Diagnostic, Prognostic, and Predictive Value of the Three Clinically Relevant Molecular Glioma Markers (26,41)

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4 Figure 1 Figure 1: Multifocal nonenhancing tumor (arrow, a-e) with histologic grade II astrocytoma in a 57-year-old woman. However, molecular profile is consistent with the glioblastoma type (phosphatase and tensin homolog mutation, epidermal growth factor receptor amplification, IDHwt ). Despite lack of enhancement, relative cerebral blood volume (rcbv) is high, corresponding to greater degree of malignancy. (a, b) Axial three-dimensional T2-weighted fluid-attenuation inversion-recovery 2-mm reconstruction images (repetition time [TR] 6500 msec; echo time [ TE ], 110 msec; inversion time [ TI] 2002 msec; flip angle, 90 ; voxel size, mm3). (c, d) Axial three-dimensional inversion-recovery fast spoiled gradient-echo contrast-enhanced T1-weighted 2 mm reconstruction images (TR, 6.1 msec; TE, 2.1 msec; TI, 450 msec; flip angle, 12 ; voxel size, mm3). (e, f ) rcbv maps overlaid on contrast-enhanced T1-weighted images derived from dynamic susceptibility contrast-enhanced imaging (TR, 2000 msec; TE, 45 msec; flip angle 90 ; voxel size, mm3; 52 dynamics; contrast material bolus of 10 mmol gadolinium contrast agent delivered after 20-second baseline acquisition without contrast material). genes. Deletion of 1p with or without deletion of 19q is the hallmark feature of oligodendrogliomas (43). While it is present in approximately 60% 90% of histopathologically diagnosed oligodendrogliomas (44), molecularly, it is now considered its defining feature. Radiology: Volume 284: Number 2 August 2017 n Histopathologically diagnosed oligodendroglioma without 1p19q codeletion is now classified as diffuse glioma of the oligodendroglial phenotype (27). This mutation, therefore, has diagnostic, prognostic, and predictive relevance (Table 2). radiology.rsna.org Clinical Relevance Results of two major randomized controlled trials have indicated the clinical relevance of this mutation. Early procarbazine, lomustine, and vincristine chemotherapy added to radiation therapy greatly increases survival in patients 319

5 Table 2 Diagnostic, Prognostic, and Predictive Value of the Three Clinically Relevant Molecular Glioma Markers (26,41) Value 1p19q Codeletion MGMT Promoter Methylation IDH Mutation Diagnostic value Prognostic value Predictive value Yes, presence confirms oligodendroglioma. Absence classifies a histopathologically diagnosed oligodendroglial tumor as diffuse glioma of the oligodendroglioma phenotype Yes, patients with codeleted grade II and III tumors have better prognosis compared with those without, regardless of therapy (median overall survival, 8.0 years for 1p19q codeleted and IDH-mutated versus 6.3 years for 1p19q-intact and IDH-mutated low-grade glioma) (17) Yes, patients with grade III codeleted tumors have better survival after treatment with combined procarbazine, lomustine, and vincristine chemotherapy and radiation therapy (median overall survival, 14.7 years) than with radiation therapy alone (median overall survival, 7.3 years). This benefit is not seen in patients with intact tumors (median overall survival, 2.7 years vs 2.6 years for combined chemotherapy and radiation therapy vs radiation therapy alone, respectively) No Yes, patients with methylated grade III tumors have better prognosis (after radiation therapy, chemotherapy, or both; median overall survival, 23 months vs 16 months in patients with unmethylated tumors). However, this is explained by the common co-occurrence of IDH mutation and MGMT promoter methylation Yes, patients with MGMT methylated glioblastomas have better survival after treatment with combined Temozolomide chemotherapy and radiation therapy (median overall survival, 22 months) than with radiation therapy alone (median overall survival, 15 months). This benefit is not seen in patients with unmethylated tumors (median overall survival, 12 months vs 13 months for combined chemotherapy and radiation therapy vs radiation therapy alone, respectively) (9) Yes, grade II and III gliomas are commonly IDH mutated, while gliosis and other glioma entities are not. In glioblastomas, the IDH mutation allows differentiation between secondary (IDH-mutated) and primary glioblastomas (IDHwt) Yes, patients with IDH-mutated gliomas have better prognosis than do those with IDHwt gliomas of similar grade. IDHwt gliomas, even low-grade, have poor prognosis similar to that of glioblastoma Probably in grade II and III diffuse gliomas, but definitive evidence is still lacking Note. There is a an important distinction between prognostic and predictive value: The prognostic value indicates the effect on outcome independent of intervention, while the predictive value indicates the benefit of one treatment over another. with anaplastic oligodendroglioma when compared with treatment with radiation therapy alone (7,8). Procarbazine, lomustine, and vincristine plus radiation therapy is now the standard of care for patients with 1p19q-codeleted anaplastic oligodendrogliomas. This improved outcome of adjuvant procarbazine, lomustine, and vincristine after radiation therapy also was established recently for patients with low-grade tumors (45). Conventional Imaging Features The structural imaging features of 1p19q codeletion are not consistently identified. Several features are more common but certainly not unique in codeleted compared with intact tumors (Table 3). Localization. Localization of codeleted tumors is most common in the frontal, parietal, or occipital lobes, while intact tumors are more likely to be found in the temporal, insular, or temporoinsular region (46,47). In