Radiomics analysis has been widely studied for its use in subtyping brain tumors, predicting prognosis and treatment planning. Combining radiomics with clinical and genetic prognostic factors yields superior can result in predicting the survival of patients than using each component alone.
Diagnosis and classification of glioblastoma:
Simple features on structural MRI such as tumor size, location and enhancement patterns have been associated with various histopathological subtypes of glioblastoma. Incorporating complex radiological features derived using image processing software and combining advanced MRI modalities can further improve the accuracy of these models (Table 1).
Tumor location:
It is well known that location of the tumor affects the outcomes in patients with glioblastoma. A “probabilistic radiographic atlas” of more than 500 glioblastoma patients showed associations between stereospecific frequency of tumor occurrence with age, extent of resection, genetic expression, and survival data. Interestingly, regions closer to subventricular zone were seen to have MGMT unmethylated, mesenchymal, and EGFR-amplified tumors,[11] supporting their invasive nature and poor prognosis.[12] A comparison between solitary and multicentric glioblastoma revealed distinct gene expression profiling and outcomes between the two types, with upregulation of genes responsible for tumor cell motility and invasiveness in the multicentric type.[13]Another study showed correlation of tumor phenotypes with spatial distribution of tumors.[14]Thus, tumor location provides important information on the cell of origin and tumor behavior.
Tumor size and contrast enhancement patterns:
The volumes of both contrast enhancement and necrosis at the time of initial diagnosis were found higher in tumors with the mesenchymal gene expression signature compared with those having proneural or proliferative signatures. A ratio of the T2/FLAIR hyperintense volume to the volume of contrast enhancement plus necrosis of less than 2.3 could predict the mesenchymal subtype with 82% sensitivity and 87% specificity.[15] VAK Classification, a scoring system developed to create phenotypes based on tumor Volumetry, Age, KPS annotation, was combined with P53 activation, MGMT promoter methylation and a group of genes and microRNAs in The Cancer Genome Atlas (TCGA) glioblastoma dataset to predict patient survival and facilitate genomics-based personalized therapy for glioblastoma patients. (Figure 2).[16]‘VASARI’, a semi-quantitative feature set named was designed to measure tumor size and volumes of components with enhancement, non-enhancement, necrosis and edema, which correlated with survival rates and subtypes.[17]Specific invasive imaging signatures including ependymal involvement, deep white matter tract involvement and enhancement across the midline predicted a decrease in OS, MYC oncogene activation and inhibition of NF-KB inhibitor-alpha.[18] These patients were found to have mitochondrial dysfunction,[18] consistent with the ‘”Warburg effect”,[19] where cancer cells rely on aerobic glycolysis facilitated by MYC oncogene upregulation.
Volumetry was combined with DNA microarray analysis to train classifiers that can predict gene-expression patterns and survival. Tumor contrast enhancement and mass effect was associated with up-regulation of specific hypoxia and proliferation gene-expression programs such as VEGF, ADM, PLAUR, SERPINE1, CA12, TOPA, CDC2, and BUB1B.[20] In another radiogenomic study based on TCGA, stratification into high and low FLAIR radio-phenotypes reflected underlying edema and cellular invasion in glioblastoma, as they were associated with genes and microRNAs involved in cancer and cellular migration.[21]MRI volumetric features are predictive of several cancer-relevant, drug-targetable DNA mutations in glioblastoma. TP53, RB1, NF1, EGFR, and PDGFRA mutations could each be significantly predicted by at least one imaging feature.[22]These studies provide a basis for genomic profiling and non-invasively selecting patients for personalized therapies using tumor volumetry.
