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To explain the principles and indications of gamma knife radiosurgery and to illustrate the correlated neuroimaging features.Between December 1999 and July 2007, 1620 patients were treated by GK for a large variety of indications (metastasis 26%, vascular malformations 7%, trigeminal neuralgia 14%, pituitary adenoma 3%, primary CNS tumour 8%, other tumours 6%, vestibular schwannoma 19%, meningioma 17%, functional disorders1%). The patients benefited from MRI follow-ups.MRI is the imaging technique of choice for Gamma knife radiosurgery (GKRS) in almost all indications. Computed Tomography, Digital Subtraction Angiography and Positron Emission Tomography have an additional role in some indications. Significant MRI data is illustrated in most indications. Evaluation of the treatment is mainly performed using MRI follow-up studies. The main features of these MRI follow-ups are described. Stabilisation or shrinking of the lesions volumes was generally observed. T2 relaxation times were also modified in and around the treated target areas, in patients responding to treatment and without any symptomatic complications. Modifications in contrast uptake were also observed in those patients. A few patients presented symptomatic complications associated with T2 signal anomalies. The interpretation of those modifications is discussed.MRI is the method of choice for GKRS planning in most indications. Imaging changes after radiosurgery provide the best quality control available to assess the response to radiosurgical treatment and to identify and monitor potential complications.

Imaging follow-up of vestibular schwannomas (VS), such as CT or MR, does not allow assessing the response of the tumor tIssue to radiosurgery. Changes in contrast enhancement are frequently observed, with a loss of contrast enhancement within the treated VS. However, this typical aspect does not anticipate the long-term success of radiosurgery for VS. New functional and metabolic image modalities could be useful to assess in vivo radiosurgery-induced tIssue changes. Such data already exist, using techniques such as MR spectroscopy, positron emission tomography (PET) and SPECT, but they concern almost exclusively the evaluation of primary SNC tumors and metastases of systemic cancers. There are, however, very sparse metabolic and functional data concerning the in vivo evaluation of the response of the tumor tIssue to radiosurgery. Moreover, such information is only anecdotal in VS. In other disorders, PET and MR spectroscopy data suggest interesting new directions for the assessment of radiosurgery follow-up. Based on the predictive information provided by PET and MR spectroscopy in primary CNS tumors, it would be worthwhile to design a prospective study evaluating the role of these imaging modalities for in vivo assessment of radiosurgery-treated SV.

Trigeminal neuralgia is a very peculiar disease. The pain, also known as "tic douloureux", is paroxystic and very severe. It can be triggered by a light cutaneous stimulus on a very localized spot on the face (the so-called "trigger zone"). The patient can sometimes benefit from long remissions without any treatment. With the exception of multiple sclerosis and of uncommon cases of posterior fossa tumours or other lesions impinging on the trigeminal nerve, ganglion or root, trigeminal neuralgia is considered as "idiopathic". Some benign abnormality had for long been suspected. The current opinion is now in favour of a "neurovascular conflict": an artery, most often a loop of the superior or anteroinferior cerebellar artery, has an offending contact with the trigeminal nerve root, which results in localized demyelination and ectopic triggering of neuronal discharges. This hypothesis is in agreement with the relief provided by antiepileptic drugs and is supported by recent neuroimaging data. Therapeutic options are reviewed: very efficient drugs are available but fail to provide a significant relief and/or have important side effects in many cases. Surgical alternatives are available, for which guidelines are proposed.

Object. This study was undertaken to assess the efficacy and safety of gamma knife radiosurgery (GKS) for the treatment of arteriovenous malformations (AVMs) located within the brainstem. Methods. The results of GKS performed in 87 patients with brainstem AVMs at two centers with experienced physicians are reviewed. The mean patient age was 37 years and the population included 19 children. The male/female ratio was 56:31. The malformation was located in the upper brainstem in 52 patients. Seventy-four percent of the patients had suffered a hemorrhage before GKS. For 70% of the patients no other treatment had been proposed before GKS. The mean AVM volume was 1.3 cm3. The lesions were treated with one to eight isocenters, with a margin dose ranging between 11.5 Gy and 30 Gy. The mean clinical follow-up period was 3.2 years. Ninety-five percent of the patients improved or remained neurologically stable. Rebleeding occurred in three patients at 3, 6, and 16 months, respectively, after GKS. Two patients in whom rebleeding occured recovered, and one died. The AVM obliteration rate was 63% at 2 years and 73% at 3 years after GKS. A second GKS was performed in six patients in whom only partial obliteration was demonstrated on angiography 3 years after the first procedure. Conclusions. Gamma knife radiosurgery may be a valuable first-choice therapy for the treatment of AVMs located within the brainstem.