Structured reasoning failures compromise LLM interpretation of clinical oncology notes.
TL;DR
Evaluating and monitoring reasoning fidelity should be a prerequisite for safe deployment of LLMs in oncology decision support, as endpoint accuracy alone may mask clinically meaningful reasoning failures.
Abstract
Large language models (LLMs) show strong performance on clinical benchmarks, yet their reasoning reliability in real-world oncology care remains unclear. We evaluated LLM reasoning on authentic oncology notes using a novel hierarchical error taxonomy across two retrospective cohorts spanning breast, pancreatic, and prostate cancer. GPT-4 produced reasoning errors in 23.1% of note interpretations, the majority reflecting cognitive bias patterns. Errors were more frequent in recommendation tasks and were strongly associated with guideline-discordant recommendations and lower clinician-rated clinical impact scores. Confirmation bias, anchoring bias, and omission errors were most strongly linked to potentially harmful outputs. Compared to GPT-4, GPT-5.1 demonstrated reduced error rates and improved clinical performance but retained structured reasoning failure patterns. Automated LLM-based evaluators detected error presence but failed to reliably classify subtypes, and a preliminary self-mitigation strategy yielded only modest improvement. Endpoint accuracy alone may mask clinically meaningful reasoning failures. Therefore, evaluating and monitoring reasoning fidelity should be a prerequisite for safe deployment of LLMs in oncology decision support.