Alarcón Paredes, Alberto (2016) Modelado de colimadores multilámina en un sistema de planificación de radioterapia con haces externos de fotones. / Modeling multileaf collimators in radiotherapy planning system with external photons. Maestría en Física Médica, Universidad Nacional de Cuyo, Instituto Balseiro.
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Resumen en español
La determinacón precisa de la transmisión de rayos X a través de un colimador multiláminas, es importante para la correcta planificación del tratamiento, especialmente en técnicas con intensidad modulada. Este tipo de técnicas presentan tiempos de irradiación mayores, respecto a técnicas convencionales. Por lo tanto, la dosis debido a la radiación de fuga a través de colimador multiláminas, puede llegar a niveles significativos de dosis en regiones que no se desea irradiar, como ser los órganos en riesgo donde la dosis debe minimizarse. El objetivo de este trabajo fue modelar distribuciones de fluencia energética para todo tipo de forma de campo generados por el colimador multiláminas y/o colimadores secundarios. Para cumplir con este objetivo se desarrollo un modelo para el cálculo de la fluencia energética considerando la influencia del sistema de colimación total, para el acelerador lineal Varian Clinac iX de 6 MV con colimadores multiláminas Varian Millenium 120, usando un algoritmo tipo ray tracing para el calculo de la atenuación y el modelo de fuentes virtuales para el modelado de la fluencia energética. El paso crucial en el desarrollo del algoritmo de ray tracing fue considerar la exacta geometría de las tres principales láminas que componen el colimador multiláminas, con detalles geométricos tales como el agujero del tornillo de conducción, el carril de soporte y las puntas redondeadas de las láminas. Esta geometría es útil para encontrar las intersecciones de rayos con las representaciones poliédricas de cada tipo de lámina, utilizando procedimientos geométricos, tales como, la intersección de una recta con planos y cilindros. Además, se utilizaron técnicas de geometría computacional, por ejemplo, el método de winding number para analizar, con alta eficiencia computacional, si las intersecciones encontradas pertenecen a los límites establecidos de las láminas. Las intersecciones encontradas sirven para calcular la distancia recorrida del rayo dentro de una o varias láminas y así poder calcular la atenuación correspondiente. La evaluación de este procedimiento para una cantidad elevada de rayos produce un mapa de atenuación propia del colimador multilámina, ubicado en el plano isocéntrico. Este mapa de atenuación y las ventanas de atenuación producidas por los colimadores secundarios representan la atenuación total del sistema de colimación del acelerador. Para modelar la fluencia energética, se implementó el modelo de fuentes virtuales basados en trabajos previos y re-validado en este trabajo. La distribución de fluencia energética final considerando el tamaño finito de las fuentes es obtenida, convolucionando el mapa de atenuación con las distribuciones de intensidad, propias del modelo de fuentes virtuales. Todos los algoritmos fueron desarrollados en Python, para su posterior optimización. Las distribuciones de fluencia energética obtenidas con el modelo propuesto fueron verificados con el código de simulación Monte Carlo BEAMnrc, mediante la validación de perfiles de fluencia para campos cuadrados e irregulares formados por los colimadores secundarios y el colimador multiláminas. Los resultados muestran que existe una buena correlación entre el modelo y Monte Carlo. La comparación de los perfiles de radiación de fuga entre láminas y transmisión a través de las puntas redondeadas, predijeron con buena precisión los detalles esperados. También, se simularon distribuciones de intensidad modulada usando técnicas estáticas con buenos resultados cualitativos. Basados en los resultados obtenidos con el algoritmo presentado en este trabajo, se puede afirmar que es una herramienta útil para cálculo de fluencia energética para cualquier forma o tamaño de campo, con varias posibles aplicaciones clínicas, incluyendo el control de calidad del colimador multiláminas y el continuo desarrollo de modelos para el cálculo de la dosis basados en fluencia energética. Se deja como trabajos futuros el acoplamiento de este método a técnicas de Convolución/Superposición para el cálculo de dosis y la implementación de algoritmos de optimización de posicionamiento de las láminas para realizar planificación inversa asociada a técnicas moduladas.
