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This paper introduces CT-$\Delta$Bench, a benchmark specifically designed for longitudinal 3D medical imaging difference reporting, addressing the critical need for models that can analyze and report on temporal changes in patient scans. The study emphasizes the importance of generating clinically meaningful reports from two temporally separated CT scans, highlighting the inadequacies of existing models that focus solely on single-study analysis. Key results include the development of change-aware metrics for evaluating longitudinal differences and the introduction of DeltaMed, a baseline model that demonstrates improved performance in generating clinically relevant reports compared to traditional methods.
Longitudinal imaging difference reporting can now be accurately assessed with a benchmark that captures clinically significant changes, setting a new standard for medical imaging models.
In medical imaging, the clinical value of Computed Tomography (CT) lies not only in depicting current disease status, but crucially in enabling longitudinal comparison of serial scans to determine disease evolution, a process that underpins response assessment, recurrence detection, and ongoing patient management. Yet, despite this central role of temporal comparison in clinical decision-making, existing medical foundation models remain largely confined to single-study understanding, leaving temporally grounded cross-examination insufficiently addressed. To address this gap, we study longitudinal imaging difference reporting, a task in which a model takes two temporally separated scans from the same patient and generates a clinically meaningful report describing interval changes between them. We introduce CT-$\Delta$Bench, a dedicated benchmark for this task with patient-level splitting to prevent information leakage. To better evaluate this task beyond surface-level text similarity, we further develop change-aware metrics specifically designed to capture clinically meaningful longitudinal changes, and conduct an independent physician validation to assess the reliability of the synthesized references and event extraction pipeline. We also compare direct paired-CT reasoning with an indirect two-stage pipeline that first generates single-timepoint reports and then performs textual differencing. Finally, we propose DeltaMed, a baseline model for direct paired-CT difference reporting, and train it on the benchmark training set. Together, these contributions lay the groundwork for temporally aware medical foundation models that better reflect real-world longitudinal clinical reasoning.
