Recent studies have revealed that the deep learning approach has potential benefits in diagnostic performance for classifying and diagnosing medical images11–13. We performed novel deep-learning algorithm models using an ANN approach to classify molecular subtypes of breast cancer in WSIs from the TCGA database19,20. Our ANN algorithm with a resolution of 120 \(\times\) 80 pixels, 40 nodes and three hidden layers showed favorable diagnostic performance to classify molecular subtypes in all datasets: accuracy, 67.8%; AUC, 0.819; sensitivity, 70.5%; specificity, 84.4%. Previously, we predicted the location of the glottis in video laryngoscopy images and the detection and classification of intracranial hemorrhage on CT images and the detection of pneumoperitoneum in abdominal radiograph images through ANN with the “Kim-Monte Carlo algorithm”. The ANN model based on the “Kim-Monte Carlo algorithm” was developed by referring to the biological evolution of animals based on Monte Carlo simulation, which is different from the back-propagation method. Our previous studies based on the ANN algorithm demonstrated relatively high classification performance19–21.
Predicting the molecular subtype of breast cancer plays a very important role in selecting the decision making about treatments and prognosis. Molecular subtyping of breast cancer by RNA-based signature assays is a well-established approach, but is not routinely performed due to cost-effectiveness, extended processing time, and need for appropriate tissue samples. To compensate for these limitations, IHC results, such as ER, PR and HER2, are widely used as references for planning breast cancer treatment. In some clinical practice, when IHC or genetic analysis cannot be performed due to poor sample stability and/or insufficient laboratory facilities, the subtype classification based on histopathological patterns is useful. Therefore, it would be a significant achievement if similar results could be obtained by replacing the classification of molecular subtypes based on gene expression with computer pattern recognition. It has recently become possible to use WSIs, digitized counterparts of glass slides obtained through special scanning devices. The subtype classification based solely on these histopathological images is available with more practical advice, and it is cost-effective and easier to access than expensive methods, such as molecular and/or IHC analyses. We attempted to apply the ANN algorithm to classify the subtypes on the basis of histopathological features of WSIs and showed relatively favorable results. Morphological patterns alone may not be sufficient for distinguishing molecular subtypes, but our analysis results could be used as a reference for planning molecular or histology studies.
Previous studies reported molecular subtype classification using breast radiologic or histologic images22–24. A study reported that image data based on a CNN model were not sufficient for predicting the molecular subtype24. Another study using a residual neural network (ResNet) was performed to classify the molecular subtypes in only 384 cases of breast cancer based on 4 pretrained models (VGG16, ResNet50, ResNet101, and Xception). The study included PAM50 molecular intrinsic subtypes and an accuracy of 68%25. Another study using tissue microarray images (TMAs) reported that identification of ER-positive subtypes could be predicted with an accuracy of 75%22. The above studies used a small number of cases or used deep learning to distinguish a single subtype in limited image data, such as TMAs. Our study was the first attempt to classify the WSIs’ molecular subtypes using the ANN algorithm. Since our approach was not a CNN-based deep learning method, data selection and preprocessing of the input image to the ANN was not essential, and it has the advantage of using a simple training process and various small ANN models. Our novel deep‑learning algorithm did not show high enough accuracy to completely replace pathologist’s clinical diagnosis in the classification of molecular subtypes of breast cance. Previous studies have confirmed that WSI-based classification and PAM 50 subtyping may not be consistent because many of the expression-based features defined by the PAM50 gene are not visually identifiable features23. Applying large-scale training datasets or applying data augmentation techniques such as image flipping, image magnification/reduction, and/or image translation to our training sets can significantly improve the diagnostic performance of our ANN models. Nevertheless, it did show favorable diagnostic performance, indicating that our deep learning-based tools and workflow system have the potential to support pathologists' prescreening and diagnostic double-reading of breast cancer subtypes to increase diagnostic accuracy and reduce effort.
Our results do not show more than 90% accuracy in classifying molecular subtypes. Several limitations of our study should be considered. First, some breast cancer cases do not have unique histopathological patterns that vary constantly according to the molecular subtypes. Second, there is a limit in that the present deep learning algorithms cannot learn minimal morphological changes according to the molecular subtypes. Third, in the WSIs, the proportion of supporting cells except for cancer cells may confuse the classification of molecular subtypes. Fourth, the resolution reduction of WSIs makes it impossible to recognize minimal histopathological changes according to the molecular subtypes. Fifth, there is an imbalance between breast cancer subtypes in the TCGA database used in the study. Sixth, the number of WSI data is insufficient to learn.
In summary, the novel deep-learning algorithm for ANNs has classified four molecular subtypes of breast cancer using WSIs from the TCGA database. Our approach is fully automated and skips preprocessing steps, resulting in faster processing times, and shows relatively good diagnostic performance and accuracy. The application of deep learning algorithms for subtyping breast cancer using pathology slide images can improve the accuracy, efficiency, and objectivity of the diagnostic process. Therefore, the clinical application of deep learning using WSI to improve the performance of these algorithms requires the development of additional programs and prospective validation studies using large datasets.