A system for monitoring changes during therapy includes a first probing segment (112) having an optical fiber sensor disposed therein. The first segment is percutaneously inserted in or near a target area and providing a local reference for one or more treatment devices. A second probing segment (114) has an optical fiber sensor disposed therein. The second segment is generally disposed apart from the first probe and provides a spatial reference point for the first segment. The first and second segments have at least one common position to function as a reference between the first and second probes. A shape determination method (107) is configured to determine a shape of each of the first and second segments based on feedback signals to measure changes in the shapes during a procedure and update a therapy plan in accordance with the changes.