Abstract: Apparatus for realtime fast reconstruction and display of dose distribution which calculates the dose of absorbed radiation in an irradiated area defined by coordinates and displays the results of the calculation. The apparatus comprises an input unit capable of setting parameters in the form of time-continuous quantities; a dedicated digital computer serving to control the input by the input unit; and a dedicated fast reconstructer for calculating the absorbed dose for each coordinate in the exposed area at a high speed by decomposing calculation formulae necessary for obtaining isodose curves on the basis of the parameter set in the input unit and effecting parallel calculation of the decomposed formulae, thereby obtaining the isodose curves; and video display for storing and displaying the isodose curves classified in terms of different isodose levels. The calculation is normally cyclically repeated to provide a motion picture of the dose distribution when the magnitude of the parameters is varied.