Frequency response compensation to optimize pretrained deep learning models in lung nodule malignancy classification

Accurate differentiation of benign and malignant pulmonary nodules on computed tomography (CT) is important for early lung-cancer assessment. However, classification remains challenging because benign and malignant nodules may exhibit subtle differences in texture, margin irregularity, boundary sharpness, and internal morphology that may not be fully represented by conventional convolutional feature extraction.

A frequency response Bilinear Lag-Lead (BT-LL) EfficientNetB3 framework was developed for benign-malignant pulmonary nodule classification. The proposed approach integrates Lag-Lead frequency compensation into the pretrained feature-extraction process to refine intermediate representations. The Lag component performs structural smoothing to preserve low-frequency information, whereas the Lead component enhances high-frequency boundary and texture characteristics. These corresponding representations are integrated with the original feature maps through a dedicated fusion operation to obtain frequency-refined features for classification. The compensation filters are fixed, thereby avoiding additional parameter learning within the filtering operation. Patient-level data partitioning was used to ensure that nodules from the same patient were not distributed across different experimental subsets, reducing the risk of information leakage.

The proposed model achieved an accuracy, sensitivity, and specificity of 0.90, an F1-score of 0.81, and an AUROC of 0.95 on the held-out patient-level test set. Performance was additionally assessed using patient-level cross-validation, cross imbalance analysis, an IQ-OTH/NCCD image-level evaluation and an independent BUS-BRA patient-level evaluation. The experimental analysis further included statistical significance testing using McNemar's test for pairwise model comparisons, followed by the Holm correction to control for multiple comparisons, while Cochran's Q test was employed to assess performance differences across multiple model configurations. The combined configuration achieved the best sensitivity and balanced accuracy relative to the branch-wise variants. The Bilinear Lag-Lead (BT-LL) framework thus provides an explicit, frequency response mechanism for integrating contributions of low- and high-frequency information in pulmonary nodule representations.