Introduction
About one-third of primary brain tumors are malignant, and of these gliomas account for about 80% [
1‐
3]. Epidemiological studies report incidence rates of gliomas from 4.8 to 7.7/100,000 per year [
4]. In Norway, around 320 patients are diagnosed with diffuse gliomas each year [
5]. Gliomas are classified according to the 2021 World Health Organization (WHO) classification of tumors of the central nervous system (CNS), based on histopathological and molecular features. The majority of malignant primary brain tumors are adult-type diffuse gliomas, which are now classified into three categories: isocitrate dehydrogenase (IDH) mutant astrocytomas (CNS WHO grade 2–4), IDH-mutant and 1p/19q co-deleted oligodendrogliomas (CNS WHO grade 2–3), and IDH-wildtype (IDHwt) glioblastomas (CNS WHO grade 4) [
6,
7]. Tumor classification is essential for treatment decisions, and for estimation of treatment response and overall prognosis [
8,
9].
The recommended diagnostic imaging modality for glioma detection according to present guidelines is magnetic resonance imaging (MRI), including T2-weighted, T2-weighted fluid-attenuated inversion recovery (FLAIR), pre- and post-contrast enhanced 3D T1 sequences and diffusion-weighted imaging (DWI) [
10]. Perfusion-weighted imaging is optional but may be beneficial, especially in the assessment of low-grade gliomas [
11]. The gold standard for diagnosis, however, remains histomolecular analysis of tumor tissue, which requires operative biopsy sampling. Due to gliomas’ heterogenous nature, tissue sampling might result in underestimation of tumor grade or misdiagnosis, as some tumors have malignant foci, not visible with conventional imaging [
12‐
14].
Radiolabeled amino acids (AAs) are important imaging agents for positron emission tomography (PET) diagnostics due to the increased levels of AA transport that occur in many tumor cells compared to normal tissue [
15]. A wide range of AA tracers have been developed for clinical PET imaging of oncological diseases such as brain tumors, neuroendocrine tumors, and prostate cancer [
16]. The AA PET tracers [methyl-
11C]-L-methionine ([
11C]MET), O-(2-
18F-fluoroethyl)-L-tyrosine ([
18F]FET), and 3,4-dihydroxy-6-[
18F]fluoro-L-phenylalanine ([
18F]FDOPA) are recommended by current international guidelines to improve brain tumor diagnostics, resection, tissue sampling, grading, treatment planning and therapy response assessment [
17,
18]. The longer half-life of
18F-labeled tracers (110 min) compared to
11C-labeled tracers (20 min) facilitates the utility of AA tracers in hospitals without on-site radiopharmaceutical production [
16].
Anti-1-amino-3-
18F-fluorocyclobutane-1-carboxylic acid (
anti-3-[
18F]FACBC) is an AA PET tracer with favorably low uptake in normal brain tissue, resulting in higher tumor-to-background ratio (TBR) compared to the current recommended tracers [
19,
20].
Anti-3-[
18F]FACBC is further mediated not only via leucine preferring transport system L (LAT1), like the above mentioned tracers, but also via alanine-serine-cysteine transporter 2 (ASCT2) which is commonly upregulated in cancer cells [
16,
21,
22]. Previous studies have shown
anti-3-[
18F]FACBC uptake in gliomas of various grades and types, all with the common conclusion that PET with this tracer is effective in the detection of gliomas, and may add complementary information, especially in tumor regions not visualized with contrast-enhanced MRI [
23‐
26]. It has also been suggested that
anti-3-[
18F]FACBC can discriminate between low-and high-grade gliomas [
27].
Even though
anti-3-[
18F]FACBC was originally developed for brain tumor imaging over twenty years ago [
28], it has not been widely used or implemented in current guidelines for this purpose. Instead, the tracer has been more commonly used in biochemical recurrent prostate cancer [
29]. More studies are therefore needed to establish the potential role of
anti-3-[
18F]FACBC PET in glioma diagnostics. It is of special interest to investigate whether
anti-3-[
18F]FACBC PET can differentiate between tumor grades and subtypes, to increase the accuracy of noninvasive diagnostics.