Radiomics was used to distinguish brain metastasis and glioblastoma using contrast-enhancing and peritumoralhyperintense masks in T2-weighted (T2W) MRI. In this model, deep learning showed best performance (area under curve AUC 0.956) compared to the traditional machine learning model (AUC 0.890) and human readers (AUC 0.774).[23]Similar performance, AUC 0.96 for support vector machine (SVM), was observed in another study which used post-contrast T1 weighted (T1CE) MRI instead. However, performance decreased when subtypes of brain metastasis were attempted to classify.[24]
Texture:
Texture is a chief radiomic feature utilized for glioblastoma phenotyping. In one study, a gray-level co-occurrence matrix (GLCM) approach was employed for extracting phenotypic texture features for necrosis, active tumor, and edema on structural MRI. Features were significant predictors (p value <0.01) of prognosis but in areas of active tumor only.[25] Another study was able to predict MGMT methylation status using space-frequency texture analysis based on the S-transform in T2W MRI, albeit with an accuracy of 71%, requiring better algorithms.26Other studies on texture features were able to predict MGMT methylation status with reasonable accuracy.[27,28]
Occasionally, high‐grade gliomas (WHO Grade III and glioblastoma) may have the same MRI appearance as low-grade gliomas. A radiomic analysis using texture along with size, shape, intensity, and histogram features was used to differentiate low-grade from high-grade gliomas, reaching a prediction performance in the cross-validation as high as AUC value of 0.932 with support vector machine. However, the accuracy decreased to 0.75 in the independent validation dataset.[29] In a similar study, Random Forest gave the highest AUC in the training cohort compared to test cohort, reflecting variation in accuracy across different ML classifiers.[30]
Advanced MRI sequences and multimodal analyses:
Perfusion MRI has been extensively used in brain tumors to evaluate angiogenesis and tumor behavior. Raw tumor features from structural MRI and delta-radiomic features from Dynamic susceptibility contrast (DSC) perfusion MRI were extracted to differentiate low-grade gliomas from high-grade gliomas, this classifier reached an AUC of 0.94.[31] A Cochrane meta-analysis on 7 studies to differentiate untreated solid and non‐enhancing low-grade from high-grade gliomas using DSC MRI features (rCBV and Ktrans) reported wide range of estimates for both sensitivity and specificity, making these parameters less reliable.[32] Diffusion MRI was employed to compare the expression of various genes between the high- versus low- Apparent Diffusion Coefficient (ADC) tumors in a subset of patients. High-ADC tumors were found to have higher expression of 13 genes, 6 of which encode for extracellular matrix (ECM) molecules including collagen or collagen-binding proteins, suggesting a role of these genes in pro-invasive phenotype.[33] In another study, physiologic MRI was correlated with stereotactic image-guided biopsies to differentiate contrast-enhancing and non-enhancing tumor areas. DSC MRI was useful for identifying tissue specimens with higher tumor proliferation, necrosis, and vascular hyperplasia in the contrast-enhancing component of the lesion, while Diffusion MRI may be used to detect infiltrating tumors in the non-enhancing region. This is of particular interest for defining tumor burden in non-enhancing regions, where distinguishing reactive edema from biologically active infiltrative tumor is clinically important. Accuracy of these results could be confounded by the misregistration arising as a result of brain shift.[34]
MR imaging features of Primary CNS Lymphoma (PCNSL) and glioblastoma overlap, with differing survival outcomes and treatment options. In one study, perfusion and diffusion-weighted MRI were used to differentiate glioblastoma from lymphoma. Mean ADC and plasma volume (rVp) were higher in the glioblastoma compared to PCNSL. Moreover, mean ADC was superior (AUC 0.83) to rVp and permeability transfer constant (Ktrans). This was true for contrast-enhancing regions only, possibly due to increases in tumor cellularity, microvascular permeability, and vascular proliferation.35 In another study, ADC was outperformed by a multi-parametric (T1-weighted post contrast T1WCE, post-contrast T2Wand T2 FLAIR) and multiregional radiomics classifier with AUC 0.921.[36]
Studies have used multiparametric MRI to create more accurate radiomic models for tumor subtyping. Rathore et al. used 267 multiparametric MRI based radiomic features, extracted from T1-weighted (T1W), T2W, T1WCE, T2 FLAIR, DSC, and DTI to train classifiers to subtype de novo glioblastoma into three imaging phenotypes. For example, the solid subtype was characterized by highly uniform vascularization, highest cell densities, small-sized edema, moderately spherical and well-circumscribed appearance with peritumoral edematous tissue having signs of heterogeneous neovascularization. This subtype had a predilection for the right temporal lobe and was associated with the worst prognosis. A personalized treatment regimen would involve very aggressive peritumoral resection and radiation dose escalation in these tumors.[14] combining various MRI sequences can improve classification accuracy for tumor grading. 37,38Accuracy also increased using MRI features from multiregional and multiparametric structural MRI to predict MGMT methylation status in glioblastoma.[39,40] Similarly, IDH 1 mutation status was predicted using radiomic features on multiparametric MRI with enhanced accuracy when age and multiple regions were included.[41]
Prognostication of Glioblastoma:
It is increasingly important for physicians to understand an individual patient’s prognosis and adjust their therapy accordingly. Radiomics alone and augmented with clinical data, genomics, and proteomics can be used to predict outcomes (Table 2).
