Data Partitioning and Statistical Considerations for Association of Radiomic Features to Biological Underpinnings: What Is Needed

See also the article by Gidwani et al in this issue.

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Over the past decade, radiomics, or texture analysis, has been increasingly investigated for its utility as a potential biomarker derived from different radiologic images (1,2). There are basically two types of radiomic features: first order (statistical features) and second order (gray-level matrix [fine and course features]). They are generated as output as either single radiomic features (3–5) or multiparametric radiomics features (6). Most studies use single handcrafted regions of interest (3–5) or full radiomic images based on the tissue of interest (6). Several studies have shown some correlation of radiomic features with other important clinical parameters, which one day could make radiomics a standard-of-care parameter used in a clinical setting. However, radiomics is still an investigative tool, with several groups actively pursuing standardization methods for more accurate radiomic features (7,8).

In this issue of Radiology, the article by Gidwani et al (9) brings into focus the careful considerations of data partitioning and statistical methods that are needed to ensure reproducible data analysis without resulting in “inflated” measures of accuracy and spurious associations when using radiomics coupled with machine learning (ML) methods. Moreover, a recent publication in Radiology has highlighted these types of concerns as well (10). These reports are very timely and needed to further progress the interpretation of radiomic features as they are related to biology for a more accurate prediction of potential clinical association or significance.

The authors (9) report and demonstrate that incorrect data partitioning can lead to a very considerable boost, at least 1.4-fold, in the performance of the radiomic features when using ML to obtain the most significant factors based on the area under the receiver operating characteristic curve (AUC) and correlation analysis in overall survival. The radiomic features were derived from two public data sets consisting of low-grade gliomas, head and neck cancer, and further testing of radiomic features with association of gene array scores. The findings reported could have implications for identifying which intrinsic features are important, and the authors provide a roadmap to strengthen the testing of radiomic-ML pipelines. For example, using a model of data leakage in their simulated radiomic feature set with the different ML models resulted in high correlations and AUC metrics. When the data leakage was “corrected” in the data set, the results become inconclusive with nondiagnostic AUC values. Another major implication of the results of this study is that Gidwani et al (9) describe significant correlations between radiomics and gene array data by using simulated radiomic features that had no biologic meaning. This is clearly demonstrated when mixing high-dimensional data sets, which can be problematic due to the sparsity of the data points and can lead to the spurious correlations. As noted in the article, care needs to be taken to ensure that no data leakage can occur and to consider if the results make practical sense.

The authors present a clear direction on how to avoid these potential pitfalls when using the radiomic-ML pipeline through a series of questions investigators may ask while designing a study. These are summarized as follows: (a) Is a sample size estimate performed to determine the significance of the result? (b) Is partitioning applied correctly, and is it consistently observed through the different steps of ML application? (c) Have reproducibility and multiple hypotheses and correction methods (if applicable) been applied; and (d) Is there an external data set available for testing the model? Also, some investigators need to test if the radiomic results and correlations make sense with any quantitative imaging metrics (eg, T1, T2, or apparent diffusion coefficient of water mapping [MRI], standard uptake values [PET], or Hounsfield units [CT]). Finally, the model design introduced by Gidwani et al (9) may be able to reduce bias and spurious correlations in radiomic research and help by giving more insight and reliability to the radiomics-ML pipeline when applied to radiologic imaging or data sets.