Intracranial stereotactic radiotherapy in off-isocenter target with SyncTraX FX4

•Setup error of off-isocenter targets tended to increase in stereotactic radiotherapy.•SyncTraX FX4 achieved similar setup accuracy with cone-beam CT.•Setup accuracy of SyncTraX was within 1 mm for targets within 7 cm.•In the patient study, setup errors during treatment were reduced using SyncTraX.•SyncTraX reduced dosimetric errors caused by intrafractional setup errors. We investigated the localization accuracy of the off-isocenter targets using SyncTraX FX4, a new image registration device. In a phantom study, we used a MultiMet-WL Cube with metal targets at different distances from the isocenter. Image registrations were performed with SyncTraX and cone-beam computed tomography (CBCT). Nineteen fields with different gantry, collimator, and couch angles were delivered to each target. Localization errors of the off-isocenter targets were then evaluated. In a clinical study, localization accuracy was evaluated for 32 patients. First, image registration was performed using SyncTraX, and the accuracy of patient positioning was evaluated using CBCT. Next, positioning corrections were performed for intracranial setup errors exceeding the threshold (0.5 mm/0.5°) in each field. Finally, total setup uncertainty was evaluated using CBCT. Differences in dosimetric errors from planned doses between no patient positioning corrections during treatment and positioning corrections with SyncTraX were also evaluated. In the phantom study, the positioning accuracy on targets up to 7 cm from the isocenter was within 1 mm. In the clinical practice, the localization accuracies of SyncTraX were 0.35 ± 0.39 mm, 0.30 ± 0.24 mm, and 0.03 ± 0.27 mm in the lateral, vertical, and longitudinal directions, respectively. Post-treatment setup errors were reduced by correcting intrafractional setup errors with SyncTraX during treatment. Positioning corrections with SyncTraX reduced the maximum dosimetric error from 1.6% to 1.0%. SyncTraX provides satisfactory localization accuracy for the off-isocenter targets within 7 cm. SyncTraX reduce dosimetric errors caused by intrafractional setup errors during treatment.