ORIGINAL ARTICLE Protection of the anterior visual pathways during gamma knife treatment of meningiomas JEREMY C. GANZ 1,2 , AMR EL-SHEHABY 1 , WAEL A. REDA 1 & KHALID ABDELKARIM 1 1 Gamma Knife Center Cairo, Nasser Institute, Shobra, Egypt, Cairo and 2 Nevrokirurgisk Avdeling, Haukeland Sykehus, Bergen, Norway Abstract The anterior visual pathway (AVP) can be at risk during gamma knife surgery (GKS). There is no standardised published methodology for protecting the AVP. This paper suggests such an approach in relation to the treatment of meningiomas. There were 67 patients with a mean age of 48.8 years and a minimum follow-up of 25 months. A recent perimetry was available on the day of treatment. The visual pathway was outlined and the maximum radiation dose was recorded. In some cases a dose volume histogram (DVH) of the AVP was constructed to assess the volume receiving more than the desired maximum dose. The aim was a maximum dose between 8 and 10 Gy. A principle of sub-optimal dose planning was used to protect vision. Follow-up included new visual field examinations. Various anatomical locations place different parts of the AVP at risk. No patient suffered a deterioration of vision. In 21 (38.7%) patients there was an improvement in the visual field and in 7 (44%) associated diplopia improved. Vision could improve without corresponding tumour shrinkage. A standard measure of radiation toleration of the AVP could be the maximum dose within its volume, probably at least 10 Gy. Computerised perimetry should be available on the day of treatment and at follow-up. There is no need to have a distance between the tumour margin and the visual pathway. Sub-optimal dose-planning has been advantageous. Improvement in vision is not necessarily a consequence of tumour shrinkage. Key words: Radiosurgery, meningioma, visual pathway, optic pathway, adverse radiation effects. Introduction The anterior visual pathway (AVP) can be at risk during gamma knife surgery (GKS). The importance of protecting the AVP has been described in many papers. 1–41 It is generally agreed that preservation of vision has a higher priority than an optimal dose plan. Thus, it is inevitable that with tumours near the visual pathway, a suboptimal dose plan may be unavoidable. Acceptable methods of protection have varied in the literature but mainly concern the dose to the visual pathway 8,17,19,20,22,23,27 or the distance between the visual pathway and the target to be treated. 3,6,12,18,25,28,35,39 The concerns arose originally in respect of treatment of lesions arising in the pituitary fossa. Thus there are several reports of AVP protection when treating pituitary adenomas 2,4,9,10–12,21,30–32,35,37,39,41 or cranio- pharyngiomas. 1,3,5,15,16 There are also reports of protecting the pathways while treating menin- giomas 6,8,14,17,18,22–27,33,34,36 and also a mixed bag of adjacent skull base tumours. 9,13,19,20,24,29,38,40 The purpose of this paper is to present potential methods of standardisation which could be used for the measurement of dose to the AVP. Moreover, details of the threat to the visual pathway are described in relation to the location of the commoner relevant meningiomas. The clinical effects of GKS on the AVP over a follow-up of over 2 years are also presented in relation to radiological change in the tumours. Materials and Methods Clinical material The inclusion criterion for this study was a visual field disturbance due to an adjacent benign menin- gioma and shown on computerised perimetry. In the majority of patients, deterioration of vision was the presenting symptom. Nonetheless, in some no subjective disturbance of vision had been noticed. To ensure adequate documentation of visual func- tion all patients with tumours near the visual path- ways had to have a fresh perimetry print out present on the day of treatment. The resulting cohort consists of all patients with a visual field defect taken from a consecutive series of 275 meningioma patients Correspondence: Jeremy C. Ganz, Overlege, Nevrokirurgisk Avdeling, Haukeland Sykehus. E-mail jcganz@gmail.com Received for publication 26 September 2009. Accepted 6 December 2009. British Journal of Neurosurgery, June 2010; 24(3): 233–243 ISSN 0268-8697 print/ISSN 1360-046X online ª The Neurosurgical Foundation DOI: 10.3109/02688690903536611