Technical Note: Characterization of clinical linear accelerator triggering latency for motion management system development.

PURPOSE: Latencies for motion management systems have previously been presented as guidelines for system development and implementation. These guidelines consider the overall system latency, including data acquisition, algorithm processing, and linac triggering time. However, during system development, the triggering latency of the clinical linear accelerator is often considered fixed. This paper presents a method to decouple the linac-only triggering latency from the total system latency such that latency can be considered in terms of only the linac-independent aspects of the system.

METHODS: The linac-only latency was investigated by considering the time at which a linac response was observed relative to the time at which a beam-on/off triggering signal was sent to the linac. The relative time between the two signals was analyzed using a multichannel oscilloscope with input signals from a custom gating box to manually trigger the beam state as well as a diode positioned at beam isocenter to monitor the linac response. The beam-on/off latency was measured at multiple energies (6/18 MV) and repetition rates (100-600 MU/min) to investigate beam setting dependencies.

RESULTS: The measured latency was observed to be dependent on the accelerator settings for repetition rate and energy, with beam-on latencies decreasing with increasing repetition rate and decreasing energy. In contrast, the opposite trend was present for the observed beam-off latency. At 600 MU/min, beam-on/off latencies were observed to be 3.37/1.45 ms for a 6 MV beam and 6.02/0.73 ms for an 18 MV beam. Negative latencies were possible for beam-off measurements due to the mechanical latency being less than the pulse separation at given repetition rates.

CONCLUSIONS: The linac latency associated with triggering the beam-on/off was determined to have a minor contribution to the total allowable system latency; thus, the majority of the total system latency can be attributed to linac-independent factors.