Machine for intraoperative radiation therapy or IORT (Intra Operative Radio Therapy), comprising a body, a radiating head (23) connected to the body, and a shield for X rays. The shield comprises a mobile stand (29) supporting an absorbing mass (13), which is coupled, through connecting means (32), to means (33) for detecting the position of the absorbing mass (13), apt to give to a processor at least a signal indicating the position of the absorbing mass (13) with respect to the body of the machine. The processor determines, on the basis of the position and/or the orientation of the radiating head (23), the position of the absorbing mass (13) with respect to the electron beam axis.