Supplementary Material for: Is Methylene Blue a Reliable Substitute in Sentinel Lymph Node Biopsy? A Systematic Review Across Oncologic Sites

Background: Sentinel lymph node biopsy (SLNB) is a pivotal technique for evaluating regional lymphatic spread in breast, melanoma, and gynecologic malignancies. It minimizes surgical morbidity by avoiding complete lymphadenectomy in early-stage disease. The standard dual-tracer approach—combining a radiocolloid such as technetium-99m with a blue dye like patent blue or isosulfan blue—has high sensitivity but is not universally accessible. Radiocolloids require nuclear medicine facilities' licensing and pose logistical burdens, while patent blue may provoke allergic reactions, including anaphylaxis. These limitations are particularly pronounced in low- and middle-income countries (LMICs), where access and affordability are ongoing challenges. Methylene blue (MB), a low-cost, widely available dye, has gained traction as an alternative tracer, offering a safer and more feasible option for SLNB in resource-limited settings. Objective: This systematic review evaluates the diagnostic accuracy, detection reliability, and safety profile of methylene blue dye in SLNB across diverse oncologic sites, emphasizing comparing outcomes with traditional dual-tracer approaches. Methods: A systematic review of PubMed, Embase, Scopus, and Cochrane Library was conducted for studies published from 2000 to 2025. Inclusion criteria comprised clinical studies utilizing MB for SLNB in breast, melanoma, gynaecological, or other solid tumours, reporting outcomes such as detection rate, false-negative rate, or tracer-related complications. Data were extracted and synthesized descriptively. Where possible, pooled performance metrics were calculated. Results: Twenty-five studies encompassing 5,240 patients were included. The pooled sentinel node detection rate using MB alone was 84.5% (72.0–96.2%), with a false-negative rate of 8.6%. When MB was combined with radiocolloid, detection rates improved to 96.1%, and false-negative rates dropped below 5%. Adverse effects were rare, with skin necrosis in fewer than 2% of cases and no serious allergic events reported. Most studies focused on breast cancer (18/25), while data on melanoma and gynecologic cancers were comparatively limited. Conclusion: Methylene blue is a viable alternative for SLNB, particularly in environments where dual-tracer methods are impractical. Though slightly less sensitive than dual-tracer techniques, MB offers substantial advantages in cost, safety, and accessibility. Further multicenter studies and long-term outcomes are needed to support its broader adoption in global oncology practice.