The present invention concerns a device for shaping an electron beam of a machine for intraoperative radiation therapy (IORT—Intra Operative Radiation Therapy) using a tubular applicator (3) having a duct through which the electron beam is transmitted, the device being characterised in that it comprises a slab (1), provided with a hole (2) corresponding to the duct of the tubular applicator (3), and at least one planar element (10) comprising an upper plate (11) and a lower plate (12) removably attachable to each other through first mechanical coupling means (13), said at least one planar element (10) being removably attachable to the slab (1) through second mechanical coupling means (13, 14) so that the arrangement of said at least one planar element (10) with respect to the slab (1) is adjustable so as to define an aperture (20) of lower area than that of the section of the duct of the tubular applicator (3), the upper plate (11) of each planar element (10) being made of a first sterilisable biocompatible material with a first atomic number, the lower plate (12) of each planar element (10) being made of a second sterilisable biocompatible material with a second atomic number larger than the first atomic number, whereby the lower plate (12) attenuates X-rays produced by the upper plate (11).The present invention further concerns an automatic method for computing distribution of dose radiated by a IORT machine through such device for shaping an electron beam.