title: CyberKnife robotic spinal radiosurgery in prone position: dosimetric advantage due to posterior radiation access?
abstract: 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 (D0.1cc and D1cc), high dose (V10Gy), and low dose (V4Gy) 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 V4Gy by 23% ± 26%. In the 2 mm prone scenario, the benefit was nullified by an average increase of V10Gy by 43% ± 24%, and an increase of D1cc to cord (four cases). Spinal cord D0.1cc was unchanged (&lt; ± 1 Gy) 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.
authors.affiliation: European Cyberknife Center Munich. christoph.fuerweger@cyber-knife.net.
authors.forename: Christoph
authors.initials: C
authors.lastname: Fürweger
authors.forename: Christian
authors.initials: C
authors.lastname: Drexler
authors.forename: Alexander
authors.initials: A
authors.lastname: Muacevic
authors.forename: Berndt
authors.initials: B
authors.lastname: Wowra
authors.forename: Erik C
authors.initials: EC
authors.lastname: de Klerck
authors.forename: Mischa S
authors.initials: MS
authors.lastname: Hoogeman
mesh_terms: D006801:Humans; D058958:Organs at Risk; D056888:Patient Positioning; D016684:Prone Position; D016634:Radiosurgery; D011880:Radiotherapy Planning, Computer-Assisted; D020266:Radiotherapy, Conformal; D013120:Spinal Cord Neoplasms / Q000601:surgery*; D013131:Spine / Q000601:surgery*; D016683:Supine Position