Cobos , Agustín C. (2017) Desarrollo e implementación de técnicas alternativas de modulación en haces externos de fotones y electrones de alta energía para radioterapia de intensidad modulada. / Research and implementation of intensity modulated radiation therapy for high energy photon and electron beams using alternative techniques. Tesis Doctoral en Física, Universidad Nacional de Cuyo, Instituto Balseiro.
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Resumen en español
La calidad de un tratamiento de radioterapia se asocia a factores clínicos y físicos. Los factores físicos se vinculan con el logro seguro de la dosis prescripta en el volumen tumoral y de las dosis de tolerancia en los tejidos aledaños. De esta manera se aumenta la probabilidad de éxito del tratamiento, se producen resultados comparables entre diferentes instituciones y se disminuye la probabilidad de ocurrencia de accidentes radiológicos. La radioterapia de intensidad modulada (IMRT) con haces externos de fotones y electrones es una técnica universalmente aceptada en la que la fluencia de energía varía en la sección de cada haz de radiación, permitiendo lograr un mayor grado de conformidad de la distribución de dosis con el volumen tumoral, en comparación con las técnicas de radioterapia convencionales. La modulación de dicha fluencia se puede lograr mediante el uso de colimadores de hojas múltiples o mediante filtros compensadores. Estos últimos son una muy buena alternativa en los países emergentes debido a su bajo costo relativo. En este sentido, la Fundación Escuela de Medicina Nuclear (FUESMEN) ha desarrollado e implementado una técnica propia que permite elaborar filtros compensadores con costos y tiempos de fabricación diez veces menores que los filtros convencionales. El mayor grado de conformidad de las distribuciones de dosis en el volumen tumoral hace que la IMRT presente más exigencias en términos de las incertezas espaciales debidas a las tolerancias mecánicas de la máquina de irradiación, el posicionamiento y los cambios anatómicos del paciente, y el movimiento de órganos. Dicha exigencia ha motivado el desarrollo de nuevas tecnologías e investigaciones enfocadas en las distribuciones de dosis absorbida en el volumen tumoral y en los órganos de riesgo. El cálculo de estas distribuciones de dosis ha sido realizado históricamente mediante los métodos de convolución y convolución-superposición, siendo el método de simulación por Monte Carlo la herramienta típicamente usada para su validación. Si bien los modelos determinísticos de la ecuación de transporte de Boltzmann pueden tener la misma exactitud que los métodos de Monte Carlo, su aplicación en problemas de radioterapia ha sido muy escasa y reciente debido a la falta de memoria computacional previa para manipular la información necesaria para su resolución. La calidad de los servicios de radioterapia también se asocia al cumplimiento y mejora de la protección radiológica del paciente, trabajadores y miembros del público. Dicha protección radiológica conlleva la disminución de las dosis en el cuerpo del paciente y en localizaciones particulares dentro y fuera de las salas de tratamientos, lo que afecta el diseño estructural de estas últimas. Otra característica de la IMRT es que el tiempo de irradiación requerido para lograr la misma dosis de planificación, es mucho mayor que el correspondiente al de radioterapia convencional, lo que produce un incremento de la dosis absorbida en el cuerpo del paciente, aunque fuera del haz primario de radiación (dosis periférica). Dicho incremento es originado básicamente por el aumento de la radiación de fuga del cabezal del acelerador y por el aumento de radiación dispersada desde los colimadores y filtros compensadores. Si bien el estudio de la dosis periférica ha sido tema de interés debido a que puede tener una variedad de efectos sobre la salud del paciente, existen pocos estudios al respecto dada la dificultad de su determinación experimental y teórica. Como primer trabajo de esta tesis, se presentó un novedoso enfoque determinístico que consistió en el uso de la ecuación de transporte dependiente del tiempo como intermediaria para la obtención de las soluciones correspondientes al estado estacionario. El método demostró tener muchas ventajas en cuanto a exactitud, sencillez y velocidad con respecto a los métodos estacionarios convencionales de resolución de la ecuación de transporte. Se obtuvieron por primeros principios los criterios suficientes para la convergencia y se compararon resultados con cálculos de Monte Carlo a los efectos de asegurar la exactitud del método. El método numérico fue usado como herramienta de cálculo recurrente en todos los modelos dosimétricos presentados en esta tesis. En primer lugar, se justificó y demostró que un modelo de transporte unidimensional con simetría azimutal es una suposición aceptable para calcular la transmisión de las barreras primarias de salas de radioterapia, necesarias para el diseño de instalaciones. Aprovechando la practicidad del modelo, se realizó un estudio sobre la conveniencia radiológica que puede haber entre distintas configuraciones de barreras laminadas de materiales mixtos. Por otro lado, como parte de la mejora y caracterización de los filtros compensadores de IMRT que han sido implementados en la FUESMEN, se determinaron experimentalmente sus contribuciones a la dosis periférica producida en el cuerpo del paciente. Con el objeto de complementar y mejorar los tratamientos de IMRT, se desarrolló e implementó en los sistemas de planificación un modelo teórico que permite el cómputo de dicha dosis periférica. Finalmente, se demostró que la dosis periférica es lo suficientemente significativa y debe, por lo tanto, ser tenida en cuenta en el diseño de barreras secundarias de salas de IMRT. Se concluye que el nuevo método numérico desarrollado en esta tesis demostró tener gran robustez, pudiéndose extender a otras áreas de la ciencia. Las aplicaciones unidimensionales presentadas resultaron muy útiles para la mejora de la protección radiológica de los pacientes, trabajadores ocupacionalmente expuestos y miembros del público involucrados en IMRT. Se propone a futuro extender el método numérico a otros tipos de partículas y a problemas tridimensionales para cálculos dosimétricos dentro del campo de radiación y en la periferia cercana.
