The invention relates to a planning tool for the interactive selection of control variables (x.sub.1, y.sub.7, .alpha..sub.1, .alpha..sub.2, .alpha..sub.3, I.sub.1, I.sub.2) of a radiation therapy plan from a database (1) comprising a plurality of pre-calculated solutions. Each solution represents a radiation therapy plan which, from a technical point of view, consists of a plurality of control variables or instructions. Each solution also has characteristic values for radiation doses for a target volume (T, Target) and at least one risk volume (risk organ, h.sub.1, h.sub.2, h.sub.3) which are stored in the database (1). A plurality of axes are visibly represented on a display device (3; 3a, 3b) as radiation dose scales (30, 31, 32, 33) for the target volume (T) and the at least one risk volume (h.sub.1, h.sub.2, h.sub.3), for the formation of at least one risk axis and one target axis. The characteristic values (50, 51, 52, 53) of the radiation doses for at least a plurality of the stored solutions are allocated to the respective corresponding axes in such a way that an acceptance interval (a.sub.30, a.sub.31, a.sub.32, a.sub.33) is created for each risk axis (31, 32, 33) and the target axis (30), said acceptance interval determining a common planning area (40; 41, 42) for all axes. Said planning area (40) is visibly emphasised on the display device (3) in relation to the surrounding field (45, 45a).