Background and rationale {6a}
Compared to other oncological diagnoses, primary brain tumours are less common, with an incidence of 8/100,000 population. Unfortunately, the most aggressive tumors, glioblastomas (GBM), account for approximately half of all primary malignant brain tumors in adults. Due to their biological behaviour, these tumours are ranked among the most difficult-to-treat diseases and therefore represent a serious health problem despite the relatively low incidence. Despite advances in the complex oncological treatment of gliomas, treatment results remain unsatisfactory [1, 2].
The reported median survival of 14–17 months with a five-year survival of 10% is observed mainly in patients with favourable prognostic factors who undergo complete adjuvant oncological therapy [3, 4]. Despite all efforts, the median overall survival has only increased by a few months over the past thirty years. For this reason, further research and development of new therapeutic procedures are necessary with the aim of ensuring better disease control and prolonging overall survival [5].
Clinical studies evaluating the role of modern targeted therapy and immunotherapy have not yet demonstrated higher effectiveness of this treatment strategy, and the results of GBM treatment have thus not fundamentally changed. The only small advance in the last 15 years has been treatment using alternating electric fields emitted from electrodes taped to the scalp (Optune). However, the broader availability of this method is limited by the enormously high cost, patient motivation, and some debatable issues in the clinical trials performed [6, 7].
The current standard of care for GBM is based on a multimodal approach combining the maximum possible and safe surgery, postoperative radiotherapy (RT), and chemotherapy (CHT) with alkylating cytostatic temozolomide (TMZ). Adjuvant therapy is usually started within 4 to 6 weeks after surgery, during which the patient recovers from surgery, and all necessary procedures (multidisciplinary indication committee, preparation of radiation plan, etc.) take place before further oncological treatment. The standard postoperative management of newly diagnosed patients with GBM has remained unchanged since the published results of the EORTC 26981–22981/NCIC CE3 trial (Stupp regimen), which completed patient recruitment as early as 2002 and published the findings in 2005. In this protocol, TMZ (75 mg/m2) is administered on days 1 to 42 concurrently with RT (60 Gy), followed by TMZ alone (150 to 200 mg/m2) on days 1–5 in six consecutive 4-week cycles. Coadministration of RT and TMZ improved survival from 12.1 months (with RT alone) to 14.6 months (with the addition of TMZ) (3). Due to a certain stagnation in the treatment of GBM in recent years, intensive research at all levels is necessary to improve patients' perspectives on this unfavourable diagnosis.
The Perry accelerated regimen [8] is a treatment option for patients aged ≥ 65 years and those for whom long conventional RT (60 Gy in six weeks) combined with chemotherapy is considered unsuitable by their treating physicians. The total RT dose for this accelerated regimen is 40.05 Gy, administered in 15-day fractions over three weeks.
Concomitant TMZ is administered here at a dose of 75 mg/m2 per day for 21 consecutive days from the start of RT until its termination. As with Stupp's regimen, adjuvant TMZ alone follows at 150 to 200 mg/m2 daily for five consecutive days of a 28-day cycle, up to 12 cycles or until disease progression. For patients who are not candidates for concomitant chemoradiotherapy (regardless of age), individual adjuvant treatment is indicated as standard, usually accelerated RT alone (total dose 30–50 Gy in a 10-20-day dose, for example, fractionation regimens of 10x3.0 Gy, 10x3.4 Gy, 15x2.67 Gy, 20x2.5 Gy and others), followed in some patients by TMZ palliative chemotherapy based on the actual overall condition of the patient.
Glioblastoma as a progressive disease:
In connection with the greater availability of magnetic resonance (MR), it is possible to optimize the planning of radiotherapy according to the findings of the so-called planning MR for an increasing number of patients. It is an MR study performed immediately (approximately a few days) before the start of radiotherapy. Following the performance of this MR study (typically approx. 4–5 weeks after surgery), a new phenomenon called very rapid progression (REP, rapid early progression) is described. The REP diagnosis is based on comparing early postoperative MR findings (usually within 48–72 hours after surgery) and planning MR before radiotherapy (preRT).
