In targeted radiation therapy such as intensity-modulated radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT), it is critical to position a patient in a treatment position accurately and reproducibly. It is also highly desirable to have accurate and continuous localization of the patient throughout radiation delivery, especially for a patient having lesions in thoracic and abdominopelvic region. It is not uncommon to observe abrupt patient motion due to patient voluntary movement, cough, irregular breathing, or dyspnea. Without accurate and continuous monitoring of the patient, it is difficult to decide if the patient displacement is within the tolerance margin.
Known x-ray based image guidance systems provide adequate accuracy for initial patient positioning, but most of them are not appropriate for real-time and continuous monitoring during the treatment. Fluoroscopy makes it possible to view internal organ motion in real time with the help of surgical clips or fiducial markers. But continuous fluoroscopy during long treatment durations is associated with the risk of excessive radiation exposure to the patient. Gantry-mounted, kilovoltage cone-beam computed tomography (CBCT) systems are quickly gaining popularity for patient positioning and target localization due to its superb capability of visualizing a soft-tissue target. However, CBCT is limited to an angle of a treatment couch of 0° and CBCT cannot acquire images in real-time.
Optical tracking systems present an attractive solution for continuous monitoring of patient during radiation delivery. There are two types of optical tracking systems developed for the purpose of radiotherapy: 1) a body marker-based optical tracking system uses infrared (IR) light to detect target position via active or passive reflective markers affixed to the patient, and 2) a three-dimensional (3D), surface image-based optical tracking system that captures patient surface images in real time and relates the surface images to target position. Both types of optical tracking systems are noninvasive and nonionizing.
Body marker-based optical tracking systems have been widely used for initial patient localization as well as for patient monitoring because of their high spatial resolution, real-time measurement and ease of implementation.
Ambiguity occurs when two cameras are coplanar with two true markers, and the cameras see four markers (two true markers and two ambiguous markers). When an ambiguous marker is present, all known body marker-based optical tracking systems fail to capture (reset zero after CBCT shifts). As a result, a time consuming trial and error process must then be used to remove one of the two true markers that caused the ambiguous marker. Additionally, a reduction in number of true markers for any reason, including the removal of a true marker to eliminate the ambiguous marker, degrades accuracy.
Several known commercially-available, marker-based, optical tracking systems have been developed for the use in radiation therapy.
The first known system, RadioCamera™ (ZMed/Varian, Inc., Ashland, Mass.), uses two ceiling-mounted charge-coupled device (CCD) infrared cameras to detect four passively reflective markers rigidly affixed to a reference array (“biteplate”) that links to maxillary dentition of the patient. There are at least two factors limiting use of the first known system in the treatment of lesions in areas such as thoracic and abdominopelvic region: 1) the first known system is specifically designed for the treatment of intracranial lesion and requires the patient to bite a biteplate, and 2) the first known system tracks a rigid, pre-defined marker pattern which disallows a user from choosing other marker patterns.
A second known commercially available optical tracking system, ExacTrac® (BrainLab AG, Munich, Germany), uses two ceiling-mounted CCD infrared cameras to track passively reflective markers. The second known system is designed to track non-rigid body markers affixed directly on the patient's skin. Patient positioning with the second known system has been reported to significantly improve overall setup accuracy at various treatment sites such as head and neck, lung, prostate and pelvic. The second known system is used for initial patient positioning as well as real-time monitoring for SBRT patients treating lung and liver lesions. Treatment flow begins with a CT simulation where six (6) to eight (8) IR markers are affixed to a patient's chest and abdominal region in an irregular pattern. Locations of IR markers and their relationship with respect to the treatment isocenter are semi-automatically identified with software. At a time of treatment, the patient is pre-positioned with the guidance of the second known system. Then, a CBCT scan is acquired with an x-ray volume imaging (XVI) system. A CT to CBCT registration is performed to correct residual target displacement. Isocenter shifts in the second known system need to be reset before continuous monitoring starts. To do this, the second known system captures current marker positions for tracking. Treatment can be interrupted when patient displacement exceeds predetermined tolerances. In such cases, the second known system provides a quick way to reposition patient without additional x-ray imaging. However, the second known system suffers from the ambiguous markers problem and may fail to capture the marker positions. Two ambiguous markers appear when two real markers fall in a same plane as the two CCD cameras. This phenomenon leads to a system failure in capturing new marker positions and consequently, a significant delay in patient treatment. It has been reported that the second known system failed to unambiguously recognize marker patterns in 21% of the fractions for prostate treatment, resulting in an inability to position the patient with the camera. It has also been reported that ambiguity was possible when more than five markers were used. For this reason, with the second known system, it has been reported that the number of external markers should be limited to five, although more markers has been deemed desirable in a study on a correlation between external markers and internal target. The second known system is used for various treatment sites including thoracic and abdominopelvic regions. Its accuracy has been proven for pre-treatment setup and real-time monitoring. However, the second known system suffers from ambiguous markers which limits flexibility in marker placement at CT simulation and leads to significant delay in patient treatment. When two markers are placed in the same plane as cameras, two ambiguous markers are created and the second known system disadvantageously cannot recognize the marker pattern for tracking.
A third known commercially-available, marker-based, optical tracking system is AlignRT® system (Vision RT Limited, London, UK). A fourth known commercially-available, marker-based, optical tracking system is the Real-time Position Management™ system (Varian Medical Systems, Inc., Palo Alto, Calif.).
All known, commercially-available, body marker-based optical tracking systems suffer from an ambiguous markers problem.
In an attempt to establish a one-to-one correspondence between reference markers and optical markers without manual marker labeling, one known method is based on 2D specificity of a marker disposition model. In such known method, all marker positions are projected onto the 2D plane of cameras for classification. This known method has two constraints: 1) missing markers or phantom markers are not allowed; and 2) patient motion must not alter the 2D geometrical peculiarities of the marker disposition model.