Study raises caution on methylene blue with ICG node mapping

A new study in Animals is putting a finer point on a practical problem in fluorescence-guided surgery: methylene blue may suppress the near-infrared signal of indocyanine green during imaging used for sentinel lymph node mapping. In small animal oncology, that’s more than a technical footnote. ICG-based near-infrared imaging is being adopted to help surgeons identify lymphatic channels and sentinel nodes in real time, and methylene blue has often been used alongside it as a familiar visual dye. If one tracer weakens the other, that could affect staging accuracy and intraoperative decision-making. (pmc.ncbi.nlm.nih.gov)

The backdrop is a broader shift in veterinary oncology toward sentinel lymph node mapping rather than relying only on expected regional nodes or gross enlargement. Reviews of canine mast cell tumor mapping have emphasized that anatomic assumptions can miss true drainage patterns, and that imaging-guided approaches can improve node identification. In canine mammary tumor work, investigators have likewise described ICG near-infrared fluorescence as a promising tool because it can support transcutaneous lymphatic visualization and more targeted node excision. (pmc.ncbi.nlm.nih.gov)

Against that backdrop, the Animals paper asks a straightforward but clinically relevant question: what happens when methylene blue and ICG are used together? Based on the study abstract and secondary indexing, the authors quantitatively and qualitatively assessed methylene blue–ICG mixtures at different ratios across three near-infrared imaging modalities and found evidence of fluorescence quenching. In plain terms, methylene blue appeared to reduce the ICG signal the camera is meant to capture. Even without full clinical outcome data yet, that finding points to a potential source of false-weak or missed fluorescence during mapping. (researchgate.net)

That concern fits with signals from human and translational literature. A 2022 Cancers paper on methylene blue fluorescence imaging in breast cancer sentinel node biopsy said quenching may help explain poor detection performance at some concentrations. A practical review on fluorescence-guided abdominal surgery has also treated both ICG and methylene blue as useful agents, but with distinct optical properties that require protocol awareness. At the same time, other clinical reports show the two can sometimes be visualized in the same operation when imaging systems and workflows are designed around their different spectra, suggesting the issue may be less “never combine” than “combine carefully.” (pubmed.ncbi.nlm.nih.gov)

There’s also a timely access angle. A 2026 JAVMA pilot study evaluated 10% fluorescein sodium as a low-cost intraoperative sentinel lymph node mapping method in six dogs with cutaneous or subcutaneous tumors. Investigators reported lymphatic tract visualization in 83% of dogs, sentinel lymph node identification in 100%, and no adverse events, positioning fluorescein as a potentially more accessible option for practices that don’t have near-infrared platforms or want a lower-cost workflow. Because the study was small, it’s best read as proof of concept, not a practice-changing comparison with ICG. Still, it arrives at a moment when clinics are weighing not just performance, but affordability and equipment requirements. (eurekamag.com)

Why it matters: For veterinary professionals, the immediate takeaway is procedural. If a team is using ICG-based near-infrared mapping, methylene blue may not be an innocent add-on. The new paper suggests dye choice, concentration, and administration strategy could materially affect fluorescence visibility. That matters most in cases where sentinel node identification guides staging, prognosis, and the extent of surgery. It also reinforces the need to standardize protocols as more practices adopt fluorescence-guided techniques, because variation in tracer combinations could be one reason results differ across studies and centers. (researchgate.net)

The study also lands in an area where veterinary evidence is still maturing. Reviews of canine mast cell tumor mapping have reported stronger node detection and metastatic node yield with near-infrared-guided approaches than unguided dissection, but they also note limitations such as penetration depth and technical variability. If methylene blue is one avoidable source of signal loss, that’s a practical variable clinicians can start scrutinizing now, even before larger prospective veterinary trials are published. That’s an inference from the available evidence, rather than a direct recommendation from a guideline. (pmc.ncbi.nlm.nih.gov)

What to watch: The next step is likely protocol-focused research, including head-to-head veterinary studies on tracer combinations, dilution ranges, injection timing, and whether alternatives such as fluorescein sodium can expand access without sacrificing staging performance. Watch, too, for conference discussion or follow-up commentary from surgical oncology groups, because this is exactly the kind of technical detail that can reshape day-to-day mapping workflows. (eurekamag.com)