Room: Room 209
Purpose: Motion management systems aid in the accurate delivery of radiotherapy treatments by accounting for intra-fraction motion. High system latencies, such that the beam response is delayed relative to the actual position of the target, can impact the effectiveness of these systems. This work developed a systematic approach to analyze the dosimetric effect that system latencies can have on a radiotherapy delivery.
Methods: To analyze the dosimetric effect of system latency, an ion chamber was traversed through a 5x5 cm field while gating was performed on a Varian Clinac 21EX with varying programmed latencies. A programmable motion stage was used to move the ion chamber through a realistic 1D motion trace during delivery. Gating was performed based on the known motion positions with a 30% duty cycle. Custom hardware enabled the addition of simulated system latencies between 0 and 500 ms during gating. Gating measurements were compared to both static and â€œfree-breathingâ€? deliveries. A total of 1000 MU was delivered for all measurements. Measurements were performed for two ion chamber positions: at the field edge and 1 cm inside of the field edge.
Results: Increased dose deviations relative to static measurements were observed with increasing system latency for the edge-of-field case, while minimal variations were observed for the in-field case. At the edge of the field, a dose deviation of 4.0% and 11.3% were observed for 0 ms latency and 500 ms latency respectively. â€œFree-breathingâ€? measurements without gating resulted in a 39.5% dose difference relative to static measurements at the edge of the field, and 0.37% in the field.
Conclusion: While still advantageous relative to free-breathing, gating with high system latencies can limit the relative impact of motion management near the field edges. System latencies should be minimized such that the full dosimetric advantages of motion management can be obtained.
Funding Support, Disclosures, and Conflict of Interest: This work was partially funded by NIH grants R01CA190298 and T32CA009206