Purpose: To evaluate the displacement of the target position caused by residual setup error (RSE) in stereotactic radiosurgery (SRS) with single-isocenter volumetric modulated arc therapy (SI-VMAT) for multiple brain metastases.
Methods: A total number of 417 gross tumor volumes (GTVs) from 72 consecutive patients who underwent SRS with SI-VMAT between April 2017 and June 2019 were included. All patients were immobilized with a frameless mask. All treatment plans consisted of 3 or 5 arcs including non-coplanar arcs. Patient positioning was performed with ExacTrac X-ray system (ETX) (Brainlab) and a six-degree-of-freedom (6-DOF) robotic couch. Verification and correction of the patient position was performed until within tolerance values (0.7 mm/1.5°) for each arc. The RSEs were regarded as the values on the ETX obtained just before irradiation. The planned center positions of each GTV from treatment isocenter for all patients were calculated. Also, the distance between the planned center position of each GTV and treatment isocenter (d) was calculated. Then, the shifted center positions of each GTV caused by RSEs were calculated using in-house program. Finally, the distance between the planned center position and the shifted center position for all GTVs (ΔD) was calculated, and the correlation of ΔD and d was analyzed.
Results: A total number of 7,270 shifted center positions were analyzed. All 1,185 RSEs were within the tolerance. The median of d was 49.5 mm (range: 4.9 – 95.0 mm). The frequency of ΔD within 1 mm was 97.0%. No correlation was observed between ΔD and d (r=0.11). The maximum value of ΔD was 1.93 mm, which was observed in a tumor with d of 73.3 mm.
Conclusion: The patient positioning with a frameless mask, ETX, and 6-DOF robotic couch is highly accurate in SRS with SI-VMAT for multiple brain metastases even when targets are far from treatment isocenter.