Calculating optic nerve planning organ at risk volume margins for stereotactic radiosurgery using optic nerve motion determined using MRI

Objectives The combination of sharp dose gradients in stereotactic radiosurgery (SRS) and minute optic nerve motion may significantly increase dose to the optic nerves when treating perioptic lesions. The aim of this study was to calculate optic nerve planning organ at risk volume (PRV) margins for CyberKnife SRS treatment planning.Methods MRI scans were taken of 10 healthy volunteers looking left, right, up, down, and straight ahead to measure optic nerve motion. The measured optic nerve motion and the uncertainties in the technical accuracy of CyberKnife were used to calculate optic nerve PRV margins.Results Two optic nerve PRV margins were calculated: a non-isotropic margin of mL/R,PRV=3 mm, mSup/Inf,PRV=2 mm, and mAnt/Post,PRV=1 mm which considers the full range of motion measured in a worst case scenario; and an isotropic margin of mPRV=1 mm which considers a scenario where patients are asked to look neutrally during imaging and treatment. Applying these PRVs to 8 historical sphenoid wing meningioma CyberKnife plans showed tolerance levels may be exceeded due to optic nerve motion.Conclusions Optic nerve PRV margins may be needed in CyberKnife planning to reduce risk to the optic nerves. The use of a mPRV=1 mm PRV to account for organ motion, along with instructing patients to hold their gaze neutrally during imaging and treatment, may be a suitable organ sparing strategy.Advances in knowledge Measured optic nerve motion and the technical accuracy of the CyberKnife system have been used to calculate optic nerve PRV margins.