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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=3mm, mSup/Inf,PRV=2mm and mAnt/Post,PRV=1mm which considers the full range of motion measured in a worst case scenario; and an isotropic margin of mPRV=1mm 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=1mm 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., (© The Author(s) 2024. Published by Oxford University Press on behalf of the British Institute of Radiology.)

Tumours involving the anterior skull base are a challenge in diagnosis and treatment. They may arise from the bony skull base itself, intracranially or from the sinonasal tract and orbit. It is often difficult to determine the site of origin of giant tumours as anatomical boundaries are frequently breached. Accurate imaging evaluation is useful in planning treatment and may help in the differential diagnosis. We review those CT and MRI features of giant anterior cranial fossa tumours which may be helpful in identifying a pre-operative diagnosis.

Ossification of the ligamenta flava is rare and has been described almost exclusively in Japanese people. We present a case in which ossification of the thoracic ligamenta flava caused compressive myelopathy in a Caucasian.