[(18)F]-fluoro-L-thymidine PET and advanced MRI for preoperative grading of gliomas.

PURPOSE: Conventional MRI based on contrast enhancement is often not sufficient in differentiating grade II from grade III and grade III from grade IV diffuse gliomas. We assessed advanced MRI, MR spectroscopy and [(18)F]-fluoro-l-thymidine ([(18)F]-FLT) PET as tools to overcome these limitations. METHODS: In this prospective study, thirty-nine patients with diffuse gliomas of grades II, III or IV underwent conventional MRI, perfusion, diffusion, proton MR spectroscopy ((1)H-MRS) and [(18)F]-FLT-PET imaging before surgery. Relative cerebral blood volume (rCBV), apparent diffusion coefficient (ADC), Cho/Cr, NAA/Cr, Cho/NAA and FLT-SUV were compared between grades. RESULTS: Cho/Cr showed significant differences between grade II and grade III gliomas (p = 0.03). To discriminate grade II from grade IV and grade III from grade IV gliomas, the most relevant parameter was the maximum value of [(18)F]-FLT uptake FLTmax (respectively, p < 0.001 and p < 0.0001). The parameter showing the best correlation with the grade was the mean value of [(18)F]-FLT uptake FLTmean (R(2) = 0.36, p < 0.0001) and FLTmax (R(2) = 0.5, p < 0.0001). CONCLUSION: Whereas advanced MRI parameters give indications for the grading of gliomas, the addition of [(18)F]-FLT-PET could be of interest for the accurate preoperative classification of diffuse gliomas, particularly for identification of doubtful grade III and IV gliomas.

Five-Year Longitudinal MRI Follow-up and (1)H Single Voxel MRS in 14 patients with Gliomatosis Treated with Temodal, Radiotherapy and Antiangiogenic Therapy.

Gliomatosis cerebri (GC) is a challenging tumor, considered to have a poor prognosis and poor response to treatments. The purpose of this study is to better understand glial tumor metabolism and post chemotherapy, radiotherapy and antiangiogenic variations in a longitudinal study to determine cerebral variation in MRS area, amplitude, and ratios of metabolites and spectral profiles during a five year longitudinal follow-up in 14 patients with gliomatosis without initial hyperperfusion and treated with chemotherapy (Temozolomide (Temodal(®))), radiotherapy and subsequent antiangiogenic therapy. The study also aimed to detect changes in infiltration, proliferation, lipids or glycolytic metabolism, as these changes could be monitored longitudinally in humans with glial brain tumors (low and high grade) after therapy, using conventional magnetic resonance imaging (MRI), spectroscopy (MRS) and MR perfusion. Most patients had first initial clinical and MRS improvement and stable MRI. After 12 to 24 chemotherapy treatment cycles MRS usually showed an increase in the Cho/Cr ratio (proliferation) and sometimes contrast enhancements. Later, the patients showed clinical deterioration and radiotherapy was started. There was an improvement with radiotherapy that lasted nine to 18 months. This was followed by a worsening that led to try antiangiogenic therapy. Later in the evolution for three patients with hyperperfusion this symptom disappeared, but proliferation, infiltration and glycolytic metabolism remained at a high level. Spectroscopic and metabolic changes often occur well before clinical deterioration and sometimes before improvement. Therefore, MRS could be more sensitive and could detect changes earlier than MRI and is sometimes predictive. Despite the difficulty, the variability and unknown factors, these repeated measurements give us a better insight into the nature of the different processes, tumor progression and could lead to better understanding of therapeutic response.

[Candida albicans cerebral granuloma in an immunocompetent patient. A case report]. / Granulome cérébral àCandida albicans chez un patient immunocompétent. Cas clinique.

We report a case of temporal lobe granuloma caused by Candida albicans in an immunocompetent patient. This 54-year-old patient had experienced headaches and some memory disorders for two to three months before his admission to the neurosurgical department. Cerebral-computer tomography and magnetic resonance imaging showed a single right-temporal lesion with a large peritumoral edema. We operated on the patient via a temporal approach using neuronavigation and resected the lesion. The anatomopathological result and the cultures of the granuloma showed C. albicans species. The patient received antifungal therapy for three months. No predisposing factors or immunosuppression was found. After seven months, he presented an ischemic cerebrovascular accident of the brain stem and then chronic meningitis complicated by hydrocephalus. The patient's condition progressively deteriorated and he died 18 months later in an other department. C. albicans can be found even in the immunocompetent patient, but is seldom observed. Surgery can provide an accurate diagnosis and therapeutic management in the initial phase, completed by antifungal therapy.

