Intrafraction verification of gated RapidArc by using beam-level kilovoltage X-ray images

Abstract

Purpose: To verify the geometric accuracy of gated RapidArc treatment using kV images acquired during dose delivery.

Methods and materials: Twenty patients were treated using the gated RapidArc technique with a Varian TrueBeam STx linear accelerator. One to 7 metallic fiducial markers were implanted inside or near the tumor target before treatment simulation. For patient setup and treatment verification purposes, the internal target volume (ITV) was created, corresponding to each implanted marker. The gating signal was generated from the Real-time Position Management (RPM) system. At the beginning of each fraction, individualized respiratory gating amplitude thresholds were set based on fluoroscopic image guidance. During the treatment, we acquired kV images immediately before MV beam-on at every breathing cycle, using the on-board imaging system. After the treatment, all implanted markers were detected, and their 3-dimensional (3D) positions in the patient were estimated using software developed in-house. The distance from the marker to the corresponding ITV was calculated for each patient by averaging over all markers and all fractions.

Results: The average 3D distance between the markers and their ITVs was 0.8 ± 0.5 mm (range, 0-1.7 mm) and was 2.1 ± 1.2 mm at the 95th percentile (range, 0-3.8 mm). On average, a left-right margin of 0.6 mm, an anterior-posterior margin of 0.8 mm, and a superior-inferior margin of 1.5 mm is required to account for 95% of the intrafraction uncertainty in RPM-based RapidArc gating.

Conclusion: To our knowledge, this is the first clinical report of intrafraction verification of respiration-gated RapidArc treatment in stereotactic ablative radiation therapy. For some patients, the markers deviated significantly from the ITV by more than 2 mm at the beginning of the MV beam-on. This emphasizes the need for gating techniques with beam-on/-off controlled directly by the actual position of the tumor target instead of external surrogates such as RPM.

Fig. 1

Flowchart for the overall methods including the clinical workflow and image/data analysis.

Fig. 2

One of the beam-level kV images acquired during a gated RapidArc treatment for patient 1. All three fiducial markers were present in the kV image and are clearly visible. The projections of the PTV and fiducial marker ITV contours as well as the reference markers (shown as circles) are superimposed on the image using in-house software.

Fig. 3

3D position (circles) of one of the fiducial markers estimated from the beam-level kV images acquired during a gated RapidArc treatment for: (a), patient 1; (b), patient 12. The horizontal line indicates the reference position of the marker defined in planning CT.

Fig. 4

3D position (circles) of one of the fiducial markers estimated from the beam-level kV images acquired during a gated RapidArc treatment for patients with multiple fractions: (a), patient 2; (b), patient 5. The horizontal line indicates the reference position of the marker defined in planning CT.