Standard nnU-Net brain-tumour segmentation drops 0.075 Dice on new data

single source· 1 articles · confidence: medium · first seen 2026-09-13 20:00 UTC

What this means for you

If you validate segmentation models on held-out data from the same source as training, expect that number to fall on a new population: this pipeline lost 0.0747 Dice, concentrated in small, fragmented enhancing tumour. It is a reason to distrust single-source validation figures, not to change tools.

A conventional 3D nnU-Net — a standard, self-configuring image-segmentation pipeline — was trained on 1,351 labelled cases in five-fold cross-validation for the BraTS-GoAT 2026 brain-tumour challenge. On the official pooled validation set its Dice scores (overlap between predicted and true tumour regions; 1.0 is perfect) were 0.7805 for enhancing tumour, 0.8288 for tumour core and 0.8854 for whole tumour. Under matched fold-0 inference, mean Dice fell from 0.9058 on the source's out-of-fold cases to 0.8310 on pooled validation, a difference of 0.0747. Flipping images and averaging predictions gave small single-fold gains but no clear ensemble benefit; failures clustered in small, fragmented enhancing tumour, and a residual-encoder variant reached 0.8282.

Key facts

  • ·A conventional 3D nnU-Net was trained on 1,351 labelled cases with five-fold cross-validation and 1,000 epochs per fold. source
  • ·Global Dice on pooled official validation was 0.7805 for enhancing tumour, 0.8288 for tumour core and 0.8854 for whole tumour. source
  • ·Under matched fold-0 inference, mean regional Dice fell from 0.9058 on source out-of-fold cases to 0.8310 on pooled validation, a difference of 0.0747. source
  • ·Test-time mirroring gave small single-fold gains but no clear ensemble benefit. source
  • ·A residual-encoder alternative reached 0.8282 mean Dice. source
  • ·Failure cases had substantially smaller reference enhancing-tumour volumes; after adjustment for ET and WT volume, lower Dice was associated with more disconnected ET components and a smaller fraction of ET in the largest component. source

What the sources say

Sources

The original reporting. Follow these — they did the work.

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