[Surgical treatment of cortical and subcortical cavernomas. General principles and personal series of 20 cases treated between 2000 and 2006]. / Traitement chirurgical des cavernomes corticaux et sous-corticaux. Principes généraux et série personnelle de 20 cas opérés entre 2000 et 2006.

The surgical treatment of cortical or subcortical hemispheric cavernomas is founded on a series of questions: Is the cavernoma located in an eloquent or non-eloquent area? Is the cavernoma tangent to the cortex and visible immediately after the dura opening or deep seated in the hemisphere? Does the cavernoma lie in the depth of a sulcus and identifiable on the pretrans-sulcal approach MRI? Will perilesional tissue have to be removed to cure the epilepsy? What is the appropriate technology for each particular case: preoperative functional MRI, angiography, preoperative stereotactic guidance, peroperative ultrasonography, neuronavigation, peroperative neurophysiology and cortical stimulation, preoperative MRI? Based on a personal series of 20 cases operated on between 2000 and 2006, we describe our personal experience.

[Metabolic imaging for supratentorial oligodendrogliomas]. / Les oligodendrogliomes supra-tentoriels: imagerie métabolique.

Metabolic imaging with positron emission tomography (PET) provides, in neuro-oncology, information complementary to that provided by anatomic imaging obtained with CT-scanner or MRI. Only a few publications have yet reported its use in oligodendroglial tumors. These findings and partial results obtained in ongoing work, suggest some preliminary conclusions: 11C-MET (L-methyl-methionine) is a more appropriate tracer than 18F-FDG (fluoro-deoxy-glucose), in terms of both specificity and sensitivity, for the assessment of patients with this category of tumor. PET/MET allows differentiation between grade II and grade III oligodendrogliomas; better targeting for stereotactic biopsy; more accurate assessment of the post-operative residual tumor; identification of progression from low-grade to anaplastic grade during the disease course; differentiation between recurrence and a post-radiation processes. PET/MET allows, to some extent, prediction of response to radiotherapy; and, probably, to chemotherapy.

[Giant extradural epidermoid cyst of the posterior fossa]. / Kyste épidermoïde extra-dural géant de la fosse postérieure.

We report an unusual case of giant extradural epidermoid cyst of the posterior fossa in a 73-year-old man. The patient presented headache and gait disturbance for 3-4 months, cerebellar ataxia, left cerebellar dysmetria, and perturbed balance. The CT-scan showed a huge posterior fossa extradural lesion with a small area of peripheral contrast enhancement. Outcome was good after total resection of the lesion via a suboccipital approach. Pathology reported an epidermoid cyst. Extradural epidermoid cyst is a rare benign tumor of the skull which sometimes can reach considerable size. It is a slow growing lesion and may cause mild neurological deficits. The goal of surgical treatment is total resection of the tumor with its capsule. The long-term prognosis is excellent after successful resection.

[The ventrolateral position]. / La position ventro-latérale.

AIM: To propose an alternative to the sitting position and the other horizontal positions while maintaining the advantages of the sitting position and assuring maximum safety for the patient particularly with respect to the risk of air embolism. POSITION: The patient is positioned as for the sitting position with the median axis of the posterior cranial fossa prolonging the spinal axis. The head is maintained by a Mayfield head holder and is inclined without force. The patient rests laterally on the apron, the thorax and the lower limbs resting on cushions. The upper limbs rest on the arm rests interdependent with the table facilitating surgical access and rolling movements. DATA: Since 1993, this position was used for 556 patients. For 81.3% of the patients lesions were located in the posterior cranial fossa and for 12.75% in the supratentorial region. Access to the cervical area was achieved for 4.7% of the patients. RESULTS: This position gave us access to the lesions without specific difficulties, irrespective of the patient's morphology. There were no air embolism events and no capnographic anomaly was reported. CONCLUSION: Since discontinuing use of the sitting position in our institution, we have found that the ventrolateral position can replace the sitting position advantageously. It allowed us to approach lesions located in a large area, from the cervical spine to the supratentorial area located behind external auditory meatus, and was compatible with anesthetic and surgical requirements.

[Diagnosis and prognosis in neuro-oncology: contribution of positron emission tomography]. / Diagnostic et pronostic en neuro-oncologie: apport de la tomographie par émission de positrons (TEP).

