Which Contrast Phases Does Breast DCE-MRI Tumour Segmentation Need? A Pre-Registered Comparison and Exploratory Analysis

Breast dynamic contrast-enhanced MRI acquires one image volume before contrast injection and several after it; each volume is a contrast phase. Which phases a tumour segmentation model needs has not been systematically tested. In 980 examinations from 22 sites of ISPY2, a neoadjuvant trial cohort in the public MAMA-MIA benchmark, we trained a 3D U-Net on all fifteen combinations of the first four phases, five seeds each, and measured overlap with the expert-corrected reference in Dice points (0–100). One comparison was pre-registered as confirmatory. The confirmatory comparison was inconclusive: a pair chosen before the sweep by a phase-selection model exceeded the clinical pre-contrast plus first post-contrast pair by 1.33 points (−0.39 to 3.12), which is too small a difference for this study to detect. Replacing the clinical pair with the best pair on average was also inconclusive (+1.56). All further findings are exploratory. Averaged across the three pairs of each kind, post-contrast pairs exceeded pairs containing the pre-contrast volume by 2.52 points (1.26–3.91), but this concerned overlap, not boundary accuracy, was not significant in a second cohort, reversed in point estimate when the ISPY2 models were applied there without retraining, and could be inflated by how the reference segmentations were generated. No pair suited every patient: the clinical pair was best for 30% of patients and the best-on-average pair for only 12%, so the average does not identify the best pair for an individual. The interval from pre-contrast to first post-contrast volume varied 2.25-fold across sites (87–196 s), so phase index cannot be assumed to mark a fixed point on the enhancement curve. For data already acquired, phase choice is cost-free, but its effect is small, patient-specific and unconfirmed beyond one cohort; it does not warrant protocol changes. Recording injection time would allow phase timing to be compared across sites.