Whereas there have been several studies that focus on the applications of advanced MR techniques to the assessment of tumor grade and type, there have been relatively few studies that have provided a detailed analysis of the metabolic and physiological properties of grade 3 gliomas. Even when they are included, the number of grade 3 lesions is often small, and they are frequently grouped with a relatively large population of grade 4 lesions under the general designation of high-grade glioma. This means that the results presented are typically dominated by the findings for grade 4 glioma. Defining the metabolic and physiological characteristics that best represent grade 3 lesions is an important clinical problem that has so far not been adequately addressed. Determining where within the region of hyperintensity on T2-weighted MR images, it would be best to take tissue samples to have the highest probability of obtaining an accurate diagnosis is an important question that requires information about the functional properties of the lesion. It is for this reason that we have focused on the evaluation of spatial variations in parameters that were extracted from metabolic and physiological MR images of patients with grade 3 glioma.
The current study indicates that the median n
ADC values were similar in enhancing and nonenhancing lesions from the patients with grade 3 lesions, with values of 1.59 and 1.67. These are considerably higher than the median values that we and others have reported in the literature for grade 4 lesions (typically 1.4–1.5) [44
] and are in the same range that we have observed for grade 2 lesions (1.6 for oligodendrogliomas and of 2.0 for astrocytomas) [45
]. The interpretation of these differences is complex because there have been a number of reports in the literature that there are inverse correlations between ADC and markers of tumor proliferation for brain tumors, including of Ki-67 labeling index [46
], cell density [47
], tumor cellularity [48
], and malignancy [33
]. When these studies are examined in the composition of the populations of patients being considered, it becomes clear that the results cannot be considered representative for grade 3 glioma because there were only 2 of 47 [46
], 2 of 18 [47
], 0 of 56 [48
], and 6 of 59 [33
] of the patients with this diagnosis. In a recent study that focused specifically on patients with grade 2 and 3 glioma, a positive rather than a negative correlation between ADC and total tumor cell number was reported for both types of lesions [49
]. In our study of grade 4 glioma, the lower 10th percentile of ADC values and larger areas of n
ADC < 1.5 were both associated with a poor survival [44
], which might indicate an increase in tumor burden. The median n
ADC levels in the contrast enhancing and T2 hyperintense areas of grade 3 patients were both found to be higher than 1.5 in our study, indicating smaller n
ADC < 1.5 volumes, that may contribute to the improved survival, which is typically observed in such patients. Our study is consistent with the diffusion properties in grade 3 glioma being more similar to those in lower-grade lesions and with the processes determining the observed levels of n
ADC and n
FA being associated with demyelination, edema, and disruption of normal brain as opposed to being direct measures of tumor cell density.
There were 21 (41%) of the 51 patients in our study with areas of low to moderate contrast enhancement. These enhancing lesions were relatively small in volume and resided mainly within the metabolically abnormal regions. This is consistent with the findings of Pirzkall et al. [19
], in which the enhancing lesion had the highest median n
Cho and the lowest median n
NAA. This was not the case for grade 4 lesions, where the highest Cho levels were observed in the nonenhancing portion of the tumor [44
]. This may be due to there being more necrosis in the contrast enhancement for grade 4 tumors, which would reduce overall tumor cellularity. The fact that the median n
ADC was significantly higher and the median n
FA was significantly lower in the enhancement of grade 3 gliomas compared with NAWM may suggest that there was breakdown of the normal tissue architecture in these regions that allowed for more rapid diffusion. The high median LL/n
NAA and low median n
Cr levels suggest that there was a tendency toward the enhancing regions being hypoxic, which may also be a factor in inducing the breakdown of the blood-brain barrier. Overall, these results suggest that, when present, the regions of enhancement are the most malignant portion of untreated grade 3 glioma and provide an appropriate target for directing biopsy or surgical sampling for diagnostic purposes.
The other anatomic region studied was the portion of the T2 hyperintensity that was nonenhancing. For 30 (59%) of 51 patients, this comprised the entire lesion. There were highly variable median levels of Cho in these regions, with 16 being lower and 35 being higher than the levels in NAWM. Although there were variations in median levels of NAA, these were all lower than the corresponding values in NAWM. Differences in the normalized metabolite levels reflect the heterogeneity in lesion characteristics between patients and underline the motivation for using the CNI index, which defines whether a spectrum is abnormal by combining the values of nCho and nNAA. The fact that the LL/nNAA was higher in both the nonenhancing and enhancing regions than the value in NAWM is consistent with the presence of regions with hypoxia. The relatively large increase in median nADC and decrease in median nFA in the nonenhancing regions may be attributed to a combination of demyelination, inflammation, edema, and tumor infiltration.
