3.10 Facultad de Física

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Desarrollo e implementación de un sistema de auditoría para dosimetría absoluta con alanina y TLD en centros de radioterapia chilenos
(2018) Faúndez Cifuentes, Daniel; Gago Arias, Araceli; Pontificia Universidad Católica de Chile. Facultad de Física
Evaluación de algoritmos comerciales de cálculo de dosis absorbida de fotones en regiones fuera del campo de tratamiento
(2018) Medina Ascanio, Karem Nathalie; Sánchez Nieto, Beatriz; Pontificia Universidad Católica de Chile. Facultad de Física
Los algoritmos presentes en los Sistemas de Planificación están optimizados para entregar una dosis veraz y precisa dentro de los límites del campo de tratamiento. Fuera de este, se ha demostrado una inexactitud que depende del algoritmo particular. Este estudio analiza, por medio de medidas experimentales, la exactitud de la dosis fuera del campo de dos algoritmos semianalíticos, Pencil Beam Convolution y Collapsed Cone Convolution, implementados en dos Sistemas de Planificación comerciales EclipseTM (Varian Medical System) y Monaco® (Elekta) respectivamente, así como también, el algoritmo de cálculo de dosis basado en métodos de Monte Carlo implementado en Monaco®. Tanto los cálculos con los Sistemas de Planificación, como las determinaciones experimentales de las dosis se realizaron en medios homogéneos (agua) para diferentes configuraciones de campos convencionales y para una distribución de dosis de un plan real de IMRT de próstata. Las determinaciones experimentales se llevaron a cabo con TLDs-100, películas radiocrómicas EBT3 ycámara de ionización Semiflex. Los resultados obtenidos al comparar perfiles dedosis laterales de campos abiertos calculados por los algoritmos respecto a los medidos experimentalmente muestran que, a partir del 9% de la dosis los cálculos realizados por el algoritmo Pencil Beam subestiman la dosis real en un promedio de 53%. Mientras que, se obtuvo un buen acuerdo con los cálculos realizados por el algoritmo Collapsed Cone, con una desviación local promedio de aproximadamente 2,5%. En el caso del algoritmo Monte Carlo, de los resultados obtenidos no se puede inferir ninguna conclusión ya que cálculos de este tipo (conformados) donde se logra la dosis deseada con una incertidumbre de 1% en un punto (generalmente el isocentro), emplean pocas partículas para lograr dicha incertidumbre y, por lo tanto, el ruido estadístico se hace significativo. Con respecto al desempeño de los algoritmos Pencil Beam y Monte Carlo en el caso de un plan clínico de irradiación de próstata en relación a la medida experimental con EBT3, al hacer un análisis gamma local (3% - 3 mm) ambos algoritmos fallaron en regiones de dosis bajas (> 5%), siendo el algoritmo MC el que obtuvo la mayor cantidad de puntos que no pasaron el criterio gamma, mientras que, Pencil Beam fue el que arrojo mayor imprecisión en regiones de dosis altas (para lo cual es análisis gamma fue global, respecto al máximo).
Evaluación dosimétrica de tratamientos con Leksell gamma knife en presencia de inhomogeneidades
(2018) Elgueda Farías, Susan Pamela; Caprile Etchart, Paola F.; Pontificia Universidad Católica de Chile. Facultad de Física
Feasibility study : use of cine-MRI in MR-guided particle therapy
(2018) Sepúlveda Muñoz, César; Pfaffenberger, Asja; Sánchez Nieto, Beatriz; Pontificia Universidad Católica de Chile. Facultad de Física
Cinematic Magnetic Resonance Imaging (cine-MRI) are fast time-resolved 2D MRI sequences that allow online visualization of organ motion without exposing patients to ionizing radiation. In this study, the potential of cine-MRI for the definition of animage-based gating criterion in particle therapy of pancreatic cancer is investigated. Transversal cine-MRI are employed to segment the pancreas, since they can provide information on tissue changes in the beam-path during delivery. Volunteer data were acquired on 10 different acquisition days. An image-based gating window was defined based on the breathing-curve (BC) of the first day (end-exhale breathing phase ±30%). The superposition of the pancreas segmentations in this window was calculated. For other days of data acquisition, the agreement with this initial superposition contour was evaluated by using different segmentation image similarity metrics. Additionally, a motion analysis of the pancreas was performed. From the metrics comparisons, a gating criterion was defined to establish when irradiation can be allowed. An evaluation of the dosimetric differences due to differences in the segmentations achieving the criterion and the spatial location of the organs was carried out for a proton treatment. Although the method applied was subjected to several limitations, it was proved to be feasible.
