Methylene blue may blunt ICG fluorescence in sentinel node imaging

CURRENT BRIEF VERSION: A new exploratory study in Animals suggests methylene blue can substantially interfere with indocyanine green, or ICG, during near-infrared fluorescence imaging, a technique increasingly used for sentinel lymph node mapping in canine and feline oncology. The authors, Elisa Maria Gariboldi, Luigi Auletta, and Roberta Ferrari, evaluated methylene blue–ICG mixtures across three imaging modalities and found evidence of fluorescence quenching, meaning the visible blue dye may reduce the fluorescent signal clinicians are trying to detect. That matters because dual-tracer workflows are common in surgical oncology, including veterinary practice, where methylene blue is often paired with imaging-based mapping approaches. Recent canine data have also shown methylene blue remains clinically useful for intraoperative mapping, but with only moderate agreement versus CT lymphangiography in dogs with oral neoplasms, reinforcing that tracer choice and technique can shape what gets identified as the sentinel node. (pubmed.ncbi.nlm.nih.gov)

Why it matters: For veterinary teams using ICG-guided lymphatic mapping, the study is a practical reminder that combining dyes isn’t automatically additive. If methylene blue suppresses ICG fluorescence in the field, surgeons could miss or under-call sentinel nodes, especially when signal is already weak, anatomy is complex, or equipment sensitivity varies. That also gives added context to emerging interest in lower-cost alternatives such as fluorescein-guided mapping in dogs. In a recent preliminary JAVMA study of 6 client-owned dogs with cutaneous or subcutaneous tumors, diluted 10% fluorescein sodium allowed visualization of lymphatic tracts in 5 of 6 dogs and sentinel nodes in all 6, with no adverse events and metastatic disease identified in 4 sentinel nodes on histopathology. The Veterinary Cancer Society has highlighted that work as a potential way to expand access to sentinel lymph node surgery, and older veterinary work also showed fluorescein can visibly stain lymphatics and sentinel nodes in dogs long before the current push for fluorescence-guided workflows. (vetcancersociety.org)

What to watch: Look for follow-up in vivo veterinary studies that test whether changing injection sequence, concentration, or tracer selection can preserve mapping accuracy without sacrificing cost or workflow. More broadly, methylene blue’s effects are context-dependent: outside oncology imaging, recent Animals research in Nile tilapia found methylene blue delivered in feed reduced methemoglobin formation, prevented mortality during acute nitrite intoxication, and lessened gill damage, underscoring that the dye remains clinically useful even if it complicates fluorescence-based mapping. (pmc.ncbi.nlm.nih.gov)