Conventional MRI features:
Studies have used various features extracted from conventional MRI to predict patient outcomes in glioblastoma. Longer median survival was associated with higher sphericity, surface-to-volume ratio and edge enhancement on T1W MRI.[42]Lao et. al divided features into ‘handcrafted features’ and ‘deep features’ to create a feature signature, which when coupled with clinical risk factors such as age and Karnofsky Performance Score, was able to predict overall survival (OS). Compared with the predictive ability of traditional risk factors, the proposed feature signature achieved a superior prediction of OS (C-index = 0.739).[43]Similar combined models reached C-index of 0.974.[44]
Texture, tumor shape and volumetric features were extracted, and combined with patient age to produce a model that would predict short-term, mid-term, and long-term OS.[44,45] Zhou et al went one step further and identified spatial-based characteristics from tumor sub-regions that can be used to predict survival time in patients.[46] Similarly, Chaddad et al found three texture features extracted from active part of the tumors that significantly predicted survival outcomes compared to necrotic and edematous parts.[25]Moreover, these radiomic models could predict survival in different molecular subtypes as well.[47]
Advanced MRI features:
Advanced MRI modalities have also been also explored to predict glioblastoma patient outcomes.[48] It was seen that high rCBV in the non-enhancing region of tumor was predictive of worsening OS and Progression-free Survival (PFS).[49]Pre-treatment ADC histogram analysis was useful to predict PFS in bevacizumab-treated patients with newly diagnosed as well as recurrent glioblastoma.33,50 In these studies, low ADC predicted poor outcomes.
Radiogenomics and proteomics:
MGMT promoter hypermethylation, associated with better prognosis and response to therapy, has been combined with radiomic features from structural MRI to stratify patients based on overall survival. Adding MGMT and IDH1 mutation status resulted in more robust radiomics-based prognostic models.[51,52] Zinn et al stratified VAK annotated cases further with molecular signatures and found a 10.5 months’ additional survival benefit for the group with MGMT promoter methylation.[16]In another study, glioblastomas were divided into groups based on vascularization (rCBV values). It was seen that MGMT methylation was a positive predictive factor for OS (p = 0.003, AUC = 0.70) in the moderately vascularized tumors. However, there was no significant effect of MGMT methylation in the highly vascularized tumors (p = 0.10, AUC = 0.56).[53]Other studies did not find any significant association of prognosis with MGMT promoter hypermethylation.[42,54] This could be due to insufficient feature selection methods.
Integrative models promise a reduction in prediction errors.[51,55]Chaddad et al created multi-omic integrative model using radiomic, clinical, protein expression and genetic features to predict the outcome for IDH1 wild-type glioblastoma patients which reached AUC of 78.24%.[56]Liao et al. us extracted First order and multi-dimensional features from segmented lesions on T2‐FLAIR MRI and gave a feature importance score for feature selection.[57] When combined with genetic expression, the Gradient Boosting Decision Tree model gave a 0.81 accurate prediction of both short-term and long-term survival. Six metagenes showed significant interactive effects with image features. However, this study was limited by unavailability of complete genomic data.[57]
Immunophenotypes in glioblastoma are important as they predict response to immunotherapy and outcomes. Hsu et al. used radiomic immunophenotyping models to predict patient prognosis.[58]The phenotype with the worst prognosis comprised highly enriched myeloid-derived suppressor cells and lowly enriched Cytotoxic T lymphocytes.[58]
Treatment of glioblastoma:
Studies have shown the benefit of radiomics analysis in planning surgical procedures, evaluating the dose of radiotherapy, predicting the effective dose of chemotherapeutic agents and stratifying patients who will benefit from therapy. After initiating therapies, radiomics can be used to differentiate mimicking entities like true progression, pseudoprogression and radionecrosis(Table 3).
