Single Model Deep Learning on Imbalanced Small Datasets for Skin Lesion Classification

Deep convolutional neural network (DCNN) models have been widely explored for skin disease diagnosis and some of them have achieved the diagnostic outcomes comparable or even superior to those of dermatologists. However, broad implementation of DCNN in skin disease detection is hindered by small size and data imbalance of the publically accessible skin lesion datasets. This paper proposes a novel data augmentation strategy for single model classification of skin lesions based on a small and imbalanced dataset. First, various DCNNs are trained on this dataset to show that the models with moderate complexity outperform the larger models. Second, regularization DropOut and DropBlock are added to reduce overfitting and a Modified RandAugment augmentation strategy is proposed to address the defects of sample underrepresentation in the small dataset. Finally, a novel Multi-Weighted Focal Loss function is introduced to overcome the challenge of uneven sample size and classification difficulty. By combining Modified RandAugment and Multi-weighted Focal Loss in a single DCNN model, we have achieved the classification accuracy comparable to those of multiple ensembling models on the ISIC 2018 challenge test dataset. Our study shows that this method is able to achieve a high classification performance at a low cost of computational resources and inference time, potentially suitable to implement in mobile devices for automated screening of skin lesions and many other malignancies in low resource settings.