Time delays and margins in gated radiotherapy

In gated radiotherapy, the accuracy of treatment delivery is determined by the accuracy with which both the imaging and treatment beams are gated. Time delays are of four types: (1) beam on imaging time delay is the time between the target entering the gated region and the first gated image acquisition; (2) beam off imaging time delay is the time between the target exiting a gated region and the last image acquisition; (3) beam on treatment time delay is the time between the target entering the gated region and the treatment beam on; and (4) beam off treatment time delay is the time between the target exiting the gated region and treatment beam off. Asynchronous time delays for the imaging and treatment systems may increase the required internal target volume (ITV) margin. We measured time delay on three fluoroscopy systems, and three linear accelerator treatment beams, varying gating type (amplitude vs. phase), beam energy, dose rate, and period. The average beam on imaging time delays were [Formula: see text] (amplitude, 1 SD), [Formula: see text] (phase); while the average beam off imaging time delays were [Formula: see text] (amplitude) and [Formula: see text] (phase). The average beam on treatment time delays were [Formula: see text] (amplitude, 1 SD), [Formula: see text] (phase); while the average beam off time delays for treatment beams were [Formula: see text] (amplitude) and [Formula: see text] (phase). The negative value indicates the images were acquired early, and the positive values show the treatment beam was triggered late. We present a technique for calculating the margin necessary to account for time delays. We found that the difference between these imaging and treatment time delays required a significant increase in the ITV margin in the direction of tumor motion at the gated level. PACS number: 87.53.Dq