimetry of radiotherapy beams. This was achieved by employing the EGSnrc Monte Carlo (MC) method to investigate certain properties of the detector, such as size, shape and electrode materials. Simulations were carried out for a broad 6 MV photon beam, and water phantoms with both uniform and non-uniform voxel dimensions. A number of critical MC parameters were investigated for the development of a model that can simulate very small voxels. For a given number of histories (100 million), combinations of the following parameters were analyzed: cross section data, boundary crossing algorithm and the HOWFARLESS option, with the rest of the transport parameters being kept at default values. The MC model obtained with the optimized parameters was successfully validated against published data for a 1.25 MeV photon beam and CVD diamond detector with silver/carbon/silver structure with thicknesses of 0.07/0.2/0.07 cm for the electrode/detector/electrode, respectively. The interface phenomena were investigated for a 6 MV beam by simulating different electrode materials: aluminium, silver, copper and gold for perpendicular and parallel detector orientation with regards to the beam. The smallest interface phenomena were observed for parallel detector orientation with electrodes made of the lowest atomic number material, which was aluminium. The simulated percentage depth dose and beam profiles were compared with experimental data. The best agreement between simulation and measurement was achieved for the detector in parallel orientation and aluminium electrodes, with differences of approximately 1%. In summary, investigations related to the CVD diamond detector modelling revealed that the EGSnrc MC code is suitable for simulation of small size detectors. The simulation results are in good agreement with experimental data and the model can now be used to assist with the design and construction of prototype diamond detectors for clinical dosimetry. Future work will include investigating the detector response for different energies, small field sizes, different orientations other than perpendicular and parallel to the beam, and the influence of each electrode on the absorbed dose.

Chemical Vapour Deposition (CVD) diamond detectors were modelled for dosimetry of radiotherapy beams. This was achieved by employing the EGSnrc Monte Carlo (MC) method to investigate certain properties of the detector, such as size, shape and electrode materials. Simulations were carried out for a broad 6 MV photon beam, and water phantoms with both uniform and non-uniform voxel dimensions. A number of critical MC parameters were investigated for the development of a model that can simulate very small voxels. For a given number of histories (100 million), combinations of the following parameters were analyzed: cross section data, boundary crossing algorithm and the HOWFARLESS option, with the rest of the transport parameters being kept at default values. The MC model obtained with the optimized parameters was successfully validated against published data for a 1.25 MeV photon beam and CVD diamond detector with silver/carbon/silver structure with thicknesses of 0.07/0.2/0.07 cm for the electrode/detector/electrode, respectively. The interface phenomena were investigated for a 6 MV beam by simulating different electrode materials: aluminium, silver, copper and gold for perpendicular and parallel detector orientation with regards to the beam. The smallest interface phenomena were observed for parallel detector orientation with electrodes made of the lowest atomic number material, which was aluminium. The simulated percentage depth dose and beam profiles were compared with experimental data. The best agreement between simulation and measurement was achieved for the detector in parallel orientation and aluminium electrodes, with differences of approximately 1%. In summary, investigations related to the CVD diamond detector modelling revealed that the EGSnrc MC code is suitable for simulation of small size detectors. The simulation results are in good agreement with experimental data and the model can now be used to assist with the design and construction of prototype diamond detectors for clinical dosimetry. Future work will include investigating the detector response for different energies, small field sizes, different orientations other than perpendicular and parallel to the beam, and the influence of each electrode on the absorbed dose.

<p>Non-invasive electromagnetic brain stimulation uses electrodes and/or coils to modulate brain activity via the induced E-fields. E-field dosimetry solvers have improved non-invasive electromagnetic brain stimulation protocol and our understanding of neuroscience. However, E-field dosimetry techniques are incomplete in that the contributions of non-linear neuron activity are left unaccounted for. To better understand the neurological effects of non-invasive electromagnetic stimulation, we introduce an integral equation formulation for modeling the non-linear behavior of neurons due to an incident E-field generated by electrode and coil sources. We formulate the new integral equation using a boundary element approach. We compare the boundary element solver accuracy with an established finite element solver and multi-scale cable equation approaches. Unlike previous approaches, this new boundary integral formulation avoids multi-scaling challenges from meshing while retaining the accuracy and the robust spatial support of integral equation-based methods. The memory savings from switching to surface meshes makes simulations with more complex morphologies computationally tractable. Additionally, we examine the ability of neurons to couple to one another via the local extracellular fields. Examples of simulations with both transcranial electric and magnetic stimulation results for simple geometries are used to illustrate the capabilities of a boundary integral approach. This boundary integral method will aid the development of better neurological understanding, delineate the mechanisms by which electromagnetic stimulation engenders neuronal activity, and aid in modeling local E-field coupling.</p>

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Two innovative radiotherapy (RT) approaches are under development at the ID17 Biomedical Beamline of the European Synchrotron Radiation Facility (ESRF): microbeam radiation therapy (MRT) and minibeam radiation therapy (MBRT). The two main distinct characteristics with respect to conventional RT are the use of submillimetric field sizes and spatial fractionation of the dose. This PhD work deals with different features related to small-field dosimetry involved in these techniques. Monte Carlo (MC) calculations and several experimental methods are used with this aim in mind. The core of this PhD Thesis consisted of the development and benchmarking of an MC-based computation engine for a treatment planning system devoted to MRT within the framework of the preparation of forthcoming MRT clinical trials. Additional achievements were the definition of safe MRT irradiation protocols, the assessment of scatter factors in MRT, the further improvement of the MRT therapeutic index by injecting a contrast agent into the tumour and the definition of a dosimetry protocol for preclinical trials in MBRT.