If variations in levels of CNI are used to define regions of abnormality, it is possible to obtain further insight into the relationships between metabolite and diffusion parameters. The three regions considered in this study represented areas that were mildly abnormal (CNI2–3), moderately abnormal (CNI3–4), and very abnormal (CNI > 4). In the mildly abnormal region, the Cho was higher than normal, the Cr was slightly reduced, the NAA was almost half of the normal value, and the LL/nNAA was significantly increased over normal. The reduction in nNAA was inversely correlated with nADC, which was increased to 1.47 and the nFA was decreased to 0.54. This suggests that the region included some tumor infiltration but that the changes in MR parameters were dominated by reduction in neuronal function and disruption of tissue architecture. In the moderately abnormal region, the Cho was significantly higher than normal but the Cr was similar to the values in the mildly abnormal region. The NAA was further reduced, the LL/nNAA further increased, the nADC further increased and the nFA further decreased. This is consistent with the presence of a larger number of tumor cells, together with a continued and more extensive disruption of normal tissue.
In the highly abnormal region, there were further significant increases in the median values of n
NAA, and n
ADC and a continued decrease in the median values of n
FA and n
NAA relative to the moderately abnormal region. This suggests that the highly abnormal region included a relatively large number of tumor cells with a higher likelihood of there being areas with reduced oxygenation. Taking into consideration the variations in metabolite and diffusion parameters, it seems that, for patients with grade 3 glioma, the area of the tumor likely to be the most malignant is at the location of the maximum CNI, irrespective of whether the lesion is enhancing or not. This is consistent with a recent study in non-enhancing grade 2 and grade 3 gliomas, which investigated correlations between spectroscopic parameters and histologic measures, and reported that both the ratio of Cho/NAA and the CNI correlated with MIB-1 proliferative index, cell density, and the ratio of proliferation to cell death [50
In the 30 patients who received lactate-edited spectroscopy, the region with highly abnormal metabolism was observed to have the highest level of lactate. This suggests that these regions exhibit anaerobic glycolysis and are poorly oxygenated. The regions with mild and moderate metabolic abnormalities and the nonenhancing lesion also had lactate levels higher than NAWM, indicating poor oxygenation even in areas of relatively lower tumor invasion. This is consistent with recent studies from our group, which have noted voxels with lactate and lipid peaks in some patients with grade 3 glioma [31
] and reported the presence of significant lactate and lipid peaks with volumes of these regions varying from 0.1 to 11.8 ml [15
]. The findings from this study suggest that the MRSI data may be valuable in predicting sensitivity to radiation therapy by delineating such poorly oxygenated regions.
Some of the diffusion and metabolic imaging patterns were different for subtypes of grade 3 gliomas than had previously been reported for subtypes of treatment-naive grade 2 gliomas. In particular, it was found that the ADC values were similar for all 3 subtypes of grade 3 gliomas, whereas they were significantly higher for grade 2 astrocytomas and oligoastrocytomas compared with grade 2 oligodendrogliomas [45
]. The higher lipid values in subregions of OD3 compared with the other two subtypes of grade 3 glioma is also a novel finding and may indicate a higher tendency to cellular breakdown and the formation of necrosis than for other histologic subtypes.
Overall, the results of the current study suggest that MRSI and diffusion parameters provide complementary information that may be valuable for characterizing nonenhancing regions of the anatomic lesion and possibly directing tissue sampling for accurate diagnosis of tumor grade. This is especially important for distinguishing patients with grade 3 glioma from those with lower-grade lesions. Regions of infiltrative tumor were differentiated by their increased water content (high nADC), altered normal tissue function (high Cho, low NAA), and structure (low nFA). Regions of macroscopic tumor could be identified by high cellularity (high CNI) and major disruptions in the tissue structure and function (low nFA and low NAA) and hypoxia (high Lac or LL/nNAA). These signatures may be of interest not only for directing tissue sampling but also for defining the targets for focal therapy and assessing response to therapy. Future studies that compare presurgery MRSI and diffusion parameters with the findings from image-guided tissue samples would be valuable for validating these biomarkers as clinical tools for managing patients with grade 3 glioma.