Visualisation of magnetic field-induced nanoparticle clusters and mechanical property changes in a breast phantom for inductive moderate hyperthermia

BACKGROUND: Visualising magnetic nanoparticle (MNP) clusters is important for inductive moderate hyperthermia (IMH) as their formation influences mechanical force transduction and heat generation in malignant tumours. An applied inhomogeneous stationary magnetic field (ISMF) can direct the arrangement of MNP clusters and the force exerted on cancer cells. Herein, we analysed MNP cluster formation and changes in mechanical properties using digital breast tomosynthesis (DBT) and ultrasound shear wave elastography (SWE) for a breast phantom containing MCF-7 breast cancer cells under the influence of IMH with ISMF. RESULTS: Texture analysis of the tumour-mimicking region revealed that ISMF induced a 13% increase in fractal dimension and a twofold decrease in lacunarity of MNP clusters on DBT images, as well as a 20% decrease in lacunarity of apparent stiffness within fixed regions of interest on SWE images, as compared with non-targeted MNPs (p < 0.05). While the addition of MNPs increased the maximum temperature in the tumour-mimicking region only by 1.3 °C, it led to a 3.8-fold decrease in lacunarity and a 10% increase in fractal dimension on thermal images, as well as an 87% lower fraction of viable MCF-7 cells than IMH with ISMF (p < 0.05). We also considered the clinical relevance for breast cancer patients, given that the heat-pain threshold (~ 42 °C) is close to the temperatures observed during IMH, whereas the forces generated by ISMF remain below typical pressure-pain thresholds. CONCLUSIONS: ISMF initiated a more uniform spatial distribution of MNP clusters, altering the SWE-derived apparent stiffness and temperature patterns in the tumour-mimicking region. While causing only a moderate temperature increase (< 42 °C), IMH combination with ISMF and MNPs significantly reduced MCF-7 viability, indicating the additional role of magneto-mechanical effects.