Surface guidance compared with ultrasound-based monitoring and diaphragm position in cone-beam computed tomography during abdominal stereotactic radiotherapy in breath-hold.

Background and purpose: Spirometry induced deep-inspiration-breath-hold (DIBH) reduces intrafractional motion during upper abdominal stereotactic body radiotherapy (SBRT). The aim of this prospective study was to evaluate whether surface scanning (SGRT) is an adequate surrogate for monitoring residual internal motion during DIBH. Residual motion detected by SGRT was compared with experimental 4D-ultrasound (US) and an internal motion detection benchmark (diaphragm-dome-position in kV cone-beam computed tomography (CBCT) projections). Materials and methods: Intrafractional monitoring was performed with SGRT and US in 460 DIBHs of 12 patients. Residual motion detected by all modalities (SGRT (anterior-posterior (AP)), US (AP, craniocaudal (CC)) and CBCT (CC)) was analyzed. Agreement analysis included Wilcoxon signed rank test, Maloney and Rastogi's test, Pearson's correlation coefficient (PCC) and interclass correlation coefficient (ICC). Results: Interquartile range was 0.7 mm (US(AP)), 0.8 mm (US(CC)), 0.9 mm (SGRT) and 0.8 mm (CBCT). SGRT(AP) vs. CBCT(CC) and US(CC) vs. CBCT(CC) showed comparable agreement (PCCs 0.53 and 0.52, ICCs 0.51 and 0.49) with slightly higher precision of CBCT(CC). Most agreement was observed for SGRT(AP) vs. US(AP) with largest PCC (0.61) and ICC (0.60), least agreement for SGRT(AP) vs. US(CC) with smallest PCC (0.44) and ICC (0.42). Conclusions: Residual motion detected during spirometry induced DIBH is small. SGRT alone is no sufficient surrogate for residual internal motion in all patients as some high velocity motion could not be detected. Observed patient-specific residual errors may require individualized PTV-margins.

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 ultrasound-real-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 ±â€¯3.3 mm in LR (left-right), 0.2 ±â€¯4.3 mm in CC (cranio-caudal) and 1.0 ±â€¯3.0 mm in AP (anterior-posterior) directions (vector magnitude 5.4 ±â€¯3.3 mm, MV ±â€¯SD). Over 544 DIBHs, target displacement was 1.3 ±â€¯0.5 mm, 0.7 ±â€¯0.3 mm, 1.6 ±â€¯0.6 mm for CC, LR and AP directions, respectively (3D-vector 2.5 ±â€¯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.