Ex vivo sentinel lymph node mapping in colorectal cancer using a magnetic nanoparticle tracer to improve staging accuracy: a pilot study

Aim: Nodal status is the most important prognostic factor in colorectal cancer (CRC). Small occult metastases may remain undetected on conventional histopathological examination, potentially resulting in undertreatment. Ex vivo sentinel lymph node mapping (SLNM) can be used to improve nodal staging accuracy, but currently used tracers suffer from drawbacks, which hampers implementation of the technique in routine clinical practice. Magnetic tracers have an optimal size for sentinel lymph node (SLN) retention and allow objective quantitative selection of SLNs and therefore have great potential for SLNM in CRC. The study evaluates the feasibility of ex vivo magnetic SLNM and compares the performance of this technique to blue dye SLNM.

Method: Twenty-eight ex vivo SLNM procedures were performed in 27 histological node-negative patients with CRC using a magnetic tracer and blue dye. A magnetometer was used to select magnetic SLNs after formalin fixation of the CRC specimen. Both magnetic and blue SLNs were subjected to serial sectioning and immunohistochemical staining to reveal occult metastases.

Results: At least one SLN was successfully identified in 27/28 (96%) and 25/28 (89%) of the cases with the magnetic technique and blue dye. Isolated tumour cells were detected in 10 patients. This was predicted with 100% sensitivity and accuracy using the magnetic technique, and with 91% sensitivity and 96% accuracy using the blue dye technique.

Conclusion: This study demonstrates that ex vivo magnetic SLNM is a feasible technique for use in routine clinical practice, which improves nodal staging accuracy of CRC patients.