Redefining the role of carbon nanoparticles in colorectal cancer surgery: From lymph node yield to diagnostic paradox.

Accurate lymph node (LN) staging is critical in colorectal cancer (CRC). Carbon nanoparticles (CNPs), used as a tumor localization agent, incidentally stain LNs, creating uncertainty about their utility for improving staging and guiding oncologic resection. This study evaluates the surgical and oncologic outcomes of CNP use and determines the diagnostic accuracy of CNP staining for metastasis. We conducted a dual-component study: a retrospective, propensity score-matched analysis of 556 patient pairs (CNP vs. control) undergoing CRC surgery between 2020 and 2024, and a prospective cohort of 119 patients in which each LN was analyzed for staining and metastatic status. In the matched cohort, CNP use improved surgical efficiency (shorter operative time, less blood loss) and increased total LN yield (mean, 21.5 vs. 17.7; p < 0.001). However, CNP guidance did not increase metastatic LN detection (p = 0.5) or improve 3-year survival (OS, p = 0.074; DFS, p = 0.31). The prospective analysis revealed a paradoxical finding: unstained LNs had a significantly higher rate of metastasis than stained LNs (3.7% vs. 2.1%; p = 0.02). Consequently, CNP staining was an unreliable diagnostic tool, failing to identify 67.5% of metastatic LNs. All CNP-stained extra-regional LNs (found in 23.5% of patients) were benign. CNPs have a paradoxical dual role in CRC surgery. They are a valuable localization aid that improves surgical efficiency. However, the incidental staining of lymph nodes is diagnostically unreliable and should not guide oncologic decision-making. The practice of extending dissection to pursue extra-regional black-stained nodes is not supported by evidence and should be avoided.