Surgical Resection:
A recent study examined the correlation of tumor surface regularity on T1W MRI and OS of 165 glioblastoma patients who underwent surgical resection and highlighted that patients with surface-regular tumors had a higher survival rate and benefit from total tumor resection as compared to surface-irregular tumor patients.[59]Gaw et al used machine learning models to better predict tumor cell invasion before resection was conducted.[60] Their aim was to allow for more effective surgery and radiation planning and created a hybrid model with Proliferation-Invasion (PI) model of glioma growth and pre-operative MRIs.[60] Thus, radiomics can help plan a targeted and personalized surgical treatment.
Radiation Therapy (RT) planning:
Radiomics shows immense potential to guide precision radiotherapy. Prediction models can estimate the extent of tumor infiltration and can help identify areas that are at a higher risk of tumor recurrence for targeted RT.[14,61]Rathore et al. worked on a method for estimating peritumoral edema infiltration using radiomics by testing on pre- and post-operative multiomodal MRI sequences in 90 de novo glioblastoma patients and found that recurrent tumor regions revealed higher vascularity and cellularity when compared with the non-recurrent regions.[14] A similar study done on 31 de novo glioblastoma patients confirmed these findings and also highlighted the importance of using multiparametric pattern analysis methods for planning a focused treatment approach to decrease recurrence rate.[61] Thus, radiomics can guide in planning radiation therapy dose escalation in areas with higher risk of tumor recurrence as well as increasing gross total resection. This method can also help prevent dose-related toxicities seen with RT, salvaging the neural tissue at lower risk areas from damage.[62]
Chemotherapy with Temozolomide (TMZ):
Chemotherapy with TMZ along with adjuvant RT increases median OS.[63] However, TMZ resistance arises due to tumor heterogeneity. Yan et al. confirmed the importance of radiomics analysis in predicting disease progression in 57 glioblastoma patients treated with TMZ post-surgery using structural, diffusion and perfusion MR. The study found lower ADC, higher FLAIR and contrast enhanced T1 signals in areas with a higher risk of tumor progression.[64] In another study assessing the efficacy of using a deep-learning based survival-prediction model of 118 patients undergoing concurrent chemoradiotherapy with temozolomide post-surgery, radiomics features including T1W with and without contrast, T2 FLAIR and ADC images were used to assess the OS. While there was no difference observed between the two groups, it highlighted that both clinical and radiomic features should be used hand in hand to predict OS of glioblastoma patients.[65] This reiterates the importance of radiomic models predictive of treatment response to identify suitable treatment regimens.
Therapy with Bevacizumab:
However, variations in genetic makeup of VEGF among individuals can lead to resistance to bevacizumab and limiting its use.[66]Radiomics analysis can provide important biomarkers for selecting patients and to predict the treatment outcome. T1W and T1WCE MRI of 172 patients with recurrent glioblastoma prior to treatment with bevacizumab were used to develop radiomics-based survival predictor as a low-cost instrument for identifying treatment response in these patients.[66] In patients with recurrent glioblastoma receiving bevacizumab treatment, radiomics features from T1WCE obtained at baseline and post-treatment showed prognostic value for survival and progression.[67]Using ADC and CBV of 54 patients with recurrent glioblastoma that were treated with RT and temozolomide, and subsequently treated with bevacizumab, was effective in segregating patients into responders and non-responders to bevacizumab treatment.[68] To predict which patients will benefit from bevacizumab therapy for brain necrosis after radiotherapy, a stratification model was created which integrated the pre-treatment MRI radiomics signature, the interval between radiotherapy and diagnosis of brain necrosis, and the interval between diagnosis of brain necrosis and treatment with bevacizumab. This model achieved AUC 0.912 in the validation set.[69]
Evaluating response to Radiation Therapy (RT):
Texture features derived from enhancing component and peri-lesional edema on structural MRI were used to differentiate pseudoprogression from true progression in glioblastoma.[70] Another model achieved high sensitivity and moderate specificity; incorporating the MGMT status further increased accuracy.[71] While these studies were based on post-RT MRI, pre-RT MRI scans to predict the development of future pseudoprogression in glioblastoma patients gave an AUC of 0.82.[72]Recent studies show that incorporating diffusion and perfusion-weighted MRI, which reflect hypercellularity and hypervascularity of tumor, improves the accuracy in detecting pseudoprogression than conventional MRI alone.[73-75]
Radiation necrosis, another post-RT effect that is difficult to differentiate from true progression, can be detected using ML classifiers based on traditional and delta radiomic features derived from MRI.[76]