Anastasia Janas ^1,2,3 Jakob Jordan ⁴ Gergely Bertalan ⁴ Tom Meyer ⁴ Jan Bukatz ^1,3 Ingolf Sack ⁴ Carolin Senger ³ Melina Nieminen-Kelhä ¹ Susan Brandenburg ¹ Irina Kremenskaia ¹ Kiril Krantchev ^1,3 Sanaria Al-Rubaiey ^1,3 Susanne Mueller ^5,6,7 Stefan P. Koch ^5,6,7 Philipp Boehm-Sturm ^5,6,7 Daniel Zips ³ Peter Vajkoczy ⁸ Güliz Acker ^2,3,8*

• ¹ Clinic for Neurosurgery with Pediatric Neurosurgery, Charité University Medicine Berlin, Berlin, Germany
• ² Berlin Institute of Health, Charité Medical University of Berlin, Berlin, Baden-Württemberg, Germany
• ³ Department of Radiation Oncology and Radiotherapy, Charité University Medicine Berlin, Berlin, Baden-Wurttemberg, Germany
• ⁴ Department of Radiology, Charité University Medicine Berlin, Berlin, Baden-Württemberg, Germany
• ⁵ Clinic for Neurology with Experimental Neurology, Charité University Medicine Berlin, Berlin, Baden-Württemberg, Germany
• ⁶ NeuroCure, Charité University Medicine Berlin, Berlin, Baden-Wurttemberg, Germany
• ⁷ Charité 3R – Replace | Reduce | Refine, Charité-Universitätsmedizin Berlin (Corporate Member of Freie Universität Berlin and Humboldt-Universität zu Berlin), Berlin, Germany
• ⁸ Pediatric Neurosurgery Division, Charité University Medicine Berlin, Berlin, Baden-Württemberg, Germany

Magnetic Resonance Elastography (MRE) allows the non-invasive quantification of tumor biomechanical properties in vivo. With increasing incidence of brain metastases, there is a notable absence of appropriate preclinical models to investigate their biomechanical characteristics. Therefore, the purpose of this work was to assess the biomechanical characteristics of B16 melanoma brain metastases (MBM) and compare it to murine GL261 glioblastoma (GBM) model using multifrequency MRE with tomoelastography post processing. Intracranial B16 MBM (n=5) and GL261 GBM (n=7) mouse models were used. Magnetic Resonance Imaging (MRI) was performed at set intervals after tumor implantation: 5, 7, 12, 14 days for MBM and 13 and 22 days for GBM. The investigations were performed using a 7T preclinical MRI with 20 mm head coil. The protocol consisted of single-shot spin-echo-planar multifrequency MRE with tomoelastography post processing contrast-enhanced T1and T2-weighted imaging and diffusion-weighted imaging (DWI) with quantification of apparent diffusion coefficient of water (ADC). Elastography quantified shear wave speed (SWS), magnitude of complex MR signal (T2/T2*) and loss angle (φ). Immunohistological investigations were performed to assess vascularization, blood-brain-barrier integrity and extent of glucosaminoglucan coverage.Volumetric analyses displayed rapid growth of both tumor entities and softer tissue properties than healthy brain (healthy: 5.17 ± 0.48, MBM: 3.83 ± 0.55, GBM: 3.7 ± 0.23, [m/s]). SWS of MBM remained unchanged throughout tumor progression with decreased T2/T2* intensity and increased ADC on days 12 and 14 (p<0.0001 for both). Conversely, GBM presented reduced φ values on day 22 (p=0.0237), with no significant alterations in ADC. Histological analysis revealed substantial vascularization and elevated glycosaminoglycan content in both tumor types compared to healthy contralateral brain. In summary, our results indicate that while both MBM and GBM exhibited softer properties compared to healthy brain, imaging and histological analysis revealed different underlying microstructural causes (hemorrhages in MBM, increased vascularization and glycosaminoglycan content in GBM, further corroborated by DWI and T2/T2* contrast). These results underscore the complementary nature of MRE and its potential to enhance our understanding of tumor characteristics when used alongside established techniques. This comprehensive approach could lead to improved clinical outcomes and a deeper understanding of brain tumor pathophysiology.