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This research addresses a critical limitation in training medical Large Vision-Language Models (LVLMs), where standard preference optimization methods fail to distinguish clinically decisive phrases from stylistic noise. The authors propose FiRe-MPO, a method that aligns reward signals with specific visual evidence by minimizing edits to model generations and using sparse, span-localized rewards. This approach effectively prevents reward hacking while improving clinical accuracy over existing techniques like DPO.
Analyzing and Improving Fine-grained Preference Optimization in Medical LVLMs
arXiv:2606.12590v2 Announce Type: replace-cross Abstract: Preference optimization is increasingly used to post-train medical large vision-language models (LVLMs), yet it operates at a much coarser granularity than the one that defines clinical correctness. Whether one response is clinically better than another usually comes down to a few decisive phrases, such as an anatomical laterality or a lesion attribute, and to whether each is supported by the image region the question concerns. Direct Preference Optimization (DPO) and its variants, by contrast, reduce the comparison to a single response-level scalar, an objective structurally unable to represent which tokens carry clinical meaning. We show that this mismatch is costly: of the objectives we compare, response-level DPO places the least reward on the phrases that decide clinical correctness, and substituting supervised references for preferred responses opens a stylistic gap that the model exploits as a reward-hacking shortcut, raising preference accuracy but not clinical accuracy. To capture the full feedback in a clinical comparison, we propose Fine-grained Regularized Medical Preference Optimization (FiRe-MPO). Preference pairs are built by minimally editing the model's own generations, so that preferred and rejected responses differ only on clinically decisive spans, and each is paired with a lesion-corrupted image withholding the supporting visual evidence. Because span-localized rewards are sparse, we stabilize optimization with a bidirectional token-wise KL regularizer. Across medical visual question answering and report generation, FiRe-MPO outperforms DPO, RRPO, and competing fine-grained objectives on two popular LVLMs, one medical and one general-purpose, while strengthening visual grounding and placing more reward on the medical phrases than the alternatives.
Abstract:Preference optimization is increasingly used to post-train medical large vision-language models (LVLMs), yet it operates at a much coarser granularity than the one that defines clinical correctness. Whether one response is clinically better than another usually comes down to a few decisive phrases, such as an anatomical laterality or a lesion attribute, and to whether each is supported by the image region the question concerns. Direct Preference Optimization (DPO) and its variants, by contrast, reduce the comparison to a single response-level scalar, an objective structurally unable to represent which tokens carry clinical meaning. We show that this mismatch is costly: of the objectives we compare, response-level DPO places the least reward on the phrases that decide clinical correctness, and substituting supervised references for preferred responses opens a stylistic gap that the model exploits as a reward-hacking shortcut, raising preference accuracy but not clinical accuracy. To capture the full feedback in a clinical comparison, we propose Fine-grained Regularized Medical Preference Optimization (FiRe-MPO). Preference pairs are built by minimally editing the model's own generations, so that preferred and rejected responses differ only on clinically decisive spans, and each is paired with a lesion-corrupted image withholding the supporting visual evidence. Because span-localized rewards are sparse, we stabilize optimization with a bidirectional token-wise KL regularizer. Across medical visual question answering and report generation, FiRe-MPO outperforms DPO, RRPO, and competing fine-grained objectives on two popular LVLMs, one medical and one general-purpose, while strengthening visual grounding and placing more reward on the medical phrases than the alternatives.
Computer Vision and Pattern Recognition (cs.CV); Artificial Intelligence (cs.AI)
arXiv:2606.12590 [cs.CV]
(or arXiv:2606.12590v2 [cs.CV] for this version)
https://doi.org/10.48550/arXiv.2606.12590 Focus to learn more arXiv-issued DOI via DataCite
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