Study flags methylene blue interference in ICG node mapping

CURRENT FULL VERSION: A new study in Animals is putting a spotlight on a practical issue in veterinary surgical oncology: methylene blue may interfere with indocyanine green fluorescence during near-infrared sentinel lymph node mapping. The paper, titled When Blue Turns the Green Off, examined methylene blue–indocyanine green mixtures across multiple imaging modalities and found evidence that the blue dye can dampen the fluorescent signal clinicians rely on to identify draining lymphatics and sentinel nodes. (researchgate.net)

That question matters because sentinel lymph node mapping is becoming more common in dogs and cats, especially in oncology cases where accurate nodal staging can change surgery, prognosis, and follow-up treatment. Reviews of the field describe growing use of indocyanine green near-infrared fluorescence because it offers real-time visualization, better tissue penetration than blue dye alone, and improved tracking of afferent lymphatics. But those same reviews also note important limitations, including equipment access, protocol variability, and the lack of standardization across veterinary centers. (pmc.ncbi.nlm.nih.gov)

The new paper lands in a space where combined-dye workflows are already in use. The University of Florida, for example, has run a clinical trial in dogs with oral tumors using both methylene blue and indocyanine green intraoperatively alongside CT-based mapping, underscoring that dual-tracer approaches are not just theoretical in veterinary medicine. At the same time, a recent feline model study from the University of Guelph found that indocyanine green alone produced significantly greater fluorescence intensity in sentinel lymph nodes than indocyanine green plus methylene blue, even though transit times were similar. That earlier result now looks more clinically relevant in light of the new quenching-focused report. (research.vetmed.ufl.edu)

The broader backdrop is that veterinary teams are still working out which mapping approach gives the best balance of accuracy, cost, and workflow. Methylene blue remains attractive because it is simple and inexpensive, and a recent study in dogs with low-grade mast cell tumors described it as a cost-effective single-technique option in resource-constrained settings. Indocyanine green, meanwhile, is being adopted because it can improve real-time intraoperative visualization and may support more accurate staging. In canine mammary tumor work, investigators reported that indocyanine green near-infrared fluorescence improved visualization of lymphatic drainage and helped optimize node selection, with 1.0 mg/mL performing best in that study. (pmc.ncbi.nlm.nih.gov)

Industry and expert commentary in the veterinary space has been moving in the same direction: use mapping thoughtfully, and don’t assume one tracer or protocol fits every case. A 2024 Today’s Veterinary Practice review emphasized that methylene blue is the most described blue dye in the veterinary literature, but also stressed the need for preoperative mapping and careful technique because direct visualization is required and local tissue staining can complicate surgery. That article also highlighted how protocol choices, including dilution and timing, can affect what the surgeon is actually able to see in the operating room. (todaysveterinarypractice.com)

Why it matters: For veterinary professionals, the immediate takeaway isn’t that methylene blue should disappear from practice. It’s that combining methylene blue with indocyanine green may come with a real optical tradeoff. If the fluorescent signal is reduced, surgeons could miss lymphatic channels or sentinel nodes, particularly in small, deep, or anatomically complex fields where near-infrared guidance is supposed to add value. That could affect staging accuracy, node selection, operative time, and confidence in negative findings. It also reinforces a larger point in veterinary oncology: as sentinel node mapping expands, protocol validation has to include not just whether a tracer reaches the node, but whether one tracer alters the performance of another. (researchgate.net)

The study also arrives as access questions are becoming harder to ignore. Not every practice has near-infrared imaging equipment, and not every pet parent can absorb the added cost of advanced mapping. That makes the newly reported JAVMA fluorescein sodium approach worth watching. In that preliminary study, 6 client-owned dogs with cutaneous or subcutaneous tumors received intradermal peritumoral injections of 10% fluorescein sodium diluted 1:120 with saline. Using blue and black light sources rather than near-infrared equipment, investigators visualized lymphatic tracts in 5 of 6 dogs and identified sentinel lymph nodes in all 6. Mean time to lymphatic tract fluorescence was 6.7 minutes, and mean time to sentinel node fluorescence was 8.0 minutes. No adverse events were reported. Histopathology found metastatic mast cell disease in 4 of the mapped sentinel nodes, while the remaining nodes showed reactive changes; one of two second-tier nodes also contained HN2 metastasis. Those early results support fluorescein sodium as a safe, feasible, and genuinely accessible intraoperative option that could help bring sentinel node mapping into more general practices. (todaysveterinarypractice.com)

There is also a useful reminder here that methylene blue’s effects are context-dependent. In a separate Animals paper outside oncology, investigators studying Nile tilapia in recirculating aquaculture systems found that methylene blue delivered in feed at 0.1% mitigated acute nitrite intoxication. After five days on the medicated diet and 48 hours of nitrite exposure, control fish had five deaths, while methylene blue–treated fish had no mortality or behavioral distress. The treated group also had lower circulating methemoglobin concentrations and less severe gill damage on histology. That work does not change the oncology-specific concern about fluorescence quenching, but it does underscore that methylene blue remains a biologically active and clinically useful compound in other veterinary settings. (mdpi.com)

What to watch: The next step is likely prospective clinical work in dogs and cats comparing indocyanine green alone, methylene blue alone, combined protocols, and lower-cost alternatives in real tumor cases. Watch for studies that tie imaging performance to outcomes that matter in practice, including sentinel node detection rate, metastatic node yield, false-negative risk, procedure time, and cost. It will also be worth watching whether fluorescein sodium can reproduce its early safety and detection results in larger cohorts, and whether methylene blue protocol changes can preserve its practical advantages without compromising indocyanine green signal quality. (research.vetmed.ufl.edu)