Degradation of proton and carbon Bragg peaks due to density inhomogeneities

Abstract

Current analytical treatment planning of radiotherapy with protons and heavier ions neglect degradation of the sharp distal dose falloff (Bragg peak, BP) caused by inhomogeneous tissue. There is no appropriate model of this effect - which in turn allowed reduction of related dose deposition uncertainties. This thesis develops a comprehensive analytical model of the degradation resulting from static lung parenchyma. To do so, it adopts Monte Carlo (MC) simulations, validated by a series of transmission experiments on lung-like phantoms. Fluctuations in the water equivalent thickness (WET) were found the major degradation factor, contributing more than 75% (40%) to the distal falloff widening for a carbon (proton) BP - while energy and particle type were found to have no considerable impact. Also, it was found that the plateau of a clinical spread-out BP remains unaffected but the distal falloff is degraded and that the impact on the biological effect is driven by changes to the physical dose. The model was parametrized with respect to lung specific parametr (alveolar dimension and tissue density) and breathing state parameters (thickness traversed, air filling). Formulation of a Gaussian filter provided a unified, compact and complete description that can readily be implemented in a treatment planning system.