Abstract: The invention relates to a mobile building unit which is assembled to a building including at least one room (3) enclosed by walls (6), a roof (7) and a floor (12) for accommodating radiating equipment (5) and for treatment, therapy or diagnosing by means of ionizing radiation. The walls, the roof and floor of said building (1, 1′) serve as a radiation shielding barrier for preventing radiation at health-impairing levels from escaping to the outside of the building structure. At least two of the walls and the roof of the building, has the form of a double walled structure comprising an inner (8) and an outer (9) partition element with a space (10) therebetween.