The invention provides a device for thermal ablation at a site in a subject. The apparatus comprises an expandable bladder； a displacement chamber having a variable volume； an elongated catheter forming fluid communication between the bladder and the displacement chamber； and a heater for heating the fluid. To obtain an improved pressure control and a reliable structure, the chamber comprises a syringe structure including a piston movable in a cylinder by power driven means for varying the volume of the displacement chamber.
