Sonolucent Cranial Windows for Bedside Ultrasound Identification of MRI-concordant Glioblastoma Recurrence During Outpatient Surveillance: an Imaging Correlation Pilot Study

Glioblastoma (GBM) is characterized by near-universal recurrence after surgical resection, necessitating close postoperative surveillance. Although MRI remains the gold standard for detecting tumor progression, fixed imaging intervals may delay recognition of recurrence. Sonolucent cranial implants enable transcranioplasty ultrasound (TCUS), offering a potential point-of-care adjunct for interval monitoring between scheduled MRI studies. We evaluated the feasibility of identifying sonographic findings concordant with MRI-confirmed GBM recurrence during routine outpatient follow-up. We conducted a retrospective review of prospectively registered consecutive GBM patients who underwent tumor resection with placement of a sonolucent cranial implant (ClearFit®, Longeviti Inc., Baltimore, MD) at a single tertiary center (February 2023–November 2025). Patients were included if they developed MRI-confirmed recurrence and underwent TCUS within 6 weeks before or after the recurrence-detecting MRI. TCUS examinations were performed during routine outpatient visits and interpreted by a clinician blinded to MRI findings. MRIs were independently reviewed by board-certified neuroradiologists. TCUS findings were qualitatively compared with MRI-defined recurrence. Of 13 patients with sonolucent implants, 8 developed MRI-confirmed recurrence within 13 months of resection, with a range of 1 to 13 months. Four underwent TCUS within six weeks of the recurrence-detecting MRI. In all four cases, TCUS demonstrated hyperechoic abnormalities spatially concordant with MRI-defined recurrence. Examinations were safely performed (n = 4) in the outpatient setting. TCUS through sonolucent cranial implants may demonstrate concordant findings with MRI-confirmed GBM recurrence. This preliminary series supports its spatial correlation to inform future studies that evaluate TCUS as a point-of-care adjunct for interval surveillance. Prospective studies are warranted to define standardized protocols and diagnostic performance.