RESUMEN
Density heterogeneities can have a profound effect on dose distributions for proton therapy. Although analytical calculations in homogeneous media are relatively straightforward, the modelling of the propagation of the beam through density heterogeneities can be more problematical. At the Paul Scherrer Institute, an in-house dedicated Monte Carlo (MC) code has been used for over a decade to assess the possible deficiencies of the analytical calculations in patient geometries. The MC code has been optimized for speed, and as such traces primary protons only through the treatment nozzle and patient's CT. Contributions from nuclear interactions are modelled analytically with no tracing of secondary particles. The MC code has been verified against measured data in water and experimental proton radiographs through a heterogeneous anthropomorphic phantom. In comparison to the analytical calculation, the MC code has been applied to both spot scanned and intensity modulated proton therapy plans, and to a number of cases containing titanium metal implants. In summary, MC-based dose calculations could provide an invaluable tool for independently verifying the calculated dose distribution within a patient geometry as part of a comprehensive quality assurance protocol for proton treatment plans.
Asunto(s)
Neoplasias/radioterapia , Protones , Planificación de la Radioterapia Asistida por Computador/métodos , Radioterapia Asistida por Computador/métodos , Algoritmos , Femenino , Humanos , Masculino , Modelos Teóricos , Método de Montecarlo , Aceleradores de Partículas , Fantasmas de Imagen , Radiometría , Dosificación Radioterapéutica , Radioterapia Conformacional/métodos , Radioterapia de Alta Energía , AguaRESUMEN
A method was developed for calculating range spectra of charged particles after passing through an inhomogeneous structure whose thickness was comparable to the range of the incident particles. It was shown that the spectra are strongly affected by the influence of multiple Coulomb scattering at interfaces parallel to the beam direction of two media with different relative stopping power. The calculations are in agreement with Monte Carlo simulations. The degraded Bragg peak was calculated on the basis of the computed range spectra behind the inhomogeneity interface. The method can be included into charged particle treatment planning systems where broad pencil beams are used to predict the deteriorated Bragg peak behind inhomogeneity interfaces more precisely.
