Ultrasound-based repositioning and real-time monitoring for abdominal SBRT in DIBH

Aim: Ultrasound-based repositioning and real-time-monitoring aim at the improvement of the precision of SBRT in deep inspiration breath-hold (DIBH). Accuracy of ultrasound-based daily repositioning was estimated by comparison with DIBH-cone-beam-CT. Intrafraction motion during beam-delivery was assessed by ultrasoundreal-time-monitoring. Patients/methods: Residual error after ultrasound-based interfractional repositioning (85 fractions, 16 SBRT-series; 14 patients) was assessed by marker-based (7 series) or liver-contour-based (9 series) matching in DIBH-CBCT. During beam-delivery, the percentage of 3D misalignment vector below 2 mm, between 2 and 5 mm, 5-7 mm and over 7 mm was estimated. Percentage of relevant target-displacements was analyzed as a function of DIBH-duration. Results: Residual error after ultrasound-based positioning was 0.4 +/- 3.3 mm in LR (left-right), 0.2 +/- 4.3 mm in CC (cranio-caudal) and 1.0 +/- 3.0 mm in AP (anterior-posterior) directions (vector magnitude 5.4 +/- 3.3 mm, MV +/- SD). Over 544 DIBHs, target displacement was 1.3 +/- 0.5 mm, 0.7 +/- 0.3 mm, 1.6 +/- 0.6 mm for CC, LR and AP directions, respectively (3D-vector 2.5 +/- 0.7 mm). 3D misalignment vector length was below 2 mm in 49.8%, between 2 and 7 mm in 46.3%, and over 7 mm in 3.9% of the beam-delivery-time. During the first 5 s of the DIBH, 3D-misalignment vector length was always below 10 mm. Percentage of target displacements over 10 mm was 0.2%, 0.5% and 0.8% for 10 s, 15 s and 20 s DIBH-duration. Conclusions: Ultrasound-based interfractional repositioning is an accurate method for daily localization of abdominal DIBH-SBRT targets. Residual motion is < 7 mm in 96% of the beam-delivery-time. Deviations > 10 mm occur rarely and can be avoided by gating the beam at a predefined threshold. Ideal DIBH-duration should not exceed 15 s.