CyberKnife robotic spinal radiosurgery in prone position: dosimetric advantage due to posterior radiation access?

CyberKnife spinal radiosurgery suffers from a lack of posterior beams due to workspace limitations. This is remedied by a newly available tracking modality for fiducial‐free, respiration‐compensated spine tracking in prone patient position. We analyzed the potential dosimetric benefit in a planning study. Fourteen exemplary cases were compared in three scenarios: supine (PTV=CTV), prone (PTV=CTV), and prone position with an additional margin (PTV=CTV+2 mm), to incorporate reduced accuracy of respiration‐compensated tracking. Target and spinal cord constraints were chosen according to RTOG 0631 protocol for spinal metastases. Plan quality was scored based on four predefined parameters: dose to cord ([Formula: see text] and [Formula: see text]), high dose ([Formula: see text]), and low dose ([Formula: see text]) volume of healthy tissue. Prescription dose was 16 Gy to the highest isodose line encompassing 90% of the target. Results were related to target size and position. All plans fulfilled RTOG 0631 constraints for coverage and dose to cord. When no additional margin was applied, a majority of eight cases benefitted from prone position, mainly due to a reduction of [Formula: see text] by 23% ± 26%. In the 2 mm prone scenario, the benefit was nullified by an average increase of [Formula: see text] by 43% ± 24%, and an increase of [Formula: see text] to cord (four cases). Spinal cord [Formula: see text] was unchanged ([Formula: see text]) in all but two cases for both prone scenarios. Conformity (nCI) and number of beams were equivalent in all scenarios, but supine plans used a significantly higher number of monitor units (+16%) than prone. Posterior beam access can reduce dose to healthy tissue in CyberKnife spinal radiosurgery when no additional margin is applied. When a target margin of 2 mm is added, this potential gain is lost. Relative anterior‐posterior position and size of the target are selection criteria for prone treatment. PACS numbers: 87,53.Ly, 87.55.D‐