Development and Validation of a 3-Dimensional Breathing Motion Tracking System Using Camera and Laser

Purpose: Respiratory motion during radiotherapy causes geometric uncertainty, compromising tumor targeting and exposing healthy tissue to unintended radiation. Existing three-dimensional (3D) motion tracking systems are either limited to one dimension or prohibitively expensive. This study developed and validated a cost-effective 3D respiratory motion tracking system based on a laser distance sensor and an optical camera, modified for clinical compatibility with linear accelerators. Methods: A Python/OpenCV system integrated a USB camera for two-dimensional (anterior-posterior (AP), left-right lateral (LAT)) motion tracking and an Arduino-interfaced Leuze ODSL 96B laser distance sensor for the superior-inferior (SI) axis. Chessboard-based camera calibration and normalized cross-correlation template matching tracked a chest marker as the moving reference, while a manually selected laser dot served as the fixed spatial anchor. A custom three-axis phantom validated the system at displacements of 5, 10, and 15 mm (AP, LAT) and 10, 20, and 30 mm (SI), compared simultaneously with a clinical Real-Time Position Management (RPM) system under identical conditions. The system was also tested on 8 healthy volunteers (4 males, 4 females) and simultaneously compared with the RPM system. Results: The developed system measured 5.07±0.06 mm, 10.00±0.10 mm and 15.00±0.10 mm (AP), 4.97±0.06 mm, 9.70±0.12 mm and 14.93±0.06 mm (LAT) and 10.00±0.2 mm, 20.03±0.15 mm and 30.00±0.21 mm (SI). The RPM system measured 5.00±0.00 mm, 10.00±0.00 mm and 15.00±0.00 mm (AP), 5.00±0.00 mm, 9.70±0.58 mm and 15.00±0.00 mm (LAT) and 6.70±0.58 mm, 16.33±0.58 mm and 25.33±0.58 mm (SI), underestimating SI motion by 3.30, 3.67 and 4.67 mm respectively. Volunteer testing confirmed that respiratory amplitude waveforms from the developed system were consistent with those simultaneously recorded by the RPM system. Conclusion: The laser and camera-based 3D respiratory motion tracking system demonstrated accuracy comparable to that of the commercial RPM system in AP and LAT, while exceeding the RPM particularly in the SI direction where the RPM showed a clinically relevant underestimation. This non-ionizing, cost-effective solution shows strong potential for clinical integration in respiratory-gated radiotherapy, especially in resource-limited cancer treatment centers.