Accuracy and consistency of respiratory gating in abdominal cancer patients.
OBJECTIVE
To evaluate respiratory gating accuracy and intrafractional consistency for abdominal cancer patients treated with respiratory gated treatment on a regular linear accelerator system.


METHODS
Twelve abdominal patients implanted with fiducials were treated with amplitude-based respiratory-gated radiation therapy. On the basis of daily orthogonal fluoroscopy, the operator readjusted the couch position and gating window such that the fiducial was within a setup margin (fiducial-planning target volume [f-PTV]) when RPM indicated "beam-ON." Fifty-five pre- and post-treatment fluoroscopic movie pairs with synchronized respiratory gating signal were recorded. Fiducial motion traces were extracted from the fluoroscopic movies using a template matching algorithm and correlated with f-PTV by registering the digitally reconstructed radiographs with the fluoroscopic movies. Treatment was determined to be "accurate" if 50% of the fiducial area stayed within f-PTV while beam-ON. For movie pairs that lost gating accuracy, a MATLAB program was used to assess whether the gating window was optimized, the external-internal correlation (EIC) changed, or the patient moved between movies. A series of safety margins from 0.5 mm to 3 mm was added to f-PTV for reassessing gating accuracy.


RESULTS
A decrease in gating accuracy was observed in 44% of movie pairs from daily fluoroscopic movies of 12 abdominal patients. Three main causes for inaccurate gating were identified as change of global EIC over time (∼43%), suboptimal gating setup (∼37%), and imperfect EIC within movie (∼13%).


CONCLUSIONS
Inconsistent respiratory gating accuracy may occur within 1 treatment session even with a daily adjusted gating window. To improve or maintain gating accuracy during treatment, we suggest using at least a 2.5-mm safety margin to account for gating and setup uncertainties.
Department of Radiation Oncology, Washington University School of Medicine, St Louis, Missouri, USA.
Jiajia
J
Ge
Lakshmi
L
Santanam
Deshan
D
Yang
Parag J
PJ
Parikh
D000008:Abdominal Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D000465:Algorithms; D019583:Dose Fractionation, Radiation; D057918:Fiducial Markers; D005471:Fluoroscopy / Q000379:methods; D006801:Humans; D009068:Movement; D056888:Patient Positioning; D061089:Radiotherapy, Image-Guided / Q000379:methods / Q000592:standards; D055425:Respiratory-Gated Imaging Techniques / Q000379:methods / Q000592:standards*