Intro -- POPULÄRVETENSKAPLIG SAMMANFATTNING -- ABSTRACT -- Acknowledgments -- Contents -- List of Figures -- List of Tables -- Introduction -- Aim -- Research questions -- Thesis outline -- Abbreviations -- Radiotherapy -- Radiobiology -- Radiotherapy modalities -- Physics of Cobalt-60 radiation -- Treatment planning -- Magnetic resonance imaging -- The origin of the MR signal -- Pulse sequences -- Signal localization -- Diffusion MRI -- Diffusion -- Measuring diffusion with MRI -- Diffusional variance decomposition (DIVIDE) -- Probabilistic modelling -- Regression models in diffusion MRI -- Bayesian linear regression -- Gaussian processes -- The connection between Bayesian linear regression and Gaussian processes -- Summary of papers -- Bibliography -- Papers.