Resumen en inglés
Radiotherapy treatment quality depends on physical and clinical factors. The physical factors influence how the medically prescribed dose to the tumoral volume and an adequate dose to the surroundings tissues are achieved in an assured and controlled fashion. Following strict protocols and high fidelity models, the probability of success of the treatment increases, different institutions produced readily intercomparable results, and the probability of radiological accident decreases. Intensity Modulated Radiation Therapy (IMRT) with external photons and electrons beams is a universal and accepted technique in which the energy fluence changes with the beam section. Therefore, this technique allows a higher control of the dose conformity mapping over the tumoral volume compared to other conventional radiotherapy techniques. Fluence modulation can be achieved by multileaf collimators or compensator filters. The latter ones are a good option for developing countries, especially due to its relative low cost. In this sense, the Fundación Escuela de Medicina Nuclear (FUESMEN) has already developed compensator filters that can be manufactured at ten times lower cost and faster than conventional filters. The higher degree of conformity in the dose mapping around the tumoral volume results in higher requirements of precision for the IMRT planning protocols due to mechanical tolerances in the irradiation machine, positioning and anatomical changes in the patient, and organs movement. Following these demands, new technologies and researches have been developed focused on the distribution of the absorbed dose in the tumoral volume and the organs at risk. In this regard, dose distribution calculations have historically been obtained by convolution and convolution-superposition methods. Monte Carlo simulations were then applied as a validation method only. Even though Boltzmann transport equation deterministic models could have the same accuracy as Monte Carlo methods, their application has been sparse in the past. The main reason deterministic methods were relegated in the past is the high volumes of machine memory needed for the calculations to reach an adequate resolution in the results. With the advent of accessible powerful and large memory computers, this historical trend may be reverting and deterministic calculation applications in commercial planning software are recently appearing. Radiotherapy services quality is also associated with the compliance and improvement of the radiological protection of patients, workers and members of the public. Such radiation protection principles lead to the reduction of doses in the patient's body and in particular locations inside and outside the treatment rooms, which affects the structural design of the latter. Another characteristic of the IMRT is that the irradiation time required to achieve the same planning dose is much higher than that of conventional radiotherapy, which results in an increase of the absorbed dose in the patient's body outside the beam primary radiation (peripheral dose). This increase is basically caused by the increase in the leakage radiation from the accelerator head and by the increase in radiation dispersed from the collimators and compensator filters. Although the study of the peripheral dose is a topic of recent interest because it can have a variety of effects on the health of the patient, there are few studies in this regard given the difficulty of their experimental and theoretical determination. In the context of the above presented background, this thesis first proposes a novel deterministic approach which consists in the use of the time-dependent transport equation as an intermediary to obtaining the solutions of the corresponding steady state problem. The method has proved to have many advantages in terms of accuracy, simplicity and speed with respect to the conventional stationary methods of solving the transport equation. Sufficient convergence criteria were obtained from first principles and results were compared with Monte Carlo calculations in order to ensure the accuracy of the method. The newly developed numerical method has been used as a recurrent calculation tool in all the dosimetric models presented in this thesis. On the one hand, a onedimensional transport model with azimuth symmetry is shown to be an acceptable assumption to calculate the transmission of the primary barriers of radiotherapy room, necessary for the design of facilities. Taking advantage of the practicality of the model, a study was made on the radiological suitability and convenience of different configurations of laminated barriers of mixed materials. On the other hand, as part of the improvement and characterization of the compensator filters of IMRT that have been implemented in the FUESMEN, their contributions to the peripheral dose produced in the patient's body were experimentally determined. In order to complement and improve the planning of IMRT treatments, a theoretical model was developed and implemented in the planning system that allows for the computation of this peripheral dose. Finally, the peripheral dose is shown to be sufficiently significant and should therefore be taken into account in the design of secondary barriers of IMRT rooms. In conclusion, the new numerical method developed in this thesis proved to have great robustness and could be easily extended to other areas of science. The onedimensional applications presented were very useful for improving the radiological protection of patients, occupationally exposed workers and members of the public involved in IMRT. Future work to extend the numerical method to other types of particles and to three-dimensional problems for dosimetric calculations within the radiation field and in the near periphery, is proposed.
| Tipo de objeto: | Tesis (Tesis Doctoral en Física) |
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| Palabras Clave: | Radiotherapy; Radioterapia [Primary barrier; Barrera primaria; Secondary barrier; Barrera secundaria; Peripheral dose; Dósis periférica] |
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| Materias: | Medicina > Radioterapia |
| Divisiones: | Gerencia Centro Integral de Medicina Nuclear y Radioterapia de Bariloche |
| Código ID: | 648 |
| Depositado Por: | Tamara Cárcamo |
| Depositado En: | 16 Abr 2018 15:42 |
| Última Modificación: | 16 Abr 2018 15:52 |
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