Magnetic Staging in Sentinel Lymph Node Procedures: Insights from the LowMag Trial

Accurate identification of metastatic sentinel lymph nodes (SLNs) is essential for cancer prognosis and treatment planning. In the LowMag clinical trial, a magnetic sentinel lymph node biopsy (SLNB) procedure was implemented in patients with invasive breast cancer. This study evaluates the iron content and AC magnetic susceptibility of SLNs containing magnetic tracer to differentiate between metastatic and non-metastatic SLNs. Two magnetic detection devices, the superparamagnetic quantifier (SPaQ) and the differential magnetometer handheld (DMH) probe, were used to measure iron content and magnetic susceptibility of individual SLNs. Additionally, ex vivo low-field MRI and detailed histopathology were conducted. A total of 37 SLNs from 20 consecutive patients, including four metastatic SLNs, were analysed. Iron quantification by low-field MRI and histopathology correlated well with measurements from the DMH probe and SPaQ. A statistically significant difference in iron content was observed between metastatic SLNs (N = 4; DMH: 203.12 ± 87.67 ?g , SPaQ: 131.28 ± 53.38 ?g) and non-metastatic LNs (N = 33; DMH: 92.47 ± 89.8 ?g, SPaQ: 42.45 ± 46.9 ?g). Moreover, the combination of DiffMag counts with two features derived by SPaQ from the AC susceptibility (ACS) curve - Full Width at Half Maximum (FWHM) and peak value - achieved 66.7% sensitivity for detection metastatic SLNs, 93.3% specificity for non-metastasis detection, and overall accuracy of 90.9%. These numbers for the DMH probe were 75.0% sensitivity, 78.8% specificity, and 78.4% accuracy. These findings suggest that the DiffMag method offers more precise detection of non-metastatic SLNs compared to metastatic ones. Notably, in both DiffMag and ACS modes, the SPaQ device outperformed the DMH probe in terms of specificity and overall accuracy, likely due to its specialized design.