BACKGROUND: Implementation of cone beam computed tomography (CBCT) in frameless stereotactic body radiotherapy (SBRT) of lung tumours enables setup correction based on tumour position. The aim of this study was to compare setup accuracy with daily soft tissue matching to bony anatomy matching and evaluate intra- and inter-fractional translational and rotational errors in patient and target positions.

MATERIAL AND METHODS: Fifteen consecutive SBRT patients were included in the study. Vacuum cushions were used for immobilisation. SBRT plans were based on midventilation phase of four-dimensional (4D)-CT or three-dimensional (3D)-CT from PET/CT. Margins of 5 mm in the transversal plane and 10 mm in the cranio-caudal (CC) direction were applied. SBRT was delivered in three fractions within a week. At each fraction, CBCT was performed before and after the treatment. Setup accuracy comparison between soft tissue matching and bony anatomy matching was evaluated on pretreatment CBCTs. From differences in pre- and post-treatment CBCTs, we evaluated the extent of translational and rotational intra-fractional changes in patient position, tumour position and tumour baseline shift. All image registration was rigid with six degrees of freedom.

RESULTS: The median 3D difference between patient position based on bony anatomy matching and soft tissue matching was 3.0 mm (0-8.3 mm). The median 3D intra-fractional change in patient position was 1.4 mm (0-12.2 mm) and 2.2 mm (0-13.2 mm) in tumour position. The median 3D intra-fractional baseline shift was 2.2 mm (0-4.7 mm). With correction of translational errors, the remaining systematic and random errors were approximately 1°.

CONCLUSION: . Soft tissue tumour matching improved precision of treatment delivery in frameless SBRT of lung tumours compared to image guidance using bone matching. The intra-fractional displacement of the target position was affected by both translational and rotational changes in tumour baseline position relative to the bony anatomy and by changes in patient position.