Resumen en inglés
The precise determination of X-ray transmission through a multileaf collimator is important for a correct treatment planning, especially in intensity modulated techniques, given the longer irradiation time compared to conventional techniques. Therefore, this type of technique can produce significant dose levels in protected regions, such as the organs at risk where the dose must be minimized, due to leakage radiation through the multileaf collimators. This work’s aim was to model the energy fluence distributions for every type of radiation field generated by the multileaf collimator and/or secondary collimators. In order to reach this goal, an energy fluence model for the Varian Clinac iX 6 MV with Millenium 120 multileaf collimator system was developed. The implemented model employs a ray tracing algorithm and includes geometric details, such as the screw hole for leaf conduction, support rail, and rounded end tips. The crucial step in the development of the ray tracing algorithm was to consider the exact geometry of the three main leaf type that make up the multileaf collimator, with geometric details such as the hole of the driving screw, the support rail and the rounded tips of the leaf. This geometry is useful for finding intersections of the rays with the polyhedral representations of each type of leaf, using geometric procedures, such as the intersection of a straight line with planes and cylinders. In addition, computational geometry techniques were used, for example, the winding number method, to analyze, with high computational efficiency, if the intersections found belonged to the geometric limits of the leaves. The intersections found are used to calculate the distance traveled by the ray within one or several leaves and thus calculate the corresponding attenuation. The evaluation of this procedure for a high amount of rays produces an attenuation map of the multileaf collimator. In order to model the energy fluence, a virtual source model was implemented based on previous works, which parameterize the various components that are present in the accelerator’s head, and allow to model the energy fluence. In turn, using the method of ray tracing, an algorithm was developed which calculates the path length travelled by the rays inside the multileaf collimator using the exact geometry of the leaves. This results allows obtaining a total attenuation map generated by the jaws and the multileaf collimator in the isocentric plane considering a point source of radiation. The final energy fluence distribution considering the sources’finite size is obtained, convolving the attenuation map with the intensity distributions, characteristic of the virtual source model. All the algorithms were developed in Python, for later optimization. As an initial code validation, the leavesındividual geometries were visualized through the rays´ıntersection points with each leaf. This test’s result shows a good visual correlation between the implemented geometry and the leavesactual shape. The energy fluence distributions obtained with the proposed model were verified with the Monte Carlo BEAMnrc simulation code, by validating fluence profiles for square and irregular fields formed by the jaws and the multileaf collimator. The results show a good correlation between the model and Monte Carlo simulations. The comparison between the interleaf, intraleaf leakage and transmission through rounded end tips accurately predicted the expected details. Also, modulated intensity distributions were simulated using static techniques with good qualitative results. Based on the results obtained with the algorithm presented in this work, it can be affirmed that it is a useful tool for calculating energy fluence for any shape or field size, with several possible clinical applications, including the multileaf collimator’s quality assurance and the continuous development of models for dose calculation based on energy fluence. Prospective work includes the coupling of this method to Convolution/Superposition techniques for dose calculation, and the implementation of algorithms for the optimization of leaves positioning to perform inverse planning associated with modulated techniques
| Tipo de objeto: | Tesis (Maestría en Física Médica) |
|---|---|
| Información Adicional: | Este trabajo fue realizado en: Laboratorio de Física Médica computacional del Centro Atómico Bariloche. Área Temática: Física médica computacional. |
| Palabras Clave: | Collimators; Colimadores; Radiotherapy; Radioterapia; Photons; Fotones; [Multileaf collimator; Colimador multilámina] |
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| Materias: | Medicina > Radioterapia |
| Código ID: | 580 |
| Depositado Por: | Tamara Cárcamo |
| Depositado En: | 19 Abr 2017 15:16 |
| Última Modificación: | 19 Abr 2017 15:30 |
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