The number of publications on deep learning for cancer diagnostics is rapidly increasing, and systems are frequently claimed to perform comparable with or better than clinicians. However, few systems have yet demonstrated real-world medical utility. In this Perspective, we discuss reasons for the moderate progress and describe remedies designed to facilitate transition to the clinic. Recent, presumably influential, deep learning studies in cancer diagnostics, of which the vast majority used images as input to the system, are evaluated to reveal the status of the field. By manipulating real data, we then exemplify that much and varied training data facilitate the generalizability of neural networks and thus the ability to use them clinically. To reduce the risk of biased performance estimation of deep learning systems, we advocate evaluation in external cohorts and strongly advise that the planned analyses, including a predefined primary analysis, are described in a protocol preferentially stored in an online repository. Recommended protocol items should be established for the field, and we present our suggestions.
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The authors thank M. Seiergren for assembling all figures, T. S. Hveem for discussions, T. Ystanes, H. A. Inderhaug and B. M. Sannes for setting up and maintaining our computer network and computational infrastructure, and the authors of Inception-v3 for making their code freely available under an open source licence (Apache License, version 2.0). The authors of this Perspective acknowledge funding from the Research Council of Norway through its IKTPLUSS Lighthouse programme (project number 259204).
The authors declare no competing interests.
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- Area under the receiver operating characteristic curve
(AUC). A performance metric measuring the concordance between a dichotomous outcome and the ranking of subjects provided by a continuous or categorical marker. An AUC of 50% indicates random guessing and 100% indicates perfect prediction. For dichotomous markers, the AUC and balanced accuracy are equivalent.
- Artificial neural networks
Mathematical functions mapping input data to output representations, structured as a directed graph of nodes and edges.
- Balanced accuracy
A classification performance metric calculated by averaging the proportion of true predicted outcomes across all possible outcomes. For dichotomous outcomes, this reduces to the average between the sensitivity and the specificity.
The ability of a model class, for example a particular network architecture, to express complicated correlations between input data and target output. Model classes with high capacity have the potential to produce models that are able to map training data to target outputs with a high degree of accuracy, but are also more prone to overfitting.
- Concordance index
(c-index). A performance metric measuring the concordance between a target outcome, usually defined by time to event data, and the ranking of subjects provided by a continuous or categorical marker. A c-index of 50% indicates random guessing and 100% indicates perfect prediction. For dichotomous outcomes, the c-index and the area under the receiver operating characteristic curve are equivalent.
- Deep learning
A class of machine learning methods that make use of successively more abstract representations of the input data to perform a specific task, typically implemented using artificial neural networks. They also consist of an objective function that compares the final output with a target output as well as an optimization method that is used to optimize the objective function.
- Deep learning models
Computational models obtained by training deep neural networks. Note that a single training of a neural network produces a sequence of models as each new optimization iteration produces a model slightly different from the previous one. A tuning data set may be used to select among these models.
- Deep learning systems
Systems utilizing one or more deep learning models to make predictions. A system’s output may be a function of the outputs of the models, for example by averaging and thresholding the model outputs.
- Development cohort
A cohort used for training and, sometimes, tuning and internal validation of a system.
- External cohorts
Also known as independent cohorts, these differ non-randomly from the development cohort. In cancer diagnostics, the external cohorts will often contain patients suspected of having the same disease or disease attribute, at risk of developing the same event or suspected to respond to the same treatment as patients in the development cohort. However, external cohorts may be intentionally more different from the development cohort.
- External validation
An evaluation of a system’s performance on an external cohort that did not influence the development of the system.
The ability of a system to perform similarly on subjects not included in training to on those included in the training. Poor generalizability can be caused by overfitting to the training data or by the lack of generally relevant features in the training data.
Utilizing noise or features in the training data that are not generally relevant for the prediction task but cause the system to perform better on the training sample.
- Supervised machine learning
A methodology in which learning occurs by mimicking the mapping of input data to target output labels. By contrast, the input data are not associated with any output labels in unsupervised learning.
Although frequently used by the machine learning community to refer to an evaluation of a system’s performance, we use ‘test’ to refer to evaluations other than external validations, for example internal validations.
Optimization of model parameters based on data.
Informed selection of hyperparameter values (parameters not optimized during training) based on data. Examples include the network architecture, optimization method and threshold for a model’s continuous output. The nomenclature in machine learning is to use ‘validation’ instead of ‘tuning’.
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Kleppe, A., Skrede, OJ., De Raedt, S. et al. Designing deep learning studies in cancer diagnostics. Nat Rev Cancer 21, 199–211 (2021). https://doi.org/10.1038/s41568-020-00327-9
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