3.3. Training from Scratch
As already stated, fine-tuning is a promising method for training deep neural networks. On the other hand, it can only be applied to well-known architectures that are already pre-trained. When designing a custom CNN architecture, it needs to be trained from scratch. In
[39][36], the authors proposed a method based on CNN with residual blocks (Res-Net) referred to as DeepLRHE to predict lung cancer recurrence and the risk of metastasis. Later in
[40][37], scholars established the new DeepIMHL model consisting of CNN and Res-Net to predict mutated genes as biomarkers for targeted-drug therapy of lung cancer. In addition, the authors in
[41][38] trained and optimized EfficientNet models on images of non-Hodgkin lymphoma and evaluated its potential to classify tumor-free reference lymph nodes, nodal small lymphocytic lymphoma/chronic lymphocytic leukemia, and nodal diffuse large B-cell lymphoma. In
[42][39], the authors proposed three architectures of ResNet differing in the construction of residual blocks trained from scratch. Their suggested model achieved accuracy comparable to other state-of-the-art approaches in the classification of oral cancer histological images into three stages. To classify kidney cancer subtypes, in
[43][40] the authors developed an ensemble-pyramidal model consisting of three CNNs that process images of different sizes. The authors in
[44][41] demonstrated that CNN-based DL can predict the gBRCA mutation status from H&E-stained WSIs in breast cancer. According to researchers in
[45][42], CNN can be employed to differentiate non-squamous Non-Small Cell Lung Cancer versus squamous cell carcinoma. To classify the tumor slide, they pooled information using the max-pooling strategy. Moreover, they added quality check with a threshold for predictions to select only tiles with a high prediction level. Additionally, to improve the prediction, they also used a virtual tissue microarray (circle from the centroid based on the pathologist’s hand-drawn tumor annotations) instead of WSI.
To compare the performance of pre-trained networks with the custom ones trained from scratch, researchers in
[46][43] used images of three cancer types: melanoma, breast cancer, and neuroblastoma. Unlike others using patches, the authors applied the simple linear iterative clustering (SLIC) to segment images into superpixels which group together similar neighboring pixels, as shown in
Figure 4. Thus, these superpixels were classified into multiple subtype categories based on the type of cancer. To make WSI-level predictions, they used multiple specific quantification metrics such as stroma-to-tumor ratio. Although the custom NN achieved comparable results, pre-trained networks performed better on all three cancer types. A similar comparison was carried out in
[47][44] for the classification of subtypes in lung cancer biopsy slides. Results showed that a CNN model built from scratch fitted to the specific pathological task could produce better performances than fine-tuning pre-trained CNNs.
A comparison of training from scratch versus transfer learning was performed in
[48][45]. The authors compared three approaches for training the VGG16 network: training from scratch, transfer learning as a feature extractor, and fine-tuning on images of breast cancer to detect Invasive Ductal Carcinoma. According to the results, the model trained from scratch achieved better results in terms of accuracy (0.85). However, using transfer learning, they were able to train a comparable model (accuracy 0.81) ten times faster. Furthermore, among the transfer learning approaches, transfer learning via feature extraction (accuracy 0.81), which involved retraining some of the convolutional blocks, yielded better results in less time compared to transfer learning via fine-tuning (accuracy 0.51).
Figure 4. WSI image segmentation using the SLIC superpixels algorithm.