A dose error determination device (100) 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-error determining unit (106) configured to generate, using dose plan data for the respective region of interest and an input 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.
