SU‐FF‐J‐21: An External‐Marker Based Non‐Respiratory Body Motion Monitoring System

The development of new technologies has leaded a trend of tighter target margin and longer treatment procedures in the new era of image‐guided radiotherapy. This prompts a concern of potential body motion of a patient during the treatment, especially for stereotactic body radiosurgery. The purpose of this study is to develop a technique to monitor the non‐respiratory motion of a patient, separated from the relatively large respiratory motion. BrainLab ExacTrac system is used as the platform for the study. Multiple infrared external markers are placed on a patient's chest and abdominal surfaces. The position of each marker is monitored with two infrared cameras. The motion signal of each marker, which is the mixture of respiratory and non‐respiratory motions, is recorded. The markers on the chest surface show much smaller, but correlated respiratory signals than those on the abdominal surface. The non‐respiratory component can be derived from the motion signal at the chest surface subtracting the respiratory component represented by the motion signal at the abdominal surface. A computer program has been written to calculate the two motion components in real‐time and was retrospectively tested in patients enrolled in a 4D‐gated‐CT protocol. Total of 24 patients were tested. The duration of recorded motion signals ranged from 10 to 25 minutes for different patients. The extracted non‐respiratory motion signal showed reduced respiratory ripples of less than 0.5 mm for most of the patients, with the maximal ripple of about 1.2 mm. It also showed slow‐drifts as well as some sharp spikes or irregular fluctuations. The slow‐drifts were <1.5mm for 8 patients, >1.5mm and <2.5mm for 6 patients, >2.5mm and <3.5mm for 5 patients, and >3.5mm for 3 patients with maximal drift of 6.5mm. This result suggests that monitoring non‐respiratory motion during treatment might be necessary.