Training Beyond Convergence: Grokking nnU-Net for Glioma Segmentation in Sub-Saharan MRI

TL;DR

This study demonstrates that training nnU-Net beyond convergence can trigger a grokking phenomenon, significantly improving glioma segmentation performance on Sub-Saharan MRI data without additional labels.

Contribution

It introduces a novel application of grokking in medical image segmentation, showing that extended training can boost accuracy in resource-limited settings.

Findings

Grokking can be triggered in glioma segmentation tasks.

Extended training improves Dice scores for all tumor regions.

Strong baseline performance achieved with limited training epochs.

Abstract

Gliomas are placing an increasingly clinical burden on Sub-Saharan Africa (SSA). In the region, the median survival for patients remains under two years, and access to diagnostic imaging is extremely limited. These constraints highlight an urgent need for automated tools that can extract the maximum possible information from each available scan, tools that are specifically trained on local data, rather than adapted from high-income settings where conditions are vastly different. We utilize the Brain Tumor Segmentation (BraTS) Africa 2025 Challenge dataset, an expert annotated collection of glioma MRIs. Our objectives are: (i) establish a strong baseline with nnUNet on this dataset, and (ii) explore whether the celebrated "grokking" phenomenon an abrupt, late training jump from memorization to superior generalization can be triggered to push performance without extra labels. We evaluate two training regimes. The first is a fast, budget-conscious approach that limits optimization to just a few epochs, reflecting the constrained GPU resources typically available in African institutions. Despite this limitation, nnUNet achieves strong Dice scores: 92.3% for whole tumor (WH), 86.6% for tumor core (TC), and 86.3% for enhancing tumor (ET). The second regime extends training well beyond the point of convergence, aiming to trigger a grokking-driven performance leap. With this approach, we were able to achieve grokking and enhanced our results to higher Dice scores: 92.2% for whole tumor (WH), 90.1% for tumor core (TC), and 90.2% for enhancing tumor (ET).