Determination of gate-on and -off timing in respiration-gated radiotherapy.

The purpose of this study was to develop an efficient method of determining gate-on and -off timing in respiration-gated radiotherapy. Gate-on and -off timing in a breathing cycle were defined as the respiratory signal level for the start of irradiation (Ls) in the expiration phase and that for the end of irradiation (Le) in the inspiration phase, respectively. Thirty subjects participated in this study. The diaphragm was used as the tracking target, and time-dependent changes in the position of the target were measured together with those in the respiratory signal level. For each subject, the following maps were created by varying the combination of Ls and Le: absolute target displacement (ATD) map, relative target displacement (RTD) map, and gate-on duty cycle (GDC) map. By classifying respiratory signal waveforms, three respiratory types were derived (A: the length of end-expiration level >40% of a breathing cycle, B: the length of end-expiration level 20% of a breathing cycle, and C: the length of end-expiration level