their retrospective study of 50 grade II III oligodendrogliomas and mixed oligoastrocytomas, Fellah et al (48) confirmed that codeleted tumors were more likely to be found in the frontal lobe (nine of 19), but nine of 31 intact tumors also were located in the frontal lobe. Although none of the codeleted tumors were in the temporoinsular region, six of 19 tumors could be found in the insular region. Appearance. In codeleted tumors, the tumor margin is generally indistinct, signal intensity on both T1- weighted and T2-weighted MR images is heterogeneous, and calcifications are commonly seen (Fig 2) (47,49,50). The presence of contrast material enhancement is not a distinguishing feature between codeleted and intact tumors (46,47). In their large study of 86 patients with grade II and III oligodendrogliomas and oligoastrocytomas, in which imaging findings were directly correlated with stereotactically obtained tissue biopsy results, Jenkinson 320 radiology.rsna.org n Radiology: Volume 284: Number 2 August 2017

6 Table 3 Imaging Features of 1p19q Codeleted versus Intact Oligodendrogliomas Feature 1p19q Codeletion 1p19q Intact Localization Frontal, parietal, and occipital predominance Temporal, insular, and temporoinsular predominance Tumor margin Indistinct May be sharp Signal intensity Heterogeneous* Homogeneous Calcifications Common (.40%) Uncommon (20%) ADC No difference lower maximum ADC No difference rcbv Mildly elevated in grade II Not elevated in grade II Fluorodeoxyglucose Mildly increased uptake Not increased * More prominent on T2-weighted than on T1-weighted image. et al (50) demonstrated that a sharp tumor border was only rarely associated with 1p19q codeletion (two of 42). An indistinct border, present in almost all codeleted tumors, however, also was seen in the majority of intact tumors (24 of 38). Similar findings were reported by Chawla et al (51). Therefore, the presence of an indistinct border does not allow codeleted tumors to be distinguished from intact tumors, but a sharp border makes a 1p19q-intact tumor more likely. Susceptibility artifacts on MR images, presumably due to calcification and hemorrhage, were found significantly more frequently in codeleted than in intact tumors in a study (51) of 40 histopathologically proven grade II III oligodendrogliomas. Although these artifacts were seen in the majority of codeleted tumors (18 of 23), they also were commonly present in intact tumors (eight of 17). Authors of other studies (47,50) did not find a significant difference in the presence of susceptibility effects. In addition, Fellah et al (48) only noted hemorrhage in grade III and not in grade II tumors. In the Fella et al study, no difference in signal intensity was observed, which the authors attributed to interrater variability related to the subjective, qualitative nature of the assessment. Texture analysis. It is, however, possible to quantify signal intensity patterns by using texture analysis. Texture analysis allows characterization of the local pattern of image signal intensity because it allows the local spatial frequency content to be determined. Low frequencies indicate smooth and homogeneous signal intensity, while high frequencies are seen in heterogeneous and detailed regions. Building on their earlier work of subjective visual assessment of signal intensity characteristics, Brown et al (52) used S-transform texture analysis to distinguish 1p19q-codeleted (n = 31) from intact (n = 24) low-grade oligodendrogliomas or mixed oligoastrocytomas, respectively. The findings were compared with visual assessment of signal intensity heterogeneity. No differences in texture were found on the T2-weighted fluid-attenuation inversion-recovery images, and only moderate differences were found on the contrast material enhanced T1- weighted images. The midfrequency domain of the T2-weighted sequence allowed differentiation of codeleted from intact tumors with great accuracy (93%). Both sensitivity and specificity were high (93% and 92%, respectively) and were much higher than those obtained with visual assessment (67% and 75%, respectively). Advanced Imaging MR imaging and spectroscopy. Chawla et al (51) found that combining dynamic susceptibility contrast-enhanced MR perfusion imaging with proton ( 1 H) MR spectroscopy added accuracy for distinguishing codeleted from intact grade II III oligodendrogliomas, because it allowed metabolite ratios to be obtained from tumor regions with maximum relative cerebral blood volume (rcbv) (51). Of all measured metabolite ratios, the choline-to-creatine ratio had the highest predictive value, with moderate accuracy of 69% when combined with maximum rcbv. Fellah et al (48) used diffusionweighted imaging, dynamic susceptibility contrast-enhanced MR perfusion imaging, and 1 H MR spectroscopy to distinguish codeleted from intact tumors in a retrospective cohort of 50 grade II III oligodendrogliomas and mixed oligoastrocytomas and found no significant differences in apparent diffusion coefficients (ADCs), rcbv, and metabolite ratios independent of tumor grade. Multiparametric assessment was slightly better than conventional MR imaging for distinguishing codeleted from intact tumors, but the misclassification error rate was still high (48% vs 40%, respectively). When only grade II oligodendrogliomas were selected, rcbv was significantly higher in codeleted tumors, a result in line with those of other studies (53 55). Jenkinson et al (56) reported that an rcbv ratio of greater than 1.6 is predictive of the codeleted genotype with 92% sensitivity and 76% specificity (56). The finding is attributed to the microvascular proliferation present in even low-grade oligodendrogliomas. By