Supervise Less, See More: Training-free Nuclear Instance Segmentation with Prototype-Guided Prompting

TL;DR

SPROUT is a training-free, prototype-guided prompting framework that enables precise nuclear instance segmentation in pathology without any supervision or retraining.

Contribution

It introduces a novel training-free approach using histology-informed prototypes and prompting to achieve competitive nuclear segmentation performance.

Findings

SPROUT achieves competitive results across multiple benchmarks.

It operates without any supervision or retraining.

The method leverages domain-specific priors for effective segmentation.

Abstract

Accurate nuclear instance segmentation is a pivotal task in computational pathology, supporting data-driven clinical insights and facilitating downstream translational applications. While large vision foundation models have shown promise for zero-shot biomedical segmentation, most existing approaches still depend on dense supervision and computationally expensive fine-tuning. Consequently, training-free methods present a compelling research direction, yet remain largely unexplored. In this work, we introduce SPROUT, a fully training- and annotation-free prompting framework for nuclear instance segmentation. SPROUT leverages histology-informed priors to construct slide-specific reference prototypes that mitigate domain gaps. These prototypes progressively guide feature alignment through a partial optimal transport scheme. The resulting foreground and background features are transformed into positive and negative point prompts, enabling the Segment Anything Model (SAM) to produce precise nuclear delineations without any parameter updates. Extensive experiments across multiple histopathology benchmarks demonstrate that SPROUT achieves competitive performance without supervision or retraining, establishing a novel paradigm for scalable, training-free nuclear instance segmentation in pathology.