The aim of this study was to evaluate whether addition of anti-3-[18F]FACBC PET to conventional MRI could improve diagnostic accuracy for patients with primary and recurrent gliomas. We also evaluated the ability of anti-3-[18F]FACBC to differentiate between histopathological and molecular entities in gliomas.
Discussion
The key finding in this study was that anti-3-[18F]FACBC PET improved the proportion of correctly predicted glioma grades, types, and IDH status, as well as the overall diagnoses compared to MRI only.
A more trustworthy pretreatment diagnosis can be useful in clinical decision making. For example, asymptomatic grade 1 gliomas may not necessarily need treatment, but may still undergo treatment if mistaken for a higher-grade lesion, like in the current study where two grade 1 gliomas were included since they were scheduled for treatment based on suspicion of being diffuse gliomas (grade 2–4). Furthermore, the prognostic difference between grade 4 glioblastomas and grade 2–3 astrocytomas and between grade 2–3 oligodendrogliomas and grade 2–3 astrocytomas could potentially affect surgical decision making. Oligodendrogliomas often exhibit a better response to adjuvant treatment, and the impact of surgery is more documented for astrocytomas [
41]. While surgically induced deficits may reduce survival in grade 4 glioblastomas [
42], patients with lower grade astrocytomas have far more to gain from extensive resections [
43] and have more time for rehabilitation.
It is an advantage that
anti-3-[
18F]FACBC has lower uptake in normal brain parenchyma and thus higher TBR values than other AA tracers [
19,
20,
44,
45]. PET hotspots appear very distinct, and in most cases, the regions with PET uptake and contrast-enhancement coincide well. However,
anti-3-[
18F]FACBC detected malignancy in 4/14 (30%) of patients where no contrast-enhancement were found on MRI in the current study (two grade 2 oligodendrogliomas, one grade 3 oligodendroglioma, and one grade 4 glioblastoma), suggesting that
anti-3-[
18F]FACBC PET could be particularly useful in cases without contrast-enhancement on MRI.
The overall sensitivity for
anti-3-[
18F]FACBC in detection of gliomas was 72.2%. This is slightly lower than reported from studies using [
11C]MET (76–100% (14 studies,
n = 556) [
46]), [
18F]FET (89% (1 study,
n = 236) [
47]), and [
18F]FDOPA (90–100% (3 studies,
n = 114) [
48‐
50]). A possible explanation could be that almost 80% of the WHO grade 2 gliomas were PET negative in this study, while for other AAs this is reported to between 20 and 30% [
47,
51,
52]. Thus, another advantage with
anti-3-[
18F]FACBC over the other AAs is a significant difference between the PET uptake in diffuse low-grade and high-grade gliomas (Fig. 4b). Parent et al. [
27] also demonstrated that
anti-3-[
18F]FACBC PET could discriminate between low-and high-grade glioma, although for a smaller sample size (
n = 18 and only one grade 3 glioma).
All glioblastomas had uptake in this study, and similar results have been demonstrated for [
18F]FET [
47]. However, for grade 2–3 astrocytomas and grade 2 oligodendrogliomas, [
11C]MET and [
18F]FET demonstrate higher sensitivities [
47,
53], and are probably better suited than
anti-3-[
18F]FACBC for evaluation and follow-up of these subtypes.
The challenge to discriminate grade 2–3 astrocytomas from oligodendrogliomas remains, even though the fraction of PET positives among grade 2–3 oligodendrogliomas were larger. Oligodendrogliomas, especially grade 3, have a higher AA metabolism compared to IDH-mutated astrocytomas and are most commonly positive at AA PET compared to IDH mutated astrocytomas, as demonstrated both in this study and by Ninatti et al. [
54]. In this study, the group of included grade 2 oligodendrogliomas is larger than the group of grade 2 astrocytomas, and the group of grade 3 astrocytomas is larger than the group of grade 3 oligodendrogliomas. Consequently, this unbalance may have masked statistical differences in
anti-3-[
18F]FACBC uptake values between these glioma subtypes.
