Palimpsest: Reconciling the CISS Trilemma for Incremental Nuclei Segmentation

Adapting computational pathology models to evolving clinical diagnostics via Class-Incremental Semantic Segmentation (CISS) is critical. However, this task imposes a unique CISS Trilemma: a simultaneous failure to preserve the intricate tissue background (stability), distinguish morphologically similar new nuclei (plasticity), and maintain a constant model size (scalability), all under a strict exemplar-free constraint. To resolve this, we introduce $\textit{Palimpsest}$, a novel framework that systematically decouples these conflicting demands. $\textit{Palimpsest}$ integrates three synergistic mechanisms: a Parameter-Conserving Synthesis (PCS) module merges lightweight adapters to ensure scalability; a novel Similarity-Aware Centroid Recalibration (SCR) module executes differentiated recalibration to counteract non-uniform foreground drift, securing plasticity; and an Adaptive Residual Shading (ARS) module performs logit-space decoupling to preserve background integrity, ensuring stability. Extensive experiments on two histopathology datasets demonstrate that $\textit{Palimpsest}$ significantly outperforms state-of-the-art methods, achieving a superior stability-plasticity balance, particularly in challenging long-term incremental scenarios. Code will be made publicly available.