using histogram analysis, Emblem et al (57) also were able to identify codeleted tumors in an unselected, albeit retrospective population of patients with grade II-IV gliomas. Their histogram analysis also allowed high-grade gliomas to be distinguished from low-grade gliomas (which included oligodendroglial tumors), and high-grade oligodendroglial tumors from low-grade ones. The conventional hot-spot technique, measuring rcbv only in selected tumor regions where perfusion appears to be high, generally does not allow this distinction to be made. There is conflicting evidence regarding the value of diffusion-weighted imaging. While Fellah et al did not find any differences in ADC, Jenkinson et al (58) reported lower maximum ADC and mean histogram-derived ADC in 1p19q codeleted tumors. Nuclear medicine. Metabolic imaging with fluorodeoxyglucose positron emission tomography (PET) or single Radiology: Volume 284: Number 2 August 2017 n radiology.rsna.org 321

7 Figure 2 methionine, reflecting cell and microvascular proliferation, also was found to be increased in grade II but not in grade III codeleted tumors in a study of 102 consecutive patients (61). In a prospective study (62) of 144 unselected patients with gliomas, increased fluoroethyltyrosine uptake was reported in tumors with oligodendroglial components compared with astrocytic tumors and in codeleted compared with intact tumors. However, fluoroethyltyrosine PET did not allow reliable identification of the 1p19q codeletion in individual patients, mostly because of overlapping findings between oligodendroglial tumors and high-grade astrocytic tumors. Summary In summary, conventional imaging features that traditionally are considered typical of oligodendrogliomas are commonly associated with 1p19q codeletion (Tables 3, 4), namely a heterogeneous signal intensity (particularly on T2- weighted images) and the presence of calcifications. A sharp margin is indicative of a 1p19q-intact tumor. Advanced imaging techniques have modest value for distinguishing codeleted from intact tumors. Perfusion and metabolism seem to be mildly increased in codeleted tumors, and these parameters particularly allow these to be distinguished from intact tumors when only low-grade tumors are selected. The choline-to-creatine ratio may add diagnostic accuracy when both grade II and grade III oligodendrogliomas are considered. IDH Mutation Figure 2: 1p19q codeletion versus intact gliomas. (a, b) 1p19q codeleted glioma (oligodendroglioma) in a 47-year-old man. (c, d) 1/19q intact glioma in a 29-year-old woman. Note indistinct border of codeleted tumor (arrowheads) versus sharp border of intact tumor (arrow). Also, there is marked signal intensity heterogeneity on both T2-weighted and contrast-enhanced T1-weighted images in codeleted tumor, while signal intensity is homogeneous in intact tumor. (a) Axial fast spin-echo T2-weighted image ( TR, 4130 msec; TE, 93 msec; flip angle, 150 ; voxel size, mm 3 ). (b) Axial 4.8-mm reconstruction of contrast-enhanced threedimensional magnetization-prepared rapid gradient-echo T1-weighted image ( TR, 2250 msec; TE, 3.9 msec; TI, 1100 msec; flip angle, 15 ; voxel size, mm 3 ). (c) Axial spin-echo T2-weighted image ( TR, 4560 msec; TE, 100 msec; flip angle, 90 ; voxel size, mm 3 ). (d) Axial contrast-enhanced spin-echo T1-weighted image ( TR, 450 msec; TE, 14 msec; flip angle, 90 ; voxel size, mm 3 ). photon emission computed tomography, or SPECT, has shown low uptake in intact and high uptake in codeleted grade II tumors (59,60). Uptake of Background IDH mutation is an early event in the development of diffuse glioma and is present in virtually all 1p19q-codeleted tumors and also in other tumor lines. IDH-mutated tumors can be divided into mutually exclusive groups of astrocytic (with TP53 mutation) and oligodendroglial (with 1p19q codeletion) tumors (63,64). The IDH mutation is implicated in glioma genesis and is present in most grade II-III and secondary glioblastomas (23). It is rare in de novo glioblastomas 322 radiology.rsna.org n Radiology: Volume 284: Number 2 August 2017

8 Table 4 Distinctive Imaging Features of 1p19q Codeletion, IDH Mutation, and MGMT Promotor Methylation 1p19q Codeletion IDH Mutation MGMT Promoter Methylation T2-weighted heterogeneity, especially when assessed with texture analysis Calcifications Mildly elevated rcbv Increased fluorodeoxyglucose uptake Note: sharp margin indicates 1p19q intact tumor (, 5%). Most tumors are IDH1 mutated (codon R132); the mutually exclusive IDH2 mutation (codon R172) occurs in less than 3% of glial tumors. The IDH1 gene encodes cytosolic IDH1, whereas IDH2 encodes mitochondrial IDH2 (65). IDH is a major enzyme in the citric acid cycle and may also play an important role in the cell s defense against oxidative stress (63). Both mutant IDH1 and IDH2 result in the production of 2-hydroxyglutarate (2-HG), which is considered an oncometabolite and may serve as a biomarker for assessment of tumor progression and response to treatment. It is unclear whether it is the oncogene (IDH mutation), the oncometabolite (2-HG), or both that causes the tumor (63). Clinical Relevance The IDH mutation has diagnostic, prognostic, and possibly predictive relevance (Table 2). IDH mutation helps distinguish secondary from primary glioblastomas. IDH-mutated glioblastomas are commonly secondary and of the proneural gene expression subtype (66). IDHwt, on the other hand, is seen in primary glioblastomas of the classic gene expression subtype, which is further characterized by epidermal growth factor receptor overexpression. In grade II and III tumors, the IDH mutation is associated with a better prognosis (24,64), while the IDHwt is associated