Accuracy of a photogrammetry-based patient positioning and monitoring system for radiation therapy

A photogrammetry system designed to reduce simulator-to-treatment and treat ment-to-treatment patient positioning errors has been developed. Two comple te systems have been installed in our department: one in the simulator room and one in a treatment room. Each system consists of three charge-coupled device (CCD) cameras; a ring of infrared LEDs around the lens of each camer a; and several small, circular, retroreflective markers that are applied to the patient. The markers reflect infrared light directly back to the camer as, producing a binary image of oval hot spots when the image is thresholde d. The three-dimensional position of each marker is calculated by conventio nal photogrammetry methods. At simulation, marker positions are measured, t hen transferred to the treatment room system. The system may be used to act ively position patients, and to passively monitor a patient's position and motion during treatment. Studies have focused on measuring the system's tem poral stability, precision, and accuracy; on optimal positioning of markers and cameras; and on assessing the system's capability to reduce the positi oning error. The repeatability of measuring a marker's position is <0.1 mm in each orthogonal direction. The accuracy is approxi mately 0.5 mm over a 40 X 40 X 40 cm(3) field of view. The system drift over four hours is appro ximately +/-0.2 mm. The photogrammetry system has been used to actively pos ition a lead BE, embedded within a head phantom, at the isocenter; repeatab ility was +/-0.3mm, as determined radiographically. The system has also bee n used to passively monitor the positioning of several head and neck patien ts that were set up by a therapist; setup errors of up to 10 mm in each ort hogonal direction were measured, as well as the motion of the patient durin g treatment. (C) 1999 American Association of Physicists in Medicine. [S009 4-2405(99)00105-4].