Detector-Stage Failures Account for Most of the Increase in the External Deficit of a Box-Prompted Glioma Segmentation Pipeline on BraTS-Africa: A Three-Arm Decomposition Across Two External Cohorts

Background/Objectives: When a box-prompted segmentation pipeline loses accuracy on new data, the detector proposing the box or a later stage may be responsible; this study measured which.

Methods

A real-time detection transformer (RT-DETR) feeding the Segment Anything Model 2.1 was compared with a detector-free model (MedSAM3 with a low-rank adapter) on whole-tumor Dice in 158 patients with glioma: held-out Brain Tumor Segmentation (BraTS) 2020 cases (n = 24), the RHUH-GBM glioblastoma cohort (n = 39), and BraTS-Africa (n = 95, glioma presumed). Giving the segmenter the ground-truth box splits the Dice gap between the systems into a detector-stage term and a residual.