with poor prognosis similar to that of primary glioblastomas (18). This is also the case with glioblastomas, but only in combination with MGMT promoter methylation status (67). Patients with 1p19q intact Presence of 2-HG ( 1 H MR spectroscopy) Frontal lobe Large unenhancing portion Sharp border Increased K trans but an IDH-mutated grade III tumor also may show improved survival after combined alkylating chemotherapy with radiation therapy, indicating that not only 1p19q status, but also IDH status may help identify patients who would benefit from this therapy (14). Conventional Imaging Features Localization. In a retrospective study of 193 patients with astrocytoma grade II-III, IDH-mutated tumors were reported to be more frequently confined to a single lobe (68). Wang et al (64) found a significant preferential localization of low-grade IDH1-mutated gliomas in the frontal lobe, specifically in the area surrounding the rostral extension of the lateral ventricles and surrounding the left hippocampus. Grade II-III IDHwt low-grade tumors, on the other hand, more commonly have a multilobar (68), frontotemporal-insular localization (86% vs 30% in IDHwt vs IDH-mutated tumors, respectively) (69). IDH-mutated glioblastomas were also found most frequently in the frontal lobe (67), specifically around the rostral extension of the lateral ventricles (70), which also has been linked to the proneural gene expression glioblastoma subtypes that are hypothesized to develop from a specific glial precursor cell (31,64). The topographic distribution is consistent with the widely held hypothesis that IDH-mutated gliomas originate from distinct precursor cells in the subventricular zone, which is the largest germinal region in the brain and lines the lateral ventricles and the hippocampus. The predilection for the same lobe as 1p19q codeleted tumors is not surprising given the observation that the proneural signature tends to be enriched in oligodendrogliomas (31) and has given rise to the speculation that these genetic mutations may be linked (67) with the IDH mutation that occurs before and potentially facilitates 1p19q codeletion (64). Appearance. The presence of large portions of nonenhancing tumor in glioblastomas is reported to be strongly associated with IDH1 mutation (67). The combination of this feature and frontal lobe localization, larger tumor size, and presence of cysts and satellite lesions allowed IDH-mutated (n = 14) to be distinguished from IDHwt (n = 188) tumors with 98% accuracy in a retrospective study (67) of 202 patients with glioblastomas (Fig 3). The specificity for identification of IDH-mutated tumors was very high (. 99%), but sensitivity was moderate (73%). The prevalence of nonenhancing portions in the tumor suggests that IDH mutation is linked to low vascular endothelial growth factor levels, although this notion is controversial (67). In their series of grade II-III astrocytic tumors from which oligodendroglial tumors were excluded, Qi et al (68) also noted that IDH-mutated tumors were significantly more likely to show less contrast material enhancement. In addition, they reported that sharp tumor margins and homogeneous signal intensity were significantly more common in mutated tumors, regardless of tumor grade. No difference was seen in edema or mass effect in this study. Metellus et al (69) described seven patients with IDHwt low-grade tumors in whom both 1p19q codeletion and TP53 expression were lacking. Thus, despite being low-grade tumors, they were of the glioblastoma type (Table 1). Tumors in these patients more commonly had an indistinct border (100% vs 45% in IDHwt vs IDH-mutated tumors, respectively), in keeping with findings from the Qi et al study (68), and a diameter larger than 6 cm (86% vs 45% in IDHwt vs IDH-mutated tumors, respectively). Radiology: Volume 284: Number 2 August 2017 n radiology.rsna.org 323

9 Figure 3 Figure 3: IDHwt vs IDH-mutated astrocytomas. (a, b) Grade III IDHwt astrocytoma in a 50-year-old woman. (c, d) Grade III IDH-mutated astrocytoma in a 42-year-old man. Note multilobar frontotemporal-insular localization, heterogeneous signal intensity, and multifocal enhancement (arrows) in IDHwt tumor. IDH-mutated tumor has typical frontal localization, in close proximity to rostral ventricle. There is only minimal enhancement (arrowhead). (a) Axial spin-echo T2-weighted image ( TR, 5610 msec; TE, 120 msec; flip angle, 90 ; voxel size, mm 3 ). (b) Axial 2-mm reconstruction of three-dimensional gradient-recalled contrast-enhanced T1-weighted image ( TR, 12 msec; TE, 5.8 msec; flip angle, 30 ; voxel size, mm 3 ). (c) Axial spin-echo T2-weighted image ( TR, 6342 msec; TE, 120 msec; flip angle, 90 ; voxel size, mm 3 ). (d) Axial 2-mm reconstruction of three-dimensional gradient-recalled contrast-enhanced T1-weighted image ( TR, 12 msec; TE, 5.8 msec; flip angle, 30 ; voxel size, mm 3 ). Baldock et al (71) developed a patient-specific mathematical model of tumor growth, with which a biologic aggressiveness ratio could be obtained. A high biologic aggressiveness is characteristic of more nodular, less diffuse tumors, while more diffuse, less nodular tumors are characteristic of low biologic aggressiveness. By applying this model to 172 patients with newly diagnosed enhancing gliomas, Baldock et al were able to distinguish IDH-mutated (low biologic aggressiveness index) from IDHwt (high biologic aggressiveness index) tumors with greater than 90% accuracy. A significant difference in survival also was seen when patients were stratified according to this index. The difference in biologic aggressiveness was driven by the high proliferation rate of the IDHwt tumors. There was no observable difference in growth velocity between IDHwt and mutated tumors, but the latter grow more diffusely. Advanced Imaging MR imaging and spectroscopy. Diffusion-weighted imaging may have some value for distinguishing IDH-mutated from IDHwt tumors, although evidence is very limited. Elkhaled et al (72) found a significant negative correlation between ADC and 2-HG levels detected with high-resolution magic angle spinning, which suggests that ADC may be decreased in IDH-mutated tumors. In comparison to the limited value of advanced MR imaging techniques, MR spectroscopy is a promising method to help identify IDH-mutated tumors noninvasively. 