## 1 Introduction  
Fluorescence-guided resection (FGR) using 5-ALA is well established for surgery of malignant gliomas [1, 2]. Not only has the value of FGR been tested prospectively for finding residual tumor [3] and improving the extent of resection, but 5-ALA-induced tumor fluorescence also provides help in defining surgical strategy, since tissue fluorescence is usually interrogated repeatedly during surgery, e.g., for initially finding subcortical tumors and for defining the surgical route along tumor margins. The question has frequently been raised regarding which other brain tumors might profit from fluorescence-guided tumor detection, and it is known in the literature that different brain tumors show various degrees of fluorescence [4]. Among these, meningiomas are potential candidates for fluorescence-guided resection, as they have been shown to demonstrate fluorescence in almost all cases [5; 6; 7; 8; 9; 10]. At first glance, meningiomas might be considered circumscribed and easily distinguishable lesions in which fluorescence might not play a meaningful role during surgery. However, considering tumors that may have bony infiltration, tumors infiltrating extracranial tissues, satellite lesions, recurrent tumors with scar tissue, those with a dural tail, or tumors infiltrating the arachnoid, additional reliable tissue information based on fluorescence might be beneficial. Such benefit may be derived from identifying residual tumor overlooked during conventional surgery. In meningiomas, progression-free survival has been linked to the degree of resection [11], and any additional detection of unexpected tumor will serve to increase the degree of resection. Several investigators have described the use of 5-ALA for resection of meningiomas, citing a high diagnostic accuracy for fluorescing tissue to predict tumor and the propensity for using this fluorescence to help identify residual tumor otherwise missed [7; 8; 9; 10]. However, all reports have been based on monocentric evaluations and unintentional bias cannot be ruled out. Specifically, bias might be involved in sampling strategies for determining diagnostic accuracy (sensitivity, specificity), or while identifying residual tumor. To this end, we have recently published possible pitfalls and biases which need to be observed for the demonstration of correlation between fluorescent agent and tumor tissue, especially in infiltrative lesions [12]. Regulatory approval will therefore depend on prospective, well-controlled studies with a minimum of bias, showing fluorescence to be closely associated with the presence of tumor cells and imaging agents to be toxicologically safe. A consensus report by the International Society of Image Guided Surgery recently compiled several recommendations for clinical trial designs regarding intra-operative imaging agents [13]. These recommendations reflect that the toxicity of such agents should be acceptable and at a level below that of a therapeutic agent but not as low as required for a purely diagnostic agent. Fluorescence should be strongly correlated with the presence of tumor, as assessed by standard detection methodology. Furthermore, there is agreement that for the agent to advance to routine clinical use, methods for detection should be widely adopted and standardized. The use of such imaging agents will also have to benefit patients, i.e., such agents will have to demonstrate clinical usefulness [13; 14]. Widely used for fluorescence-guided resection of high-grade gliomas, 5-ALA is a well-characterized agent with low toxicity. Devices for visualizing fluorescence are similarly widely distributed, including neurosurgical microscopes adapted for fluorescence (e.g., Blue400, Zeiss Meditech, Oberkochen, Germany; FL400, Leica Microsystems, Heerbrugg, Switzerland) and a validated surgical loupe and illumination device [15].