Glioblastoma (GB) is the most aggressive diffuse glioma that corresponds to grade 4 based on the 2016 WHO Classification of Tumors of the Central Nervous System. GB is the most common primary brain malignancy with the incidence of 3 per 100,000 persons per year, accounting for 45 % of malignant primary brain tumors and 54 % of all gliomas. Despite the considerable improvements in surgical techniques, which enable more extensive degree of resection, wide application of more precise radiotherapy (RT) and novel chemotherapeutic agents, GB remains an incurable disease with a median survival of 15 months and 3-year overall survival (OS) of less than 10 % in real clinical practice. Clinical trials evaluating the role of modern targeted therapy did not prove superiority of this treatment strategy and results of GB treatment remains poor.

Current standard of care is based on multimodality treatment combining surgery, RT and chemotherapy with alkylating agent temozolomide (TMZ). Standard post-surgery treatment of newly diagnosed GB patients has remained unchanged since implementation of the recommendations of the EORTC 26981-22981/NCIC CE3 trial (Stupp regimen) that finished enrolling patients in 2002 and was published in 2005. Co-administration of TMZ improved survival from 12,1 months (with RT alone) to 14,6 months (with addition of TMZ). Continuing effort how to improve treatment outcomes is urgent clinical as well as research need.

The phenomenon of postoperative REP has only recently been explored with increasingly available MRI for both postsurgery and preRT indication and is currently of high interest. REP diagnosis is based on a comparison of early postoperative MRI findings (up to 72 hours postoperatively) and planning preRT MRI. Our retrospective analysis of 95 patients with GB treated during 2014-2017 revealed that 52% patients developed suspected progression at MRI performed for RT planning purposes. These patients may represent a subset of patients with a particularly aggressive phenotype of GB. It was consistently confirmed that the presence of early recurrence on planning MRI examination was associated with a more aggressive form of glioblastoma and worse overall survival. Higher risk can be expected in patients after fewer radical resections.

Currently, it is not clear what is the optimal approach in patients with REP. Whether to indicate reoperation of recurrence, to choose accelerated RT regimes with or without concurrent chemotherapy or administration of more aggressive and intensive chemotherapy with combined alkylating cytostatics. Treatment of these patients today is not different from patients without REP, and if so, it is purely an individual approach. Clearly, these patients biased previous clinical trials where no routine preRT MRI examination was performed. Currently, these patients are usually excluded from clinical trials, moreover, recent studies often randomize patients after the competition of standard adjuvant chemoRT without any clear progression on the first post chemoRT MRI. REP in MRI planning is a significant negative prognostic factor that should be a stratification factor in future clinical trials. The basic problem is the postoperative prediction of early recurrence.

Amino acid Carbon-11-labeled methionine PET (MET PET) is the most widely studied tracer for molecular imaging in glioma. PET is currently becoming progressively more established part of brain imaging in both pretreatment as well as follow up examination. There is increasing evidence supporting implementation of PET imaging into brain cancer management. Amino acid tracers´ uptake reflects amino acid transport and proteosynthesis which are increased in most types of tumors including gliomas when compared to normal surrounding tissues. Resulted higher tumor-to-normal brain ratio (T/N ratio) provides higher contrast and tumor discrimination comparing to FDG even through lower absolute standard uptake values (SUV). However, because amino acid tracer transport is independent of blood brain barrier breakdown, there is visible PET uptake for tumors that do not enhance on MRI or for aggressive parts of tumor with no MRI contrast uptake yet.

MET PET plays an especially important role in improving diagnostic procedures for treating brain tumors. \[11C\] Methionine is not taken up by normal brain tissue to a marked degree, and the sensitivity of MET PET for detecting glioma tumors appears to be high. It has been suggested that MET PET may more precisely outline the true extent of viable tumor tissue than MRI, whereas MRI has the capability to better delineate the total extent of associated pathologic changes, such as edema, in adjacent brain areas. MET PET tumor/normal tissue index of 1,3 was considered the threshold for malignant activity based on correlation to stereotactic histopathology examination. Usage of MET PET is limited by its short half-life to centers with its own cyclotron enabling the manufacture of radiopharmaceuticals. Patients with REP of GB need to start oncological treatment as soon as possible and it is not ethical to wait for other commercially available radiopharmaceuticals (FET, FLT and others) that have a longer half-life but are only available in limited ordering schedule. In the comprehensive neurooncological centers, however, the individual rapid preparation of methionine tracer, the most studied substance in brain tumors, is the unique option how to improve outcomes of patients with REP, particularly aggressive GB.