Positron emission tomography (PET) assesses the uptake and distribution of a radiopharmaceutics labelling tissue metabolism, in tumour and in presumably healthy brain, respectively. This technique provides data which are complementary from those obtained with anatomical imaging (CT-scanner, MRI) and with histology performed on a biopsy sample. The most frequently used tracers, in neuro-oncology, are 18FDG and 11C-methionine. However, for most of the applications of this technique, the latter tracer appears to be more effective than the former, both from the point of view of sensitivity and specificity. The main applications of PET in clinical neuro-oncology are the following

Increased expression of transforming growth factor-beta after cerebral ischemia in the baboon: an endogenous marker of neuronal stress?

There has been an increasing interest in recent years in the evaluation of the neuronal and glial responses to ischemic insult. Some cytokines, including transforming growth factor-beta (TGF-beta), that are overexpressed after experimental stroke in rodents are thought to be implicated in the neuronal processes that lead to necrosis. Thus, such cytokines could predict tissue fate after stroke in humans, although data are currently sparse for gyrencephalic species. The current study addressed the expression pattern of TGF-beta1 in a nonhuman primate model of middle cerebral artery occlusion. Focal permanent ischemia was induced for 1 or 7 days in 6 baboons and the following investigations were undertaken: cerebral oxygen metabolism (CMRO2) positron emission tomography studies, magnetic resonance imaging, postmortem histology, and reverse transcription-polymerase chain reaction. The aim of the current study was to correlate the expression of TGF-beta1 to the underlying metabolic and histologic state of the threatened cerebral parenchyma. The authors evidenced increased TGF-beta1 mRNA levels (up to 25-fold) in those regions displaying a moderate (20% to 49%) reduction in CMRO2. The current findings suggest that the greatly enhanced expression of TGF-beta1 in the penumbral zones that surround tissue destined to infarction may represent a robust index of potentially salvageable brain. The current investigation, in the nonhuman primate, strengthens the authors' hypothesis, derived from rodent models, that TGF-beta1 may be involved in the physiopathology of human stroke.

Non-invasive grading of oligodendrogliomas: correlation between in vivo metabolic pattern and histopathology.

Several studies have shown that the prognosis of oligodendrogliomas is dependent on their histological grade. In order to identify a non-invasive method for the primary diagnosis and follow-up of these tumours, we investigated the relationship between their in vivo metabolism, assessed by positron emission tomography (PET), and their histological grade assessed at the same time. Forty-seven patients with histologically confirmed oligodendrogliomas were investigated. Conventional neuroradiological assessment by computed tomography and magnetic resonance imaging (MRI) was performed in all the patients. All the histology slices were reviewed by the same pathologist after referral from various pathology laboratories. The PET investigation included a carbon-1 methionine (11C-MET) uptake study and, in the majority of cases, a fluorine-18 fluorodeoxyglucose (18F-FDG) uptake study, in order to investigate at the same time both amino acid metabolism and glycolysis. The sampled tumour region of interest (ROI) was defined from the T1-weighted 3D MR scan matched with the PET scan. Tracer concentration in each voxel of the tumour ROI was divided by the mean concentration in an ROI of the same size located in the healthy brain tissue. For each tumour and each tracer, we characterized the metabolic pattern on the basis of the mean and the maximum tumour to healthy tissue concentration ratio, and also the standard deviation and range of the ratios, which indicate the degree of metabolic heterogeneity of the tumour. The histological criteria for differentiating between high- and low-grade tumours were those of the WHO and, partially, of the Sainte-Anne-Daumas-Duport classification. Highly significant differences between high- and low-grade oligodendrogliomas (Mann-Whitney test: P<0.0001) were observed for all the assessed parameters of 11C-MET uptake. On the other hand, the pattern of 18F-FDG uptake showed only moderate differences between the two tumour groups.

Voxel-based mapping of irreversible ischaemic damage with PET in acute stroke.

