Magnetic procedure for sentinel lymph node detection and evaluation of metastases: Design and rationale of the Lowmag trial

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Abstract Background Primary tumour biology and axillary lymph node status are crucial prognostic factors in breast cancer treatment. The LowMag trial introduced a magnetic sentinel lymph node biopsy (SLNB) procedure using a Superparamagnetic Iron Oxide (SPIO) tracer and handheld magnetometer as a radiation-free alternative for axillary staging. The objective of LowMag trial was to assess a low-dose magnetic tracer for non-invasive preoperative evaluation of lymph node metastases and intraoperative SLN detection. Methods Patients with confirmed invasive breast cancer or ductal carcinoma, eligible for SLNB, were included in the LowMag trial (NTR 4858, http://www.trialregister.nl) consisted of radioactive and magnetic SLNB. The SLNs were perioperatively detected by Sentimag®, and inked to keep track of spatial orientation between MRI and histopathology. Amount of iron was estimated post-surgery using two magnetic devices: SPaQ and Sentimag®. Additional ex vivo MRI was acquired using low-field MRI system (Pure Devices). After buffered in formalin fixation, the LN were lamellated perpendicular to the MRI planes, consecutively sectioned at 2 µm distance and stained with H&E, Perls Prussian blue, CK8/18 and CD68. Results In an interim assessment of 20 LNs (nine patients) two LNs from two patients contained a metastasis. The analysis reviled good uptake concordance between magnetic and radioactive tracer with median amount of iron trapped was 19.21 µg (ranging from 0.1 pg to 109 µg). Within healthy LNs, iron particles were observed in both the subcapsular space and sinusoids, with macrophages located in their proximity. Healthy regions within metastatic LNs exhibit a behaviour similar to that of the healthy LNs. In the metastatic sites, there was a reduced presence of iron pigment, particularly in the areas occupied by tumour cells. The healthy LN with low iron content shows a large fatty region in the centre of the node without iron uptake, and confirmed iron in the sinusoidal macrophages. The metastatic LN shows subcapsular and parenchymal tumour cells located in the central region, with limited infiltration into the nearby adipose tissue, without presence of local iron enhancement. However, iron deposits are were detected within the sinusoids. Conclusion The MRI images clearly differentiate between fat, nodal tissue and SPIO tracer (either in signal intensity or in texture), demonstrating a potential of LN imaging using a low-field MRI system.

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