Token-Level Attribution for Transparent Biomedical AI.

Background: Explainability (xAI) is critical for fostering trust, ensuring safety, and supporting regulatory compliance in healthcare AI systems. Large Language Models (LLMs), with impressive capabilities, operate as "black boxes" with prohibitive computational demands and regulatory challenges. Small Language Models (SLMs) with open-source architectures present a pragmatic alternative, offering efficiency, potential interpretability, and alignment with data privacy frameworks. This study evaluates whether token-level attribution (TLA) methods can provide technical traceability in SLMs for clinical decision support. Methods: The Captum 0.7 attribution library was applied to a Qwen-2.5-1.5B model on 20 breast cancer cases from a publicly available dataset. Hardware requirements were profiled on consumer-grade GPU. Using perturbation-based integrated gradients, we analyzed how clinical input features statistically influenced token generation probabilities. Results: Attribution heatmaps successfully identified clinically relevant input features, with high-attribution tokens corresponding to expected clinical factors. The model occupied minimal storage, enabling local deployment without cloud infrastructure. This validates that SLMs can provide algorithmic traceability required for regulatory frameworks. Conclusions: This proof-of-concept demonstrates the technical feasibility of combining SLMs with perturbation-based xAI methods to achieve auditable clinical AI within practical hardware constraints. While TLA provides statistical associations, bridging toward causal clinical reasoning requires further research.