¹H-MRS-Guided Stereotactic Brain Biopsy

with a power of 0.8 and type I error probability of 0.05. Anyway, in our opinion, metabolic guidance may be rather useful during stereotactic biopsy of parenchymal brain tumors, and may be especially recommended for recurrent, particularly previously irradiated, neoplasms, or for sampling of the highly vascular lesions since in the latter cases, detection of the spectroscopic abnormalities outside the contrast-enhanced area may permit a surgeon to obtain a representative tissue specimen with reduced risk of hemorrhagic complications [6] . On the other hand, it is clear that just a confirmation of the presence of glioma with stereotactic biopsy is not sufficient enough for modern treatment objectives. These neoplasms are well known for their heterogeneity; therefore, the main requirement for tissue sampling in such cases is detailed characterization of the tumor type and histopathological grade. However, in the series of the MD Anderson Cancer Center, 63% of gliomas designated after stereotactic biopsy as being of low or intermediate grade and 60% of anaplastic astrocytomas were found to be more malignant after open surgery [7] . In our analysis, just 36% of the patients diagnosed as having low-grade glioma after stereotactic biopsy had complete diagnostic agreement in tumor typing and grading compared to subsequent resection [8] . Certainly, the diagnostic accuracy of the stereotactic procedure may be improved by multiple tissue sampling; however, it is not clear, whether or not metabolic guidance may also be helpful. Unfortunately, the referenced study by Frati et al. [1] does not provide reliable information on this issue. From our experience, addition of spectroscopic support with targeting the area of the lesion characterized by the lowest NAA/Cho ratio did not result in improved diagnostic accuracy of the stereotactic procedures [5] . Major diagnostic disagreement between biopsy and subsequent surgical resection was observed in 6 out of 18 cases and was most frequently related to initial undergrading of the nonenhancing WHO grade III gliomas [5] . Others demonstrated that the direction of tissue sampling on the area with the lowest NAA/Cho ratio may lead to erroneous diagnosis of anaplastic astrocytoma in cases of glioblastoma [9] . The NAA/Cho ratio is widely recognized as a marker of glioma presence, proliferative activity, and growth characteristics, but it seems that stereotactic targeting of the tumor area with its lowest value does not necessarily result in the precise determination of the histopathological grade. Further studies are needed to identify the optimal metabolic target during spectroscopy-guided stereotactic procedures. Presence and distribution of other metabolites, particularly lactate and mobile lipids [9] , should also be taken into consideration ( fig. 1 ). 1 H-MRS is intrinsically a multiparameter investigation and limitation of its evaluation to only one metabolite content or single ratio does not seem reasonable. Therefore, at present, during the evaluation of the spectroscopic data in addition to calculation of the various metabolite contents and ratios, we are constantly performing pattern analysis of the whole pathological 1 H-MR spectrum as well ( fig. 2 ). The recently published article by Frati et al. [1] regarding frameless stereotactic brain biopsy was read with great interest. One of the objectives of that study was the evaluation of the usefulness of proton magnetic resonance spectroscopy ( 1 H-MRS) for guidance of tissue sampling in 296 brain lesions, including 211 gliomas. In 37 cases, contrast enhancement of the mass was absent. Before the stereotactic procedure, a multivoxel spectroscopic examination was performed, metabolic data were incorporated into the neuronavigation system and fused with the structural MR images, and a target was selected in the area of the lesion corresponding to the highest value of the choline-containing compounds to N-acetylaspartate (Cho/NAA) ratio, which was identified on the visualized grid of the spectroscopic voxels showing automatically calculated numerical values. Three targets were used in each case, and the number of obtained tissue specimens varied from 4 to 12 (average, 6). Overall, a diagnosis was established in all but one case (diagnostic yield 99.7%). It was found that histopathological grades within the tissue samples taken from the same tumor are well associated with the values of Cho/NAA ratios (p ! 0.01). Regrettably, no results are presented on the correlation of the metabolic data with the MIB-1 index, while immunostaining for Ki-67 was regularly done. However, the authors concluded that spectroscopic guidance may be very useful for appropriate targeting of the stereotactic brain biopsy, particularly in cases of nonenhancing lesions. Certainly, the use of metabolic and functional data for target selection during both frame-based and frameless stereotactic tissue sampling seems very promising. The application of chemical shift imaging is especially attractive, because it can be easily attained at the time of routine MRI, is fully noninvasive and highly sensitive to the presence of pathology. Indeed, the reported diagnostic yield of 1 H-MRS-guided brain biopsy typically attains 100% [2–5] . Moreover, according to our own experience, spectroscopic support may facilitate histopathological diagnosis both on frozen and permanent tissue sections [5] . However, we were not able to demonstrate a greater diagnostic yield of 1 H-MRS-guided procedures (100% in 30 patients) compared to MRI-based ones (90% in 39 patients), seemingly due to a relatively small sample size of our series [5] . In fact, a subsequent analysis revealed that with the same parameters of the study, it would be necessary to have 85 and 111 patients in the case and control groups, respectively, to prove the presence of a difference in the diagnostic yield Received: November 5, 2011 Accepted: November 16, 2011 Published online: January 24, 2012