IDH mutation is one of the most important diagnostic and prognostic biomarkers for diffuse gliomas and is associated with a more favorable outcome compared to IDHwt [
55]. When comparing TBR
peak between diffuse IDHwt and IDH1/2 mutated gliomas, we found that IDHwt gliomas had a significantly higher uptake compared to IDH1/2 mutated gliomas. Similar results have also been demonstrated by Kudulaiti et al. [
56] for [
11C]MET. Additionally, [
18F]FET and [
18F]FDOPA shows potential as effective tools to predict IDH genotype in gliomas using radiomics with static and dynamic PET parameters [
57,
58].
Grade 4 glioblastomas and the grade 1 gliomas were among the tumors with the highest uptake of
anti-3-[
18F]FACBC, and all of them were PET positive. Common for these tumors are that they are IDHwt. This high uptake is not necessarily caused by the IDH status but could be related to the increased vascular proliferation found in both grade 1 and 4 gliomas [
59], which would also explain the high uptake in the two grade 4 IDH mutated astrocytomas in this study.
It has been suggested that the dynamic characteristics of AA PET can be useful in the classification of gliomas. However, the dynamic characteristics found with [
18F]FET (increasing curve for low-grade tumors, decreasing curve for high-grade tumors) [
60] and with [
18F]FDOPA (to predict molecular features) [
61,
62] could not be established with
anti-3-[
18F]FACBC in this study. Accordingly, dynamic imaging with
anti-3-[
18F]FACBC is probably not useful for glioma classification. However, it may still be relevant to evaluate this tracer dynamically in a follow-up setting to differentiate between recurrence and treatment-induced changes. Differences in dynamic characteristics between the tracers are probably caused by different uptake and transport mechanisms (
anti-3-[
18F]FACBC: System L and ASCT2 transport, and [
11C]MET: System L (LAT1) transport/protein synthesis, [
18F]FET: System L (LAT1) transport, and [
18F]FDOPA: System L (LAT1) transport) [
16,
21,
22].
By applying different TBR
peak threshold values, we could discriminate some glioma grades, types, and molecular features from others with excellent or outstanding performance (grade 2, grade 4, glioblastoma, and IDHwt). Oligodendrogliomas or diffuse astrocytomas could be discriminated from other gliomas with acceptable performance, where the lower performance may be caused by quite similar uptake between these two subtypes and the wide range in uptake for grade 2–4 astrocytomas. A limitation with this ROC analysis was the small sample size, which also made it impossible to split the data into training and test cohorts. This resulted most likely in overestimation of the test performance [
63]. However, by applying the obtained threshold values from
anti-3-[
18F]FACBC PET, the accuracy of the predicted glioma diagnoses improved compared to MRI alone for both readers. The importance to also include the grade 1 gliomas in the ROC analysis was confirmed in the retrospective clinical MR readings, where these two tumors were predicted to be grade 3 astrocytomas by one (patient ID_02) or both (patient ID_01) of the neuroradiologists. It is, however, important to be aware that this methodological choice influence comparability with studies that aim to include only adult-type diffuse gliomas.
The reference standard was histomolecular diagnoses based on the latest 2021 WHO classification of CNS tumors. It should be noted that this classification differs from previous versions, with the most important changes being more incorporation of molecular biomarkers for tumor classification. Tumors are further graded within types, rather than across different tumor types [
7]. Applying a more robust classification system will likely improve the evaluation of diagnostic imaging as well. According to the 2021 WHO classification, all IDH mutated diffuse, astrocytic, grade 2 and 3 gliomas should be tested for CDKN2A/B homozygous deletion, since the presence of this marker would assign a grade 4 glioma [
6,
7]. This was however not performed systematically in the current study, and must therefore be acknowledged as a limitation, even if the frequencies of this marker are low in astrocytic gliomas (grade 2: 0–12%; grade 3: 6–20%) [
64].
An overall limitation is the relatively small patient cohort. However, by merging new data with data from our previously published data [
24], we were able to perform one of the largest studies using
anti-3-[
18F]FACBC in gliomas so far.
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