Objective mapping of irreversible tissue damage in the acute stage of ischaemic stroke would be useful for prognosis and in assessing the efficacy of therapeutic manoeuvres in impeding extension of infarction. From our database of 30 patients studied with 15O-PET within 5-18 h after onset of first-ever middle cerebral artery territory stroke, we extracted a subgroup of 19 survivors (age 74.6 +/- 8.5 years) in whom late CT coregistered with PET was available to determine final infarct topography. By means of a voxel-based analysis of the PET data, we determined putative thresholds for irreversible tissue damage as the lower limit of the 95% confidence interval calculated from all voxels within the ultimately non-infarcted brain parenchyma ipsilateral to the insult. The following values were found: 8.43 ml/100 ml/min, 0.87 ml/100 ml/min, 1.64 ml/100 ml, 0.27 and 2.21/min, for cerebral blood flow (CBF), oxygen consumption (CMRO2), blood volume (CBV), oxygen extraction fraction and the ratio CBF : CBV, respectively. Voxels below these thresholds occurred significantly more frequently in the final infarct region than in the non-infarcted parenchyma for CBF and CMRO2 (P = 0.016 and P = 0.0045, respectively, Wilcoxon test), but not for the other PET variables. Furthermore, with both CBF and CMRO2, the percentage of irreversible tissue damage voxels in the affected hemisphere relative to the opposite hemisphere was significantly positively correlated to both the volume of final infarct and the neurological outcome at 2 months (all P < 0.005, Spearman ranked test). These findings validate our voxel-based CBF and CMRO2 thresholds for probabilistic mapping of irreversible tissue damage within the 5-18 h interval after stroke onset; however, whether they would be applicable to earlier intervals remains to be determined. Transfer of our procedure for determination of irreversible tissue damage thresholds to other imaging modalities such as single proton emission computed tomography and diffusion-weighted MRI should be straightforward.

[Dysembryoplastic neuroepithelial tumors. Report of 8 cases including two with unusual localization]. / Tumeurs neuro-épithéliales dysembryoplasiques. A propos de 8 cas dont deux à localisation inhabituelle.

OBJECTIVES: Dysembryoplastic neuroepithelial tumors (DNTs) are usually located within the supratentorial cortex. We present a series of eight cases of DNTs including two cases with an extracortical location, one in the caudate nucleus, the other one expanded in the lateral ventricule, septum and fornix. An origin from secondary germinal layers, as previously suggested, can explain these extracortical locations. MATERIAL AND METHODS: Of the eight patients, seven had partial epileptic seizures and one intracranial hypertension. All patients underwent clinical examination, a computed tomographic (CT) scan, a magnetic resonance imaging (MRI) and a surgical removal of the lesion with histological examination. RESULTS: Clinical examination was normal except in the case with intracranial hypertension where a bilateral papillary oedema was found. In seven cases the CT scan showed a hypodense lesion of pseudocystic appearance. All lesions were hypointense on T1-weighted and hyperintense on T2-weighted MRI. Contrast enhancement was observed in two cases. The lesion was intracortical in six cases and extracortical in the remaining two: one in the head of the caudate nucleus and one in the trigonoseptal region. Histological examination identified an appearance of DNT with a specific glioneuronal element in six cases. CONCLUSION: The diagnosis of DNT can be suspected before histological examination on radiological features, chiefly because the tumor is located in the supratentorial cortex. However, extracortical locations do exist, even if unusual. As DNTs are always benign, knowledge and accurate diagnosis of these atypical cases are mandatory in order to avoid useless and even deleterious additional treatments, such as radiotherapy.

[Combination treatment for pilocytic astrocytoma: stereotaxic radiosurgery and endocavitary radiotherapy]. / Traitement combiné d'un astrocytome pilocytique: radiochirurgie stéréotaxique et radiothérapie endocavitaire.

The treatment of a pilocytic astrocytoma located in a functional area can be performed using radiosurgery. We report a 7 year old male, right-handed, who presented with a pilocytic astrocytoma in the left parieto-occipital lobe. After a 7 year follow-up, the tumor became symptomatic (partial and generalized seizures). The CT scan and nuclear magnetic resonance imaging revealed an increased size of the mural tumor and the development of a cystic component. Multi-beam irradiation of the tumor (dose of 30 Gy at the center with 21 Gy on the isodose 70%) was performed with a LINAC for radiosurgery coupling a modified Saturne 18 MeV linear accelerator and a Talairach stereotactic frame. Following multi-beam irradiation, the increase in size of the cyst imposed further intracavitary radiation using Rhenium 186 (186Re) to deliver 400 Gy to the cyst wall. After a period of intense cerebral edema, resolutive with steroid treatment, we obtained progressive cyst disappearance and mural nodule retraction. A PET scan, performed 3 years after this treatment, revealed no metabolic activity in the persistent mural nodule. The patient remains totally asymptomatic.

