TH‐C‐BRC‐01: The Effects of Adaptive Gating on Treatment Accuracy and Delivery Time

Abstract

Purpose: To evaluate the effect of an adaptive gating system on treatment accuracy and delivery time as compared to gating based only on an external surrogate. Methods: Two studies were designed to simulate the process of treatment planning, patient setup, and treatment delivery for 1) external surrogate‐based; and 2) adaptive gating. External surrogate‐based gating used only chestwall motion to generate the respiratory gating signal, while adaptive gating updates the external/internal correlation periodically based on the location of implanted fiducial markers. Lung tumor trajectories with corresponding chestwall motions acquired from 8 patients with multiple days of radiation treatments. A total of 20,026 seconds of data from 163 treatment beams was used. The average target miss percentage is defined to be the percentage of beam‐on time that the target is thought to be within the gating window but is actually outside of it. The average duty cycle is defined as the beam‐on time divided by the sum of beam‐on and beam‐off time. Results: The average target miss percentage for adaptive gating was 17.6% (range: 8.0–23.6%) and 22.6% (range: 8.1–44.5%) without adaptation. The magnitude of the average miss was 1.7 mm (range: 0.6–4.8 mm) with adaptation and 1.8 mm (range: 0.6–4.3 mm) without adaptation. The average miss percentage for misses of over 5 mm was 0.6 % (range: 0.0–3.3%) for adaptive gating and 0.9 % (range: 0.0–3.7%) without adaptation, suggesting that additional planning margins of 5 mm would mitigate most gating errors with either method. The average duty cycle using adaptive gating was 33% (range: 23–39%), compared to 45% (range: 35–57%) without adaptation. Conclusion: We have shown that adaptive gating can improve the accuracy of gated treatments, at the cost of longer treatment times. However, both adaptive and non‐adaptive techniques perform well if an additional planning margin of 5 mm is used.