Radiosurgery: operative technique, pitfalls and tips.
BACKGROUND
From frame placement to dose administration, each step of the procedure must be optimized in every detail for better preservation of global precision, accuracy, safety and efficacy.


METHODS
Quality control for resolution, accuracy and acquisition parameter optimization of both computed tomography (CT) scanners and magnetic resonance imaging (MRI) must be performed. Inaccuracies should then be quantified through systematic combination of MRI and CT in the radiosurgery planning system. Topography of petrous structures such as cochlea, vestibulum and facial nerve canal should be visible on the CT scan. T1-weighted volumetric MRI pulse sequences (3DT1) show a contrast-enhanced signal that is useful for both the pons interface delineation in Koos III cases, and the canal ending. High-resolution CISS T2-weighted volumetric pulse sequences (3DT2) allow direct nerve visualization and give superior stereotactic definition attributable to their better resolution minimizing partial volume effects and to their lower magnetic susceptibility minimizing distortions. The 3DT2 pulse sequences with contrast injection, show improved distinction between the pons and the nerves due to signal differences within the schwannomas. Fat saturation pulse sequences are of interest in postmicrosurgery conditions. The previous technical requirements and the dose planning elaboration will be balanced depending on the lesion volume staging (Koos), treatment history (microsurgery), clinical condition (hearing quality), pathological context (NF2) or age of the patient. The recommended marginal dose is 11-12 Gy. Tumor volume delineation allows the calculation of conformity, selectivity and gradient indexes. These global indexes must be weighted according to the relationship to critical structures and functional status of the patient.


CONCLUSIONS
As an exclusively image-guided surgical method, radiosurgery requires special attention in the choice of imaging modalities and their acquisition parameters need extreme care. Technical nuances during the elaboration of the dose planning itself will directly influence both the toxicity risk and the chance of cure.
Régis
Jean
J
Service de Neurochirurgie Fonctionnelle et Stéréotaxique, Hôpital d'Adulte de la Timone, Marseille, France.
Tamura
Manabu
M
Wikler
David
D
Porcheron
Denis
D
Levrier
Olivier
O
D006801:Humans; D008279:Magnetic Resonance Imaging; D009464:Neuroma, Acoustic / Q000000981:diagnostic imaging / Q000473:pathology / Q000601:surgery*; D011786:Quality Control; D016634:Radiosurgery / Q000009:adverse effects* / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D014057:Tomography, X-Ray Computed