Clinical accuracy of the respiratory tumor tracking system of the cyberknife: assessment by analysis of log files.
OBJECTIVE
To quantify the clinical accuracy of the respiratory motion tracking system of the CyberKnife treatment device.


METHODS
Data in log files of 44 lung cancer patients treated with tumor tracking were analyzed. Errors in the correlation model, which relates the internal target motion with the external breathing motion, were quantified. The correlation model error was compared with the geometric error obtained when no respiratory tracking was used. Errors in the prediction method were calculated by subtracting the predicted position from the actual measured position after 192.5 ms (the time lag to prediction in our current system). The prediction error was also measured for a time lag of 115 ms and a new prediction method.


RESULTS
The mean correlation model errors were less than 0.3 mm. Standard deviations describing intrafraction variations around the whole-fraction mean error were 0.2 to 1.9 mm for cranio-caudal, 0.1 to 1.9 mm for left-right, and 0.2 to 2.5 mm for anterior-posterior directions. Without the use of respiratory tracking, these variations would have been 0.2 to 8.1 mm, 0.2 to 5.5 mm, and 0.2 to 4.4 mm. The overall mean prediction error was small (0.0 +/- 0.0 mm) for all directions. The intrafraction standard deviation ranged from 0.0 to 2.9 mm for a time delay of 192.5 ms but was halved by using the new prediction method.


CONCLUSIONS
Analyses of the log files of real clinical cases have shown that the geometric error caused by respiratory motion is substantially reduced by the application of respiratory motion tracking.
Department of Radiation Oncology, Erasmus MC-Daniel den Hoed Cancer Center, Rotterdam, the Netherlands.
Mischa
M
Hoogeman
Jean-Briac
JB
Prévost
Joost
J
Nuyttens
Johan
J
Pöll
Peter
P
Levendag
Ben
B
Heijmen
D000465:Algorithms; D006801:Humans; D008168:Lung; D008175:Lung Neoplasms / Q000601:surgery*; D009068:Movement; D011873:Radiology Information Systems; D016634:Radiosurgery / Q000295:instrumentation* / Q000592:standards; D012015:Reference Standards; D012119:Respiration; D012371:Robotics / Q000295:instrumentation* / Q000592:standards