Retrospectively comparing early postoperative MR with preRT MR performed approximately 30 days later, Farace et al described 30% of preRT MR with signs strongly suggestive of tumour progression during this period [9]. An even more significant proportion of patients with documented REP was described by Palmer et al., where up to 52% progressed before starting adjuvant therapy without any effect of waiting time to start RT (median time from resection to RT was 32.5 vs. 33 days in patients with REP vs. without REP (p = 0.337) [10]. These results are consistent with our retrospective analysis of 90 patients with GBM treated between 2014 and 2017, where 51% of patients had suspected REP at planning preRT MR [11]. These patients represent a subset with particularly aggressive GBM requiring further intensive research and possibly treatment modification.
According to these and several other retrospective analyses, it was confirmed that the presence of early progression on planning MR was associated with a more aggressive form of GBM and worse overall survival [12–14]. A higher risk can, of course, be expected in patients after nonradical resections [14]. It is still unclear what further influences the prognosis of patients with early progression. Palmer et al. even described significantly worse survival in patients with unmethylated MGMT (O6-methylguanine-DNA methyl-transferase), the methylation of MGMT is considered a marker of higher chemosensitivity [10]. Analysis of other potential biomarkers still needs to be improved. Due to a large number of patients with REP and the retrospective nature of the studies carried out thus far, it is essential to analyse this more aggressive subgroup of tumors prospectively and try to influence the negative course of the disease more clinically.
The optimal treatment approach for patients with REP needs to be determined. It is unclear whether it is better to indicate repeat surgery for recurrence, choose accelerated RT regimens with or without concurrent chemotherapy, or directly administer more aggressive and intensive chemotherapy with a combination of alkylating cytostatics if MGMT methylation is present [15]. Treatment of these patients with REP currently does not differ from that of patients without REP; if so, it is a purely individual approach. These patients distorted the results of previous clinical trials where routine MR examination was not performed before RT. Currently, these patients are usually excluded from clinical trials.
Moreover, current clinical trials often randomize patients until after concomitant chemoradiotherapy if there is no progression on follow-up MR after chemoradiotherapy. REP on planning MR is a significant negative prognostic factor that should be a stratification factor in future clinical trials. In general, the aggressive character of GBM is manifested by its microenvironment, the molecular background of glioma cells, or the miRNA profile, which may be essential in GBM oncogenic signalling and has the potential to serve as a biomarker of this disease and a new therapeutic target in oncology [16–22].
Radiotherapy in the treatment of glioblastoma
Due to the almost zero risk of developing distant metastases, and on the contrary, due to the nearly 100% risk of developing local recurrence in the brain, in the case of GBM, great emphasis is placed on local treatment methods, i.e., surgery and radiation. Radiotherapy has experienced a stormy development in the last decade due to improved computer technology, greater availability of imaging methods, and more advanced radiotherapy systems [7, 23, 24]. A crucial part of radiotherapy treatment is correctly determining the target area for the prescribed radiation dose. In the case of brain tumours, the gold standard for defining target volumes is MR imaging. Recently, the importance of additional imaging methods, especially PET imaging, for the closer characterization of gliomas has been emphasized [24, 25].
11C-Methionine PET in the imaging of glioblastoma
Positron emission tomography is currently the most dynamically developing area for functional brain imaging. It is used both pretreatment and as part of the follow-up of GBM patients after treatment. The development of PET diagnostics is aimed not only at constructing more sensitive and robust PET scanners but also at introducing new radiopharmaceuticals, especially in centres with their own cyclotron.
Fluorodeoxyglucose (2-deoxy-2-[18F]fluoro-D-glucose, FDG) is the most commonly used radiopharmaceutical in PET/CT diagnostics in general. However, it may not provide optimal results for brain examination mainly due to the high metabolic activity in the physiological brain tissue, the image background. That is why other radiopharmaceuticals are used in the targeted diagnosis of brain tumors, most often different amino acid derivatives. Of these, [18F]-O-(2-fluoroethyl)-L-tyrosine (fluoroethyltyrosine, FET) and L-(S-methyl-[11C])-methionine (methionine, MET) play the most important roles. Carbon 11-labelled methionine is historically the most frequently used drug for glioma imaging.