[The cerebral hyperperfusion syndrome after a carotid endarterectomy operation (a case report and review of the literature)]. / Sindrom tserebral'noi giperperfuzii posle operatsii karotidnoi éndarteréktomii (opisanie nabliudeniia i obzor literatury).

The paper describes a clinical case of the cerebral hyperperfusion syndrome, a rare complication of carotid endarterectomy. The syndrome appeared as the generalized convulsive syndrome in the patient in the early postoperative period. In the context of clinical observation, the results of analysis of the literature are presented and the pathogenesis, diagnosis, therapy, and prevention of the cerebral hyperperfusion syndrome considered.

Results of surgical management of internal carotid artery aneurysm by the cervical approach.

From 1981 to 1995 a total of 14 patients with a mean age of 52 years (range: 23-71) underwent surgery for 15 aneurysms of the extracranial internal carotid artery. Fusiform aneurysms of the carotid bifurcation were not included in this study. Aneurysm led to brain ischemia in 10 cases and rupture in one case. In the remaining four cases, aneurysm was asymptomatic including three that were detected following hemispheric stroke related to a contralateral aneurysm. The etiology was spontaneous dissection in four cases, blunt trauma in three cases, fibromuscular disease in five cases, and atheroma in three cases. The upper limit of the aneurysm was located at C1-C2 in six cases, at C1 in three cases, and above C1 (at the base of the skull) in six cases. The cervical approach was used to successfully perform 12 revascularizations and three ligations (including one after extra-intracranial bypass). There were no postoperative deaths. One transient ischemic attack (TIA) occurred after ligation. Peripheral facial paralysis (PFP) occurred in four of the nine cases in which an extended cervical approach was used. No patients were lost to follow-up. Mean duration of follow-up was 4 years (range: 2 months-10 years). Two patients died at 2 and 4 years of causes unrelated to the procedure. All carotid reconstructions are currently patent and no neurologic manifestations have occurred. PFP persisted in one case. The results of this series confirm that surgical therapy of aneurysms of the extracranial internal carotid artery achieves satisfactory short- and medium-term results and that the extended cervical approach allows treatment of lesions near the base of the skull.

Progressive impairment of brain oxidative metabolism reversed by reperfusion following middle cerebral artery occlusion in anaesthetized baboons.

A better understanding of the temporospatial evolution of ischaemic brain tissue towards necrosis would be of crucial value to establish and validate therapeutic strategies for stroke in man. By means of sequential positron emission tomographic (PET) studies performed through the acute to the chronic stages of infarction, we addressed the question of the effect of 6 h temporary occlusion of the middle cerebral artery (MCAO) on the evolution of the volume of severely hypometabolic tissue in anaesthetized baboons and compared it to that reported previously in permanently occluded baboons. Thirteen anaesthetized baboons underwent serial PET (15O steady-state technique) examinations before and 1, 4, 7, 24-48 h and 15-62 days following transorbital MCAO. Reperfusion, at 6 h post-occlusion, was assessed by Doppler sonography and cerebral blood flow (CBF) values after clip removal. In each baboon, the infarct volume was calculated by standard histological procedures 20-91 days after MCAO. The severely hypometabolic tissue volume, as defined by a threshold of oxidative metabolism, showed a progressive increase for up to 24-48 h in a not dissimilar manner to that found in baboons with permanent occlusion. However, these hypometabolic volumes regressed in the chronic stage (p < 0.05). Permanent and temporary occluded baboons, when taken together, showed a highly significant relationship between histological and chronic hypometabolic volumes (r = 0.84; p < 0.001). Moreover, the final hypometabolic volume where cerebral metabolic rate of oxygen (CMRO2) was below 40% of contralateral metabolism corresponded well to that of histological infarction volume. We conclude that, in anaesthetized baboons, restoration of blood flow will reverse (even if not immediately) the progressive derangement of metabolism after MCAO and markedly limit the final volume of consolidated infarction.)

Comparative quantitation of cerebral blood volume: SPECT versus PET.

