5-Aminolevulinic Acid Fluorescence Discriminates the Histological Grade of Extraventricular Neurocytoma

Fluorescence guidance has been actively investigated as a tool that facilitates the gross total resection of brain tumors and has emerged as an additional preoperative technique, especially for surgeries involving high-grade gliomas [1-3]. Several reports that describe the application of fluorescence-guided resection of metastatic brain tumors, meningiomas [4], hemangioblastomas [5], pleomorphic xanthoastrocytomas, and ependymomas have been published, demonstrating the usefulness of the technique.

Extraventricular neurocytoma (EVN) is a rare type of brain tumor that displays a diverse range of clinical behaviors without established predictive markers. The limited number of EVNs and the heterogeneous radiological findings make the diagnosis of these tumors difficult prior to surgery, and glioma is the most common radiological tentative preoperative diagnosis. The degree of resection, however, is very important for patient prognosis in both classic and atypical EVNs [6, 7].

We describe two cases of EVN that involved resections using 5-aminolevulinic acid (5-ALA) for fluorescence guidance. Interestingly, the fluorescent reaction of the tissue in one case differed from the reaction in the other case; however, the individual fluorescent reactions were consistent with the pathologic grading. To the best of our knowledge, this is the first report on the use of fluorescence-guided resection of EVN.

Since central neurocytomas were first described by Hassoun et al. [8] in 1982, there has been a series of reports about neurocytomas that arise from the brain parenchyma without any connection to the ventricular system [9]. These so-called 'extraventricular neurocytomas' were named as the counterpart to central neurocytomas, which are typically located in the lateral ventricle near the foramen of Monro [6]. In the recent 2007 World Health Organization classification, EVN was recognized as a new entity to distinguish it from central neurocytomas [10]. EVNs are rare, and their clinical characteristics still need to be evaluated. The radiological findings of EVNs mimic gliomas and are effectively treated with complete resection, regardless of histological grade [6, 7]. When EVNs display atypical histological features such as vascular proliferation, necrosis, increased mitotic activity and an increased proliferation index, they are classified as atypical EVNs [6, 7]. However, the impact of these histological features on predicting the clinical behavior of these tumors is unknown, and the total resection of the tumor is the best indicator of prognosis, regardless of histological grade [6, 7]. Rades et al. [7] reviewed 85 cases of atypical neurocytomas, including 21 cases of atypical EVN, and concluded that patients who underwent complete resection achieved better local control and better survival rates compared to patients who underwent incomplete resection. They also showed that the patients who underwent incomplete resection appeared to benefit from radiotherapy. Brat et al. [6] reported on 35 EVNs from three large institutes and showed that none of the 14 completely resected tumors recurred within the follow-up period, whereas ten of the 19 incompletely removed tumors recurred, regardless of adjuvant radiotherapy. Recurrence occurred in five of ten classic EVNs and five of nine atypical EVNs after incomplete removal. In the Brat report, only one of ten atypical EVNs was completely resected.

Gross total resection of benign brain tumors plays a significant role in prognosis. In malignant brain tumors, gross total resection and overall prognosis seem to be highly correlated [2]. Fluorescence-guided resection of brain tumors has been extensively studied as a potential tool to help neurosurgeons achieve maximal resection of gliomas. Recently, fluorescence-guided surgery for brain tumors using 5-ALA has become more widespread in clinical practice. The proposed mechanisms of the increased 5-ALA uptake into neoplastic brain tissue include the following: induced low permeability of the blood-brain barrier, the presence of adaptable specific membrane transporters, the reduced activity of ferrochelatase, the lowering of the available iron due to chelators, the increased activity of the enzymes that are involved in protoporphyrin IX synthesis, elevated intracellular 5-ALA uptake, and decelerated protoporphyrin IX outflow from the cell [1]. Various types of tumors were investigated for the use of 5-ALA for tumor resection, and the efficacy of the technique has been demonstrated in the literature [4, 5]. To our knowledge, however, the fluorescence reaction for EVN has yet to be reported.

In our 2 cases, the maximal safe resection under fluorescence guidance and intraoperative evoked potential monitoring was performed in both patients who had EVN adjacent to eloquent areas, and there were no permanent postoperative neurological deficits. There were some areas that exhibited the fluorescence reaction for 5-ALA within the tumor tissue, and these areas correlated with histological EVN grade. We discovered that the 5-ALA fluorescence reaction may help increase the likelihood of complete resection of highly malignant cell burden and may facilitate the intraoperative diagnosis of histological grade in EVNs. Furthermore, the histological grading system of EVNs has not been systemically determined, the 5-ALA fluorescence reaction may have a potential role as a biomarker for the prognosis of EVN. Further study on the clinical experience with the 5-ALA fluorescence reaction in EVNs is needed to confirm its role in the clinical evaluation and treatment of these tumors.

In summary, the 5-ALA fluorescence reaction discriminated the histological grade of EVN and was helpful in terms of achieving complete resection and determining the intraoperative diagnosis in atypical cases. This finding shows that the 5-ALA fluorescence reaction may be a potential biomarker of the clinical behavior of EVNs.