2-HG is not detectable under normal conditions, when concentration is too low. However, with the accumulation of 2-HG with IDH mutation, concentration can increase more than 100-fold (5 35 mm) and can reach levels at which it can be detected (73). In tissue samples, 2-HG has been detected with high-resolution magic-angle spinning (72). High-resolution magicangle spinning is a technique in which the tissue sample is physically spun at the magic angle with respect to the magnetic field to increase the resolution of the spectrum for improved identification and quantification. Ex vivo, high-resolution magic-angle spinning allowed detection of 2-HG in high (84%) concordance with tissue analysis results (72). Discordance was caused both by false-negative and false-positive findings. By using two-dimensional 324 radiology.rsna.org n Radiology: Volume 284: Number 2 August 2017

10 correlation methods, Kalinina et al (74) reached high accuracy (97%). In vivo MR spectroscopy has the great advantage of being entirely noninvasive and not requiring tissue from biopsy or resection. It may allow the follow-up of tumors treated with agents targeted against the IDH mutation. Reliable detection in vivo, however, is challenging due to the fact that the 2-HG spectrum, consisting of multiplets at three locations, largely overlaps with abundant brain metabolites such as glutamate, glutamine, and g-aminobutyric acid at the largest 2.25-ppm multiplet, and N- acetylaspartate at the 1.9-ppm multiplet. The spectral resolution obtained at 3 T is generally insufficient to separate the peaks completely, and hence tailored techniques are required for 2-HG detection and quantification. Bertolino et al (75) demonstrated in a phantom study that 2-HG is significantly confounded by glutamate and N-acetylaspartate when a standard clinical short-echo point-resolved spectroscopic technique is used. However, Bertolino et al postulated that this effect is unlikely to mask the presence of 2-HG entirely, except at very low concentrations. Great care must be taken with absolute quantification when this routine clinical technique is used. Pope et al (76) confirmed the high sensitivity of short-echo point-resolved spectroscopy, but noted that specificity of 2-HG detection may be challenging and reported a false-positive rate of 26% (four of 15 patients with IDHwt). Andronesi et al (73) also showed that fitting conventional one-dimensional spectra might provide false-positive results, and instead used two-dimensional correlation spectroscopy and spectralediting 1 H MR spectroscopy with a clinical 3-T imager. They were able to distinguish the two patients with IDHmutated tumors from eight healthy subjects and patients with IDHwt tumors after careful optimization of the sequence in phantoms and brain biopsy specimens. Choi et al (77) used an optimized point-resolved spectroscopic sequence at 3 T with asymmetric long echo times to narrow both the 2-HG and glutamate multiplets in combination with precise spectral fitting in 30 patients with IDH-mutated grade II-III tumors. They attained 100% sensitivity and specificity for detection of the 15 IDH1- and IDH2-mutated tumors and noted a highly significant difference in the 1 H MR spectroscopically measured concentrations of 2-HG between the IDH-mutated and IDHwt tumors. In keeping with the results of Bertolino et al (75), they noted that this technique is superior to a short-echo sequence (78). Focusing on a separate aspect of the IDH1 mutation induced catabolic changes, Chaumeil et al (79) used hyperpolarized carbon 13 ( 13 C) MR spectroscopy to detect the production of hyperpolarized [1-13 C] glutamate after injection of hyperpolarized [1-13 C] a- ketoglutarate. In their preclinical study of IDH1-mutated rodent tumors, the production was found to be significantly reduced, indicating the potential of hyperpolarized glutamate as an indirect biomarker of IDH1 mutation status. Nuclear medicine. While it has been postulated that IDH-mutated tumors would express a shift toward glycolysis, and hence, increased glucose consumption, Metellus et al (80) did not find a difference in fluorodeoxyglucose uptake between IDH-mutated (n = 20) and IDHwt (n = 13) grade II-III tumors. Summary Conventional imaging features of IDHmutated tumors show considerable overlap with those of IDHwt tumors, but a single frontal lobe localization and large portion of nonenhancing tumor would be indications of IDH mutation. Advanced postprocessing techniques may add diagnostic accuracy by showing a low biologic aggressiveness index in these tumors. MR spectroscopy, albeit with several challenges still to be resolved, is highly promising for measurement of 2-HG as a true, direct, rather than surrogate imaging marker of IDHmutated tumor induced changes. MGMT Promotor Methylation Background MGMT is a DNA repair enzyme, the gene of which is located at chromosome 10q26 (10). High levels of MGMT in cancer cells blunt the therapeutic effect of alkylating chemotherapy. Epigenetic silencing of the MGMT DNA repair gene by means of promoter methylation, on