Source-Causal Control of Historical Context in Longitudinal Radiology Report Generation

Longitudinal radiology report generation increasingly conditions on prior images and reports. This history is clinically valuable, but it creates a source-validity failure mode: a generator can carry prior-only findings into the current report. Weintroduce source-causal robustness, a stress-test and control framework that holds the current chest X-ray fixed while intervening on historical context. On a full eligible MIMIC-CXR longitudinal study (18,716 pairs; 93,580 generations across five source conditions), historical context consistently trades fewer missed findings for more unsupported positives. Matched wrong-history, the key negative control, increases proxy-defined false positives by +0.366 per report while reducing false negatives by −0.129; random wrong-history yields an even larger false positive stress signal (+0.465 FP/report). We propose the Source-Causal Finding Router (SCFR), a source-validity controller trained with within-evaluation subject exclusion. Across three held-out evaluations, SCFR repairs matched wrong-history false-positive inflation by −0.343, −0.550, and −0.457 per report, passes Holm corrected primary testing at the bootstrap resolution floor (Holm-adjusted p ≤ 0.0012), and satisfies fixed current-only parity margins. Cross-generator stress panels with RadInfer and Libra, together with ReXPref-Prior, RadGraph2, Chest ImaGenome, ablations, and case review, support the mechanism. These results establish a protocol-defined state-of-the-art source-causal robustness/control result for longitudinal radiology report generation under matched wrong-history stress tests.