Amino acid (AA) tracers, including FET and MET, naturally cross the blood-brain barrier through neutral amino acid transport mechanisms. Unlike naturally occurring methionine, the artificially prepared amino acid FET is not subsequently incorporated into protein fractions [26–27]. Their transport and incorporation into proteins are increased in tumour tissue. However, it is still being determined which of these mechanisms is more critical for PET imaging. Methionine is also incorporated into fats and nucleic acids; this incorporation is less significant for imaging brain tumours. Studies have confirmed the correlation between the intensity of MET accumulation, tumour grading according to WHO and proliferative activity assessed by the Ki-67 index [28] and the density of microvascularization [29]. Methionine is incorporated to an increased extent even in low-grade gliomas [30], while its accumulation is low in the background of normal brain tissue. Since the transport of AA tracers does not depend on the blood‒brain barrier violation, the accumulation of AA tracers can be detected even in tumours that do not show contrast saturation and enhancement during MR examination.
MET PET is more suited than MR to accurately delineate the target volume for radiotherapy and determine the extent of viable tumours. Conversely, MR shows the overall extent, including pathological changes associated with the tumour, such as oedema in the immediate vicinity. The widespread use of MET PET is most hindered by its short half-life, so the use of MET is limited only to centres with their cyclotron and the possibility of its production. In patients with REP glioblastoma, it is necessary to start oncological treatment as quickly as possible. It is, therefore, unethical to wait for the delivery of other commercially available radiopharmaceuticals labelled with fluorine 18 (18F-FET, 18F-FLT, 18F-DOPA, etc.), which have a longer half-life. They are also available in PET centres without a cyclotron, but only on predetermined and planned production days. In complex neuro-oncology centres, the possibility of individual preparation of MET is thus an opportunity to benefit patients with REP, extraordinarily aggressive glioblastomas.
In addition to the subjective assessment, the evaluation of the PET image is refined by quantitative measurement. For this quantitative measurement in clinical practice, drawing a suitable area of interest and software evaluation of the SUVmax parameter ("maximum standardized intensity of accumulation") are usually used. The assessment of the SUVavg ("average value of the standardized intensity of accumulation") is chosen less often. Nevertheless, it is influenced by the specific location of the area of interest, with more significant variability between evaluators. Therefore, SUVavg is less suitable for standardized assessment in studies.
The area of interest can be either circular in the plane for one selected slice, 2D-ROI, or spherical in space, 3D - VOI. The two-dimensional region of interest is also very strongly influenced by the choice of a particular slice for assessment and is also burdened by low reproducibility and high interrater variability. For our evaluation, the measurement of SUVmax in a spatial 3D-VOI defined with a size of 1.5 cm was chosen to minimize variability.
The ratio of the radiopharmaceutical accumulation in the tumour relative to the background ("tumour-to-background" ratio, T/B) is used to differentiate the areas of the physiological and pathological intensity of accumulation. There is no definite recommendation on how to choose the background, and even any recommendations may be influenced by the possibilities of a specific patient. There are also no exact values of the T/B ratio defined as a threshold for distinguishing tumour tissue from physiological tissue. According to available literature data, values between 1.3 and 1.7 are usually chosen.
Explanation for choice of comparator {6b}
This study is an open-label prospective clinical trial. The experimental drug 11C-methionine PET/CT will be administered to all patients recruited to this study and compared to the cohort consisting of a historical group of patients collected in the period 2014–2018.
Objectives {7}
Patients diagnosed with GBM who develop REP before starting adjuvant radiotherapy have an abysmal prognosis. The optimal treatment for these patients has yet to be discovered, and no prospective clinical evaluation has been performed. We assume that the lesion detected on MR is not the only area of REP development. It may be MET PET that can reveal other areas of the aggressive part of the tumour, and the subsequent RT can then be better planned to increase the time to progression in such a cohort of patients. Therefore, the purpose of this study was to refine the diagnosis in patients with GBM with proven REP and subsequent optimized planning of the following treatment procedure. Chemotherapy will be administered as standard treatment to all patients.
Trial design {8}
The GlioMET study is an investigator-initiated study. This clinical trial is a prospective, monocentric, open-label, nonrandomized clinical study with a parallel assignment initiated in June 2020.
The main aim of the clinical trial is to improve the progression-free survival of patients with GBM and REP via optimizing of diagnostic and treatment procedures. The experimental drug 11C-methionine PET/CT will be administered to all patients recruited to this study and compared to the cohort consisting of a historical group of patients collected in the period 2014–2018. The setting is a confirmatory framework against the historical cohort. A schematic overview of the design of the study is presented in Fig. 1.