the other hand, results in reduced MGMT expression, and hence, diminished DNA repair, resulting in DNA damage and cell death. This increases the sensitivity of mutated tumors to alkylating agents. IDH mutations cause a cytosine-phosphate-guanine, or CpG, island hypermethylate phenotype, of which the MGMT promoter region is usually a part. Therefore, as a rule, IDH-mutated tumors also show MGMT promoter methylation (81 83). MGMT is methylated in approximately 50% of newly diagnosed glioblastomas (10). Clinical Relevance MGMT promoter methylation status has prognostic and predictive value. In patients with grade III tumors, outcome after therapy is better with MGMT promoter methylation, regardless of whether treatment is with chemotherapy or radiation therapy. This is due to the close correlation between IDH mutations and the presence of MGMT promoter methylation (83,84). In patients with glioblastomas, MGMT promoter methylation is predictive of a better response to temozolomide (10). Conventional Imaging Features Localization. Ellingson et al (85) noted a higher prevalence of methylated tumors in the left hemisphere, mostly in the temporal lobe, and of unmethylated tumors in the right hemisphere. These findings are only in partial agreement with those in an earlier study by Eoli et al (86), who found methylated tumors to be located preferentially in the parietal and occipital lobes, and unmethylated tumors in the temporal lobes (86). In actual contrast to findings in the Ellingson et al study (85) is the reported preferential location of tumors with decreased MGMT protein expression (ie, indicative of MGMT promoter methylation) in the right hemisphere and of tumors with increased MGMT protein expression in the left hemisphere described by Wang Radiology: Volume 284: Number 2 August 2017 n radiology.rsna.org 325

11 et al (87). However, this hemispheric asymmetry was not particularly strong, with 60% (32 of 53) of methylated tumors being right hemispheric, and 59% (59 of 100) of unmethylated tumors localized in the left hemisphere. Authors of other studies (67,88) did not find any predilection for a certain localization or distribution. These discrepancies may, in part, be attributed to differences in patient selection and sample size: Ellingson et al (85) and Drabycz et al (88) only included patients with de novo glioblastomas, while Eoli et al (86) also included patients with secondary glioblastomas. In addition, the study population in the Ellingson et al (n = 358) study was, by far, the largest to date. Appearance. MGMT promoter methylated and unmethylated tumors have largely overlapping imaging features and thus, are not readily distinguishable with conventional imaging (89). In fact, in their multivariate regression analysis of several imaging features in 190 patients with glioblastomas (methylated, n = 74), Carrillo et al (67) noted that imaging features were only slightly better than chance for prediction of methylation status (accuracy, 66%). Some, but not all, authors of studies (86,88) reported that mixed nodular enhancement is more common in methylated glioblastomas, with rim enhancement more common in unmethylated tumors. Authors of most studies (67,86,87) reported no differences in edema, although Ellingson et al (85) noted that the volume of T2 abnormalities was lower in methylated tumors. Also, in patients with methylated glioblastoma specifically, limited peritumoral edema seems to be predictive of better prognosis (67). Texture analysis. Although texture analysis is more sensitive to differences in tissue signal intensity, no differences in texture can be found between methylated and unmethylated tumors (86). When only the T2 abnormalities are considered, a significant prediction of MGMT status can be made, but at low accuracy (64%). A slight improvement (up to 71% accuracy) is achieved by adding the enhancement pattern (ring vs nodular) as a variable. Advanced Imaging MR imaging and spectroscopy. In a multiparametric study in which both computed tomography (CT) and MR imaging characteristics were assessed, Moon et al (90) postulated that methylated tumors display imaging features of lower cellularity (ie, low maximum CT attenuation, high minimum ADC ratio) and more white matter destruction (low minimum fractional anisotropy ratio). Authors of other studies also noted a positive relationship between mean ADC and methylation ratio, and Romano et al (91) reported high sensitivity (84%) and specificity (91%) to distinguish methylated from unmethylated tumors at a cutoff minimum ADC of mm 2 /sec (91,92). Pope et al (93), however, noted a lower, rather than higher, mean ADC L in methylated tumors, while authors of other studies (89,94) did not find any differences in ADC. Differences in study population and size may be one source of these discrepancies. Pope et al (93) only included patients with newly diagnosed glioblastoma (n = 89), while Moon et al (90) included both grade III (n = 7) and IV (n = 17) tumors. Also, in the latter study no effect on overall survival of MGMT status was seen, contrary to expectations. Rundle-Thiele et al (95) demonstrated that these discrepancies also may be attributed to differences in methodology. In the same patient population, they used both a minimum ADC region of interest and a histogram analysis and found a trend toward higher mean minimum ADC ratio and significantly lower ADC L in methylated tumors. The ADC is highly sensitive to the effects of the inadvertent inclusion of (micro) cysts and necrosis, which increase the ADC and may obscure subtle reductions in the ADC in the solid tumor components. Misregistration between the ADC map and contrast-enhanced T1-weighted images may be