**3.9. Diffuse midline glioma, H3 K27 M-mutant**

treatment. The addition of this new modality to the standard radio-chemotherapy regimen

**Figure 25.** Preoperative T1W + C sequence (a) of a glioblastoma, almost completely removed as it was revealed by immediate postoperative CT scan (b); the patient was not responsive to the standard adjuvant treatment as it is

**Figure 24.** Preoperative axial (a), coronal (b) and sagittal (c) T1W + C sequences of a patient with glioblastoma completely removed and treated with the standard radio-chemotherapy regimen; postoperative enhanced T1W images (d–f) reveal the removal of the tumor with the persistence of a small contrasted nodule, which is stable at 5 years after surgery.

We can conclude that despite the huge efforts and investments made in research and therapies, the results still disappoint. This suggests the fact that we are possibly on the correct course, but against the tide. The future multimodality treatment may consist of patient-tailored treatments owing to the multiple individual and tumor-related factors that influence the response

improved the PFS and OS in glioblastoma-treated patients [156].

demonstrated by the postoperative enhanced MRI at 4 months (c).

128 Glioma - Contemporary Diagnostic and Therapeutic Approaches

to therapy.

**Definition**. High-grade infiltrative astrocytic tumor, with midline location and showing the K27 M mutation in the H3F3A or HIST1H3B/C genes. They are grade IV tumors.

*Clinically*, it is usually encountered in children and more rarely in adults. The symptoms may be either general (usually produced by the obstructive hydrocephalus) or focal generated by the involvement of different cranial nerves: facial paresis, diplopia, hearing loss, dysphonia, and dysphagia. The extension of the tumor in cerebellar peduncles produces ataxia. The involvement of long tracts determines motor weakness. Particularly in diffuse pontine glioma, behavioral changes such as pathological laughter and anxiety were described before the onset of other more common clinical signs [157].

**Imaging**. On MRI, these lesions have a diffuse homogenous appearance of hypointense in T1W sequences and hyperintense in T2W and FLAIR ones. The contrast is discrete or absent, but some focal enhancement into an infiltrative hypointense mass on T1W + C sequences could be detected in few cases (**Figure 26**).

*MRI spectroscopy* will differentiate these tumors from other inflammatory, vascular, or infectious lesions. The Cho/Cr and NAA/Cr ratios are typically increased. An elevation of these ratios after radiotherapy poses the significance of progression [158].

**Macroscopy**. The infiltrative pattern is reflected in the size and the changes in the shape of the nervous structures affected by the tumor. In cross section, we can see purple hemorrhagic areas or yellowish-white necrotic ones.

**Histological diagnosis**. The aspect is that of an intensely infiltrative tumor consisting of small-size monomorphic, astrocytic cells. While most are accompanied by necrosis, microvascular proliferations, and an enhanced mitotic index, these elements are absent in a small number of tumors. According to the new rule introduced by the latest WHO classification, whereby "molecular beats histology," even if histologically they are grade II, they shall nevertheless be deemed grade IV. *Immunohistochemistry*. GFAP shows a heterogeneous expression in these tumors. On the other hand, they are positive for S100, OLIG2, and NCAM1. Mutations of ATRX and TP53 can occur, but more rarely. An antibody targeting the protein modified by the presence of the H3.3 K27 M mutation can be used in the diagnosis [159].

**Figure 26.** In vitro 3D model of glioblastoma stem cells culture demonstrating an increase angiogenesis in contact with TMZ and resistance to the addition of Bevacizumab compared with the control (ctrl) (personal archive).

**Genetic diagnosis**. This entity is defined by the presence of the K27 M mutation in the H3F3A (80%), HIST1H3B (20%), and HIST1H3C genes. The genetic profile is completed by mutations in the TP53 and ATRX genes, homozygous CDKN2A deletion, and MYC, CDK4/6, and PDGFRA amplifications [160].
