Study flags methylene blue interference with ICG node mapping

A new paper in Animals is putting a finer point on a problem many surgeons may not have considered: when methylene blue and indocyanine green are used together for sentinel lymph node mapping, the blue dye may suppress the fluorescent signal teams depend on during near-infrared imaging. The study, titled When Blue Turns the Green Off: Implications of Methylene Blue Interference in Indocyanine Green Near-Infrared Fluorescence Imaging, focuses on a workflow that is becoming more common in veterinary oncology as surgeons combine visible dyes with fluorescence-guided techniques for real-time node detection. (veterinaryoncology.biomedcentral.com)

That question matters because sentinel lymph node mapping is moving from a niche technique toward a more established part of staging and surgical planning in dogs, and potentially cats, with solid tumors. In veterinary oncology, investigators have described intraoperative use of methylene blue, ICG, radiopharmaceutical guidance, CT lymphangiography, contrast-enhanced ultrasound, and combined approaches, depending on tumor type and institutional resources. A 2024 Veterinary Oncology report on dogs with apocrine gland anal sac adenocarcinoma noted that intraoperative mapping commonly involves visible dyes such as methylene blue and near-infrared dyes such as ICG, while a more recent canine mammary tumor study reported a 94.4% sentinel lymph node detection rate with ICG-NIRF imaging and identified 1.0 mg/mL as the strongest-performing concentration tested. (veterinaryoncology.biomedcentral.com)

The new Animals study adds an important caution to that momentum. Based on the abstracted source information, the authors quantitatively and qualitatively assessed methylene blue–ICG mixtures at different ratios using three near-infrared imaging modalities, including fluorescence-lifetime imaging microscopy, and found evidence of fluorescence quenching. That aligns with prior veterinary research in a simulated feline tumor model, published in 2023, that specifically examined the effect of methylene blue on ICG fluorescence intensity and transit during sentinel lymph node mapping. In other words, this isn’t an isolated theoretical concern; it sits within an emerging body of work suggesting dual-tracer protocols can change signal behavior in ways that matter clinically. (pubmed.ncbi.nlm.nih.gov)

There’s also a broader technology and access story here. While ICG is attractive because it enables real-time lymphatic visualization and can improve precision, fluorescence-guided surgery still depends on compatible imaging systems, protocol familiarity, and tracer handling that may vary across practices. At the same time, researchers are testing less expensive or more accessible options. A preliminary 2026 JAVMA study of six client-owned dogs with cutaneous or subcutaneous tumors found that diluted 10% fluorescein sodium allowed lymphatic tract visualization in five of six dogs and sentinel lymph node identification in all six, with no adverse events reported. The authors suggested that approach could help expand sentinel lymph node mapping into general practice settings. (eurekamag.com)

Industry reaction is still limited, and I didn’t find a separate press release or broad outside commentary tied to the new Animals paper. But the surrounding literature points to the same practical message: tracer choice and protocol design matter. Gariboldi and colleagues have previously reported that adding methylene blue to intraoperative gamma-probe guidance increased sentinel lymph node detection in dogs with mast cell tumors from 90% to 95%, while also warning that unstained nodes can still be metastatic. That earlier finding helps explain why methylene blue remains attractive in surgery, even as the new paper raises concern that pairing it with ICG fluorescence may come with tradeoffs. (mdpi.com)

Why it matters: For veterinary professionals, this is less about abandoning methylene blue than about understanding what a missing fluorescent signal does, and doesn’t, mean. If methylene blue quenches ICG under certain concentrations or imaging conditions, surgeons could underestimate lymphatic drainage, miss a true sentinel node, or lose confidence in fluorescence guidance at exactly the point when staging accuracy matters most. That’s especially relevant for practices building oncology workflows around a single imaging platform, or for referral centers trying to standardize protocols across tumor types. The study also reinforces a broader principle in veterinary fluorescence imaging: results are highly technique-dependent, so protocols validated for one tracer combination, concentration, or device may not translate cleanly to another. (pmc.ncbi.nlm.nih.gov)

What to watch: The next step is likely clinical validation, not just bench testing, to determine which methylene blue:ICG ratios, injection sequences, and device settings preserve usable fluorescence in live patients. It will also be worth watching whether surgeons shift toward sequential rather than mixed administration, lean more heavily on single-tracer ICG protocols, or explore lower-cost alternatives such as fluorescein sodium in settings where near-infrared imaging is impractical. (eurekamag.com)