MedMO: Grounding and Understanding Multimodal Large Language Model for Medical Images

Ankan Deria, Komal Kumar, Adinath Madhavrao Dukre, Eran Segal, Salman Khan, Imran Razzak

Abstract: Multimodal large language models have advanced rapidly, but their adoption in medicine is constrained by limited domain coverage, imperfect modality alignment, and insufficient grounded reasoning. We introduce MedMO, a medical multimodal foundation model built on a general MLLM architecture and trained exclusively on large-scale domain-specific data. MedMO uses a multi-stage training recipe that includes cross-modal pretraining to align heterogeneous visual encoders with a medical language backbone, instruction tuning with multi-task supervision spanning captioning, VQA, report generation, retrieval, and bounding-box disease localization, and reinforcement learning with verifiable rewards that combine factuality checks with a box-level GIoU signal to improve spatial grounding and step-by-step reasoning in challenging clinical settings. Across modalities and tasks, MedMO surpasses strong open-source medical baselines. MedMO-8B-Next achieves consistent gains on VQA benchmarks, improving by 6.6% on average over Fleming-VL-8B, including gains of 6.0% on MMMU-Med, 9.8% on PMC-VQA, and 21.3% on MedXpertQA. On text-based QA, it improves by 14.4% over Fleming-VL-8B, driven by gains of 8.4% on MMLU-Med and 30.1% on MedQA. For medical report generation, it improves by 6.7% on MIMIC-CXR. MedMO-8B-Next also demonstrates strong grounding performance, reaching 56.1 IoU on Bacteria, which is a 47.8 IoU gain over Fleming-VL-8B. At smaller scale, MedMO-4B-Next remains competitive and exceeds Fleming-VL-8B across VQA, QA, and report generation. Evaluations spanning radiology, ophthalmology, and pathology microscopy further confirm broad cross-modality generalization.