**9. Conclusion**

Histological classification of diffuse gliomas based on the WHO grading scheme is a prerequisite to optimal patient treatment decisions. Clinicians need to be aware that

Patients with IDH1/2 mutations in anaplastic astrocytomas and glioblastomas are usually younger than those lacking a IDH mutation (Nobusawa et al., 2009, Hartmann et al., 2009). In addition IDH1 mutations are a prognostic marker of favorable outcome in grade III and IV tumors (Yan et al., 2009, Nobusawa et al., 2009, Sanson et al., 2009). There is even a study demonstrating that IDH1-positive glioblastomas WHO grade IV have a better prognosis than IDH1-negative anaplastic astrocytomas WHO grade III (Hartmann et al., 2010). In contrast patients with IDH1 mutations in diffuse astrocytomas grade II WHO are older (Kim et al., 2010) or show at least a similar age distribution (Balss et al., 2008). The prognostic role of IDH1 in grade II diffuse astrocytomas is still to be determined. Sanson and colleagues found IDH1 to be an independent prognostic factor for longer survival in 100 samples (Sanson et al., 2009), while Kim et al. in 174 grade II tumors did not observe a more favorable outcome (Kim et al., 2010). So far IDH tumor status has not been incorporated into

Analysis of low-grade astrocytomas did not found any association with MGMT promoter methylation and overall survival (Komine et al., 2003). In anaplastic gliomas, MGMT promoter hypermethylation is associated with longer progression free survival (Wick et al., 2009). In glioblastomas, MGMT methylation status in addition as a marker of prolonged survival is a predictor to therapy response (Hegi et al., 2005). In oligodendroglial tumors there is a strong association between MGMT promoter methylation and 1p19q codeletion the latter also contributing to the improved survival of patients with MGMT methylation (Levin et al., 2006; Kesari et al., 2009). MGMT alone is useful as a prognostic marker but not useful to predict outcome of adjuvant treatment in oligodendrogliomas (van den Bent et al.,

In oligodendrogliomas 1p19q codeletion is associated with improved survival (Jeon et al., 2007, McLendon et al., 2005). Presence of oligodendroglial histopathology and 1p19q deletetion shows a better overall survival for anaplastic oligodendrogliomas treated with radiation and PCV chemotherapeutic regimen (Giannini et al., 2008). The same study also demonstrated that 1p19q deletion alone is associated with a longer progression-free survival but that this effect is independent of initial treatment of oligodendrogliomas and mixed oligoastrocytomas (van den Bent et al., 2006). There is an inverse correlation with p53 mutation and codeletion of chromosomal arms 1p and 19q in oligodendrogliomas implicating that oligodendrogliomas harbouring a Tp53 mutation have a reduced overall

In 9% of astrocytomas grade II WHO no common genetic alterations are detected (Kim et al., 2010). In small biopsy specimen these tumors may enter the differential diagnosis of pilocytic astrocytoma. The latter often show BRAF abnormalities, wich drive MAPK pathway activation (Cin et al., 2011) but are absent in diffuse astrocytoma (Korshunov et al., 2009). Another possible differential diagnosis to low-grade diffuse astrocytoma is ganglioglioma and pleomorphic xanthoastrocytoma which in addition to their unique histological properties also exhibit BRAF V600E mutations, at present not known to be in

Histological classification of diffuse gliomas based on the WHO grading scheme is a prerequisite to optimal patient treatment decisions. Clinicians need to be aware that

any current therapeutic trials but is likely to be included in the future.

survival (Jeon et al., 2006, McLendon et al., 2005).

diffuse astrocytomas (Schindler et al., 2011).

**9. Conclusion** 

**8.2 Molecular biology** 

2009).

diffuse gliomas form a histological continuum and that the four-tiered scores introduces a somewhat artificial separation. Tumors on the edge between grade II and III lesions behave different than tumors showing beginning endothelial proliferations indicating close progression to grade IV. A panel of different antibodies is very helpful to secure the diagnosis and avoids potential differential diagnosis pitfalls. Immunohistochemistry has also shown that several antibodies show divergent expression patterns. Researchers therefore should strive to clearly delineate between astrocytomas, oligodendrogliomas and oligoastrocytomas, when examining new biomarkers. Primary and secondary glioblastomas are another example of "convergent evolution showing a similar phenotype of genotypically different tumor cells" (Basanta et al., 2011). The distinction of primary and secondary glioblastomas does not immediately influence management decisions, but because of their different genetic profile, it is expected that they may also differ in response to experimental therapies. Recent years have seen a progress in supplementing histological diagnosis of diffuse gliomas with an increasing spectrum of molecular markers. The utility of MGMT and 1p/19q in predicting response to therapy has led to their inclusion in current clinical trials. Implementation of these markers into routine diagnostic setting is expected after further successful results. Especially in oligoastrocytomas they complement histological results and provide a more objective classification. However clear cut-off levels for each assay is needed to guarantee interlaboratory compatibility. Histological control of the tissue used for molecular neuroonclogy through (neuro)pathologists is indispensable to avoid false-negative test results. Determining IDH1 status in diffuse gliomas is of diagnostic and clinical relevance. Not only indicates equal presence of IDH mutations a likely common origin of astrocytomas and oligodendrogliomas, but also the strong prognostic role in high-grade gliomas is likely to be included in future revisions of the current WHO classification.