Individual breast dosimetry for mammographic exams : a back-projection approach
(2017) Rioseco Mora, Camila Belén; Sánchez Nieto, Beatriz; Dörner Yaksic, Edgardo Andrés; Häring, Peter; Pontificia Universidad Católica de Chile. Facultad de Física
The aim of this project is to propose and validate a procedure for estimating a dose at the axial plane of the breast in patients under mammographic exposure by means of electronic portal imaging device (EPID) dosimetry. In vivo transit dosimetry consists of measuring the transmitted radiation through the breast with an EPID, and then back-projecting the fluence through the same volume in order to reconstruct the dose at a certain plane. The methodology relies on the fact that from individual beam attenuation information, is possible to estimate dose deposition in the mammary gland. Beam attenuation was calculated from the exit and entrance fluence. Monte Carlo simulation of the entrance fluence was computed for three beam qualities. Exit fluence is calculated for each case, by deconvolution of the EPID image with previously determined composite kernel. A complete model (i.e., X-ray tube, breast compression paddle, different breast thickness and detector) of the GE Senograph Essential with Tomosynthesis device, was simulated with EGSnrc. Specific codes as BEAMnrc and DOSXYZnrc were used for beam and detector simulations, respectively. Three different beam qualities were simulated (only for 2D conventional mammography image acquisition) and additionally, and in-house graphical user interphase MATLAB1 program “BreDose” was designed to estimate doses at an axial mid-plane of the breast from the EPID images.
Predictors for radiation-induced xerostomia and their reliability considering random setup errors and parotid gland migration
(2018) Astaburuaga García, Rosario; Bangert, Mark; Pontificia Universidad Católica de Chile. Facultad de Física
Quantum sensing using NV centers in diamond
(2018) González Brouwer, Raúl, Manuel; Maze Ríos, Jerónimo; Pontificia Universidad Católica de Chile. Facultad de Física
This thesis is focused on understanding the interaction of nitrogen vacancy (NV) defect in diamond with its environment as well as its applications in nanotechnology and biophysics. First, we study the hyperfine interaction due to nearby 13C carbon and 15N nitrogen isotopes in the vicinity of the Level Anti-Crossing (LAC), we propose an experimental method at room temperature free of microwave, in which it is possible to find the sign of the Fermi contact term, using the coupling between nuclear spins 1/2 and electronic spin 1. For this, a three-dimensional coil system was constructed to study the fluorescence close to the LAC. Second we describe an experiment in order to use the NV center as a sensor for chemical reactions, in specific to measure the pH and relate it’s macroscopic measurement with experiments like Ramsey interferometry and change in the spectrum, in order to observe changes on the charge state and T∗2 . Finally, we show a way to functionalize a nanodiamond hosting a NV defect, and use it as a biomarker for amyloid β, an Alzheimer desease related compound. We show that using this functionalized nanodiamonds might enable the development of longer experiments in biophysics due to the photostability of the NV defect compared to regular biomarkers that are widely used in biology. This works paves the way for performing unltrasensitive and reliable detection of cells involved in the pathogenesis of the Alzheimer desease.This thesis is focused on understanding the interaction of nitrogen vacancy (NV) defect in diamond with its environment as well as its applications in nanotechnology and biophysics. First, we study the hyperfine interaction due to nearby 13C carbon and 15N nitrogen isotopes in the vicinity of the Level Anti-Crossing (LAC), we propose an experimental method at room temperature free of microwave, in which it is possible to find the sign of the Fermi contact term, using the coupling between nuclear spins 1/2 and electronic spin 1. For this, a three-dimensional coil system was constructed to study the fluorescence close to the LAC. Second we describe an experiment in order to use the NV center as a sensor for chemical reactions, in specific to measure the pH and relate it’s macroscopic measurement with experiments like Ramsey interferometry and change in the spectrum, in order to observe changes on the charge state and T∗2 . Finally, we show a way to functionalize a nanodiamond hosting a NV defect, and use it as a biomarker for amyloid β, an Alzheimer desease related compound. We show that using this functionalized nanodiamonds might enable the development of longer experiments in biophysics due to the photostability of the NV defect compared to regular biomarkers that are widely used in biology. This works paves the way for performing unltrasensitive and reliable detection of cells involved in the pathogenesis of the Alzheimer desease.

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