A method and apparatus are used to compare in real-time an intended radiation treatment plan with a delivered plan. Markers are arranged, on a subject receiving radiation, at three-dimensional (3D) positions between a two dimensional radiation detector and a radiation source and are used to reconstruct the actual position of a high dose rate (HDR) source in three-dimensional (3D) space. The detected position is compared with the intended radiation path of the treatment plan. Adjustments can be made to the treatment plan if the detected position and the intended path do not correspond.