another source of error. Histogram analysis is less sensitive to outlier values caused by such errors. A lower degree of perfusion in methylated (n = 16) compared with unmethylated (n = 9) tumors (mean rcbv ratio, 5.4 vs 9.5, respectively) was reported by Ryoo et al (96). Despite relatively high sensitivity (73%) and specificity (84%) at a cutoff value of 6.2, there was substantial overlap in values. Moon et al (90) did not find a difference in rcbv between methylated and unmethylated grade III-IV tumors. The clinical value of perfusion is, therefore, not yet established. By using dynamic contrast-enhanced MR imaging, Ahn et al (89) found an increase in the volume transfer constant (K trans ) in methylated compared with unmethylated glioblastomas, which not only has diagnostic implications, but also may be of more fundamental interest. Increased K trans may be a marker of high permeability of the blood vessel wall under certain conditions (eg, low cerebral blood flow), which, in turn, potentially enhances the delivery of drugs across the blood brain barrier. These findings are consistent with those from mouse models. Nuclear medicine. Authors of one study (97) of nonenhancing grade II-III gliomas (n = 20) reported that the carbon 11 ( 11 C) methionine uptake was increased (tumor to normal tissue ratio 1.6) in methylated tumors. No methylated tumors had a ratio of less than 1.6, but four of nine unmethylated tumors had a ratio of greater than or equal to 1.6, indicating high sensitivity but low specificity to detect methylated tumors. Summary There are no clearly distinguishing imaging features of MGMT promoter methylated compared with unmethylated tumors. Mixed nodular enhancement, limited edema, moderately rather than severely increased rcbv, and increased K trans are potentially indicative of MGMT methylation (Table 5), but none of these features is conclusive. There is conflicting evidence regarding ADC, which seems to be highly dependent on methodology. Region of interest minimum ADC measurements fairly consistently show an increase, whereas ADC L assessed with bimodal histogram analysis is lower in methylated tumors. Although histogram analysis is methodologically superior, it is not yet widely available. 326 radiology.rsna.org n Radiology: Volume 284: Number 2 August 2017

12 Table 5 Conventional and Advanced Imaging Features of MGMT Promotor Methylated versus Unmethylated High-Grade Glioma Feature MGMT Methylated MGMT Unmethylated Localization Possibly left hemisphere: temporal, parietal, or occipital lobe Possibly right hemisphere: temporal lobe, basal ganglia, subventricular zone Enhancement Not different, mixed nodular Not different, ring Edema Not different, limited; less edema associated Not different, extensive; no correlation with better prognosis with prognosis ADC Lower ADC L (bimodal histogram) Higher ADC L (bimodal histogram) rcbv Not different, lower Not different, higher K trans Increased Decreased Future Directions Many other gene expression profiles can be linked with structural and physiologic imaging features, and in combination may provide far more accurate prediction of tumor genotypes. Overexpression of genes associated with hypoxia, angiogenesis, and edema (such as epidermal growth factor receptor, vascular endothelial growth factor), for instance, has been linked with contrast material enhancement and necrosis, as well as with high perfusion and restricted water diffusion (29,35,96,98). Glioblastoma subtypes are similarly distinguishable, with a more than twofold ratio of edema versus contrast-enhancing tumor being strongly predictive of the mesenchymal subtype (41). The rapidly growing field of radiogenomics and the advanced imaging postprocessing techniques that allow quantification of imaging features and multiparametric and multimodal computer-aided assessment will undoubtedly reveal further imaging phenotypes of the molecular characteristics of gliomas. In addition, recent technologic advances in diagnostic imaging have paved the way for quantitative, multiparametric assessment of tissue at the cellular and molecular level, with clear potential to further define imaging phenotypes of the various glioma genotypes. It can be expected that techniques that have already been explored, such as the various perfusion (rcbv, K trans ) and diffusion parameters, will find increased value and application. Some further implementations of interest include vessel size imaging, tensor decomposition, and chemical exchange saturation transfer imaging. Vessel Size Imaging Perfusion imaging provides a measure of tumor vascularization and is closely associated with the genetic expression of genes involved in tumor hypoxia and angiogenesis (eg, vascular endothelial growth factor). The commonly used parameter rcbv is an indirect reflection of these processes, which can be quantified more accurately with vessel size imaging. This is based on the principle that the vascular bed is selectively sensitive to whether a gradient- or spinecho sequence is used. Hybrid imaging sequences, applying both types of sequences at the same time, allow for the acquisition of vessel-size selective images and the calculation of a voxelwise vessel size index map. Voxel-wise vessel size mapping provides a measure of angiogenesis (99) that closely resembles histologic evaluation results (100) and correlates with the expression of angiogenic factors (101) as well as with tumor grade (99). Diffusion-Tensor Imaging and Tensor Decomposition Diffusion-tensor imaging is sensitive to changes in the microstructural integrity of white matter, with which disruption of the normal brain architecture is seen as reduced fractional anisotropy and increased mean