A dose error determination device for forecasting radiation dose error for a body volume comprising one or more regions of interest subject to radiation treatment based on a dose plan, the dose error determination device comprising: - a dose plan unit configured to provide dose plan data indicative of dose values associated with voxels of a current image of the body volume comprising voxels associated with the region of interest; - a dose-error determining unit, which is configured to generate, using the dose plan data for the respective region of interest and a value of a spatial-error quantity, dose variation data for each voxel; and characterized by - an input unit configured to request and receive user input indicative of the value of a spatial-error quantity corresponding to an amount of a spatial error of voxel coordinates in the current image associated with corresponding features of the body volume; - and by the dose-error determining unit, being configured:: - to generate, using the dose plan data for the respective region of interest and the value of the spatial-error quantity, dose variation data indicative of a rate of change of the dose values in a respective modified region of interest that in comparison with the respective region of interest is expanded in volume by an amount corresponding with the value of the spatial-error quantity; and - to determine and provide a respective value of a dose-error quantity associated with the respective region of interest, using a calculation rule representing a positive correlation of the dose-error quantity with the determined dose variation data; and by - an output unit, which receives the respective value of the dose-error quantity, and is configured to provide an output indicative thereof. - a dose plan unit configured to provide dose plan data indicative of dose values associated with voxels of a current image of the body volume comprising voxels associated with the region of interest; - a dose-error determining unit, which is configured to generate, using the dose plan data for the respective region of interest and a value of a spatial-error quantity, dose variation data for each voxel; and characterized by - an input unit configured to request and receive user input indicative of the value of a spatial-error quantity corresponding to an amount of a spatial error of voxel coordinates in the current image associated with corresponding features of the body volume; - and by the dose-error determining unit, being configured:: - to generate, using the dose plan data for the respective region of interest and the value of the spatial-error quantity, dose variation data indicative of a rate of change of the dose values in a respective modified region of interest that in comparison with the respective region of interest is expanded in volume by an amount corresponding with the value of the spatial-error quantity; and - to determine and provide a respective value of a dose-error quantity associated with the respective region of interest, using a calculation rule representing a positive correlation of the dose-error quantity with the determined dose variation data; and by - an output unit, which receives the respective value of the dose-error quantity, and is configured to provide an output indicative thereof.