Monte Carlo simulations for dosimetric verification in photon and electron beam radiotherapy

Dissertação para obtenção do Grau de Doutor em Engenharia Biomédica

One of the primary requirements for successful radiotherapy treatments is the accurate calculation of dose distributions in the treatment planning process. Monte Carlo (MC) dose calculation algorithms are currently recognized as the most accurate method to meet this requirement and to increase even further dose accuracy. The improvements in computer processor technology and the development of variance reduction techniques for calculations have led to the recent implementation and use of MC algorithms for radiotherapy treatment planning at many clinical departments. The work conducting to the present thesis consists of several dosimetric studies which demonstrate the potential use of MC dose calculations as a robust tool of dose verification in two different fields of external radiotherapy: electron and photon beam radiotherapy. The first purpose of these studies is to evaluate dose distributions in challenging situations where conventional dose calculation algorithms have shown some limitations and it is very difficult to measure using typical clinical dosimetric procedures, namely in regions containing tissue inhomogeneities, such as air cavities and bones, and in superficial regions. A second goal of the present work is to use MC simulations to provide a detailed characterization of photon beams collimated by a multileaf collimator (MLC) in order to assess the dosimetric influences of these devices for the MC modeling of Intensity Modulated Radiotherapy (IMRT) plans. Detailed MC model of a Varian 2100 C/D linear accelerator and the Millenium MLC incorporated in the treatment head is accurately verified against measurements performed with ionization chambers and radiographic films. Finally, it is also an aim of this thesis to make a contribution for solving one of the current problems associated with the implementation and use of the MC method for radiotherapy treatment planning, namely the clinical impact of converting dose-to-medium to dose-to-water in treatment planning and dosimetric evaluation. For this purpose, prostate IMRT plans previously generated by a conventional dose algorithm are validated with the MC method using an alternative method, which involves the use of non-standard CT conversion ramps to create CT-based simulation phantoms.

Fundação para a Ciência e Tecnologia; Centro de Física Nuclear da Universidade de Lisboa