Treatment plan quality for stereotactic treatment of multiple cranial metastases: Comparison of C-arm and O-ring treatment platforms

C-arm linacs have been used widely to treat multiple cranial metastases using stereotactic radiosurgery (SRS). A new generation of O-ring linacs offer several workflow advantages when compared to C-arm platforms. However, O-ring linacs are not able to employ couch rotations for noncoplanar beams used in SRS treatments. This study was conducted in order to simulate further possible developments of O-ring treatment units by assessing their geometrical efficiency. In this work we compare the plan quality for C-arm versus an O-ring platform including metrics that are relevant to SRS for multiple metastases. The comparison is conducted by incorporating tilted arcs on the O-ring platform therefore introducing noncoplanarity. Total 40 patients previously treated for SRS with 20 Gy single fraction were replanned for C-arm with a standard noncoplanar 5-arc arrangement and O-ring with both coplanar and noncoplanar beams. For the O-ring plans, we considered a default 3-arc coplanar arrangement, as well as 3- and 5-arc arrangements with arcs tipped up to 10 degrees from the axial plane. Target coverage, organ-at-risk (OAR) doses, monitor unit (MU) efficiency, conformity and gradient indices were assessed for all plans. For most metrics the O-ring geometries, even the coplanar arrangement, produced statistically comparable results to the C-arm. Small but significant differences were found for the 3 arc O-ring for PTV: D90%, D2% and MU/Gy and for the 5 arc O-ring at D2% when both were compared to the C-arm. Cumulative dose volume histograms (DVHs) for normal brain showed a cross-over between the C-arm and coplanar O-ring geometry at a low dose (2.3 ± 1.8 Gy), with O-ring associated with higher volumes above this cross-over dose. However, no statistical difference was seen in the brainstem, optic pathway and volumes of normal brain receiving 12 Gy or 20 Gy. This study has found that O-ring geometry linacs can produce SRS plans of comparable quality to those from a C-arm for multiple cranial metastases.