UNLABELLED: Quantification of cerebral blood volume (CBV) measured by SPECT has been used for evaluation of cerebral hemodynamics in patients with cerebrovascular diseases. The accuracy of such quantification, however, has not been validated with PET. METHODS: CBV was assessed using SPECT and in vitro 99mTc-labeled red blood cells and PET with the 15O steady-state inhalation method and C15O. In 23 patients with carotid artery disease, we measured hemispheric (including cortical and subcortical areas) CBV, and in 11 patients, we measured regional CBV in small cortical regions. We further evaluated the interhemispheric and inter-regional asymmetry of CBV with both methods. RESULTS: Quantitative values of both hemispheric and regional CBV measured by SPECT were significantly correlated with those measured by PET in the same patients. There was a significant correlation between the side-to-side asymmetry of CBV for both methods. CONCLUSION: This study demonstrates usefulness and the accuracy of SPECT for quantitative CBV assessment in comparison with the less widely available PET procedures.

Early reperfusion in the anesthetized baboon reduces brain damage following middle cerebral artery occlusion: a quantitative analysis of infarction volume.

BACKGROUND AND PURPOSE: Because in humans the clinical benefits of reperfusion remain controversial, it is important to determine whether reperfusion per se reduces infarct volume. In the nonhuman primate, mostly semiquantitative assessments of infarction have been performed. When ischemic volumes have been calculated, it has been for the acute or subacute stages of experimental stroke and may thus not adequately reflect the total volume of consolidated infarction. METHODS: Anesthetized baboons were subjected to 6 hours of either reversible or permanent middle cerebral artery occlusion (MCAO). Approximately 4 weeks later, the brains were processed for neuropathological examination to allow assessment of the final infarct volume determined by the difference of healthy tissue between occluded and nonoccluded hemispheres. RESULTS: Reversible MCAO resulted in a small essentially subcortical infarction (mean+/-SD, 0.58+/-0.31 cm3) in 6 of 10 baboons: the infarct (pannecrosis) was restricted to the head of the caudate nucleus, internal capsule, and putamen; 4 of 10 baboons showed no evidence of macroscopic infarction. Permanent MCAO produced a larger subcortical infarct in all 7 baboons studied (2.37+/-1.32 cm3; P=.0006 by Wilcoxon-Mann-Whitney test); the lesion was more extensive and encompassed the external capsule and, in 2 baboons, the adjacent insular cortex. CONCLUSIONS: We conclude that under optimal experimental conditions, an ischemic episode of 6 hours in duration is well tolerated in the anesthetized adolescent baboon, with 4 animals showing no signs of macroscopic brain damage. Thus, early reestablishment of cerebral blood flow after a focal ischemic insult is not detrimental but indeed is beneficial in terms of the final infarct volume (both at the subcortical and cortical levels) produced by occlusion of a major cerebral artery. The data further suggest a feasible time window in which to initiate and continue therapeutic interventions.

The in vivo metabolic pattern of low-grade brain gliomas: a positron emission tomographic study using 18F-fluorodeoxyglucose and 11C-L-methylmethionine.

OBJECTIVE: The object of the present study was to identify metabolic differences between low-grade astrocytomas and oligodendrogliomas and to improve their diagnosis and noninvasive assessment, because both types of tumors look very similar from the point of view of clinical and radiological data (as assessed by computed tomography and magnetic resonance imaging). METHODS: Before any aggressive treatment, 22 patients with primary low-grade gliomas (astrocytomas in 12 patients and oligodendrogliomas in 10) were investigated with positron emission tomography for both glucose metabolism (18F-fluorodeoxyglucose) and amino acid uptake (11C-L-methylmethionine). An original software that allows a full metabolic analysis of the tumor region of interest (defined from the T1-weighted magnetic resonance image) and compares tumor tissue uptake tracer concentrations with average healthy tissue values has been implemented for data processing. Heterogeneity of each individual tumor has been taken into account and was expressed in histograms, which provided data about the mean and also extreme and intermediate values of tracer concentrations and the way these values are distributed among the full tumor mass. RESULTS: It has been shown that both tumor types exhibit a glucose hypometabolism (slightly more pronounced with astrocytomas), whereas they strongly differ in methionine uptake, which is high in all oligodendrogliomas and either decreased, normal, or moderately increased in astrocytomas. This latter metabolic difference between both tumor populations may be partially explained by their different cell densities. CONCLUSION: This study suggests that despite similar radiological and clinical presentations, these two kinds of low-grade gliomas are metabolically different and could therefore have specific responses to different therapies. Moreover, their in vivo metabolic follow-up with positron emission tomography should rely on different parameters, depending on their histological type; methionine uptake may be more relevant than glucose metabolism in the follow-up of oligodendrogliomas.