diffusivity. The degree of fractional anisotropic reduction correlates with cell density (102) and proliferation index. A sensitive measure of (occult) tumor infiltration is obtained with tensor decomposition called p/q mapping (103,104). This measure allows distinction of the tumor core, tumor infiltration, and edema, and a better assessment of tumor extent. Price et al (103) have used this technique to predict the pattern of tumor recurrence, while Mohsen et al (105) recently reported higher survival in patients with a minimally invasive pattern. It can be hypothesized that this finding is correlated with a specific genotype of glioma. Chemical Exchange Saturation Transfer Chemical exchange saturation transfer, or CEST, is an emerging and innovative MR imaging technique that currently is used only in a few centers worldwide. It is used to assess and quantify a variety of metabolites and molecules. These include amide, which is quantified with CEST in terms of amide proton transfer and is related to cell membrane repair and turnover. First reports (106) indicate the potential of amide proton transfer CEST to allow high-grade gliomas to be distinguised from low-grade gliomas and to allow differentiation between tumor recurrence and radiation necrosis (107). These findings are attributed to the increased cellular protein and peptide levels in gliomas. Most CEST studies to date have only limited clinical applicability, being performed at high field strength in preclinical animal models or in small patient populations. Its potential to provide a novel imaging contrast reflecting tumor biology, however, is clear. Conclusion In the past decade, a large amount of predominantly exploratory effort has been directed toward the definition of imaging phenotypes of the three clinically relevant adult glioma genotypes: 1p19q codeletion, IDH mutation, and MGMT promoter methylation. Certain imaging features that allow the different genotypes to be distinguished have been identified, but these commonly overlap, Radiology: Volume 284: Number 2 August 2017 n radiology.rsna.org 327

13 and their accuracy is generally only modest at best. An important limitation of the evidence to date is the lack of prospective validation studies in unselected patient populations. Most studies to date were small, retrospective, and included patients with a known tumor type or grade. A priori knowledge of tumor type or grade, however, defeats the objective of finding a noninvasive assessment of the molecular profile of a tumor with imaging. The true clinical value of the reported imaging features is still largely uncertain and remains to be demonstrated. Another important limitation is the lack of multicenter studies, and it is therefore not clear how reproducible findings are with a wide range of readers, imagers, and field strengths. A further consideration is that the ground truth for molecular diagnosis, in most cases, comes from a single tissue sample. In the context of tumor heterogeneity, in which different molecular profiles can exist in the same tumor (35), sampling bias complicates the identification and interpretation of imaging phenotypes. Furthermore, with the exception of 2-HG 1 H MR spectroscopy, none of the imaging features are true biomarkers but merely an indirect reflection of pathophysiological changes or processes. 2-HG 1 H MR spectroscopy has great potential for detection and monitoring of IDH-mutated tumor activity, but implementation is challenging, and hence, clinical applicability is limited. Further developments that more directly reflect pathophysiological processes with cellular and molecular imaging are likely to increase accuracy and are well on their way into clinical practice. The work to date has been advanced by the Cancer Genome Atlas/ the Cancer Imaging Archive initiative combined with the standardized imaging feature set of glioblastomas. As a result, many of the currently available studies are focused on glioblastomas and on routine structural imaging features, such as tumor size, shape, signal intensity, and delineation. The noninvasive assessment of tumor genotypes through imaging, however, is even more relevant in patients with nonenhancing gliomas that are presumed to be low grade. In patients presenting with enhancing tumors presumed to be of high grade, there is a clear indication for prompt surgical resection (108) and further treatment that can be directed by molecular analysis from available tumor tissue. In gliomas presumed to be low grade with a disease course that is commonly, but not always, more indolent, the decision whether and when to perform tumor resection and initiate adjuvant treatment is less clear. Especially when the need to face the risks associated with surgery is questionable, such as when patients are clinically asymptomatic, elderly, or frail; when the tumor is located in or near functionally eloquent brain regions; or when long periods of stability may be expected; a relatively conservative approach may be more appropriate. Noninvasive imaging phenotyping of the tumor would make such a personalized approach possible and is, therefore, clinically highly relevant. We now have a fundamental update of the WHO glioma classification after a dramatic increase in understanding brain tumor molecular profiles in a very short time. In parallel, radiogenomic developments have indicated the potential of imaging for noninvasive detection and monitoring of tumor genotypes. Future advances in imaging technology, postprocessing techniques, and computer-aided diagnostic technology will undoubtedly further the role of imaging in glioma classification and treatment. 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