A system for endoscopic surgery within a body lumen of a patient including a flexible catheter having an expandable balloon at a distal portion and an access opening. The expandable balloon is expandable from a collapsed insertion configuration to an expanded configuration to provide an expanded chamber on a first side of the catheter. The access opening provides a window to access target tissue. The catheter includes a lumen dimensioned to receive an endoscopic instrument therethrough such that a distal end of the endoscopic instrument is positionable within the expanded chamber and angled laterally within the expanded chamber to access the target tissue through the window.