Time delays and margins in gated radiotherapy

Abstract

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 −0.04±0.05.2emsec (amplitude, 1 SD), −0.11±0.04.2emsec (phase); while the average beam off imaging time delays were −0.18±0.08.2emsec (amplitude) and −0.15±0.04.2emsec (phase). The average beam on treatment time delays were +0.09±0.02.2emsec (amplitude, 1 SD), +0.10±0.03.2emsec (phase); while the average beam off time delays for treatment beams were +0.08±0.02.2emsec (amplitude) and +0.07±0.02.2emsec (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

Measurement of time delays in gated radiotherapy for realistic respiratory motions

“…1 Following this recommendation, several groups have determined time delays on a variety of gating systems. [2][3][4][5][6][7] Jin and Yin found a 170 ms time delay in ExacTrac-gated EPID imaging on a Novalis system (BrainLab, Heimstetten, Germany) using a regular, sinuosidal-like motion phantom. 2 Tenn et al used regular motion with a small amount of hysteresis and found a latency of 60 ± 20 ms. 4 Smith et al found mean time delays between transponder positions and linac modulation were 75 ± 13 ms (beam-on) and 65 ± 12 ms (beam-off) for Calypso Medical 4D electromagnetic tracking system (Seattle, WA) with a Varian Trilogy linac using a circular motion phantom and a single patient trajectory.…”

“…These time delays were measured on a variety of Varian linacs using the Real-time Position Management (RPM) system (Varian Medical Systems, Palo Alto, CA) and an overall temporal misalignment of the imaging and treatment systems of 200 ms was found. 5 Guana found an overall time delay of 750 ms in a prospectively gated axial 4DCT on a AcQSim CT simulator (Philips Medical Systems, Cleveland, OH) using RPM. 6 Goharian and Khan found an overall time delay of 346 ms for a Philips Brilliance BigBore TM (Philips Medical Systems, Madison, WI) scanner with the RPM.…”

Technical approach to individualized respiratory-gated carbon-ion therapy for mobile organs

“…This paper describes a sequence of carbon-ion treatment planning for respiratory-gated therapy to improve the dose coverage to a target and the dose suppression in surrounding normal tissues through appropriate margin setting and range compensator design. Some papers have reported on the delays of gating systems [10–12] and treatment planning strategies of particle beam therapy for mobile tumors [13, 14]. However, for respiratory gating, papers compiling the practical processes such as proposing the method for deriving appropriate timings of image acquisitions including delay times of the gating systems, subsequent margin setting and range compensator design in treatment planning, its clinical application, and verification of the planned margins with clinical experience have not been reported so far, especially for ion beam therapy.…”

Quality Assurance for Respiratory-Gated Radiotherapy Using the Real-Time Tumor-Tracking Radiotherapy System

“…A time delay in a respiratory gating system could cause unexpected phase mismatch for respiratory-gated RT; thus, geometric and dosimetric errors might occur [19] [20]. However, the time delay between recognition of the marker position and the start or stop of dose delivery was 0.03 s for respiratory-gated RT using the RTRT system [21].…”