Apparatus and methods for treating a patient using radiation therapy. In one embodiment, an apparatus comprises a tube configured to receive a radiation source and an expandable member. The tube has a first end configured to be inserted into a patient and a second end that is generally configured to remain external to the patient. The expandable member is at the first end of the tube, and it is configured to contain the radiation source. The expandable member can comprise a balloon, flexible bladder, mechanical linkage (e.g., a cage), a mesh, or other suitable expandable systems. The apparatus further includes a marker associated with the expandable member such that the marker moves with the expandable member.