Sodium MRI at 7T for Early Response Evaluation of Intracranial Tumors following Stereotactic Radiotherapy Using the CyberKnife

Changes in the tissue sodium concentration seen on sodium MR imaging can be used to detect radiobiologic alterations in brain tumors at early time points after CyberKnife radiation therapy. BACKGROUND AND PURPOSE: Conventionally, early treatment response to stereotactic radiotherapy in intracranial tumors is often determined by structural MR imaging. Tissue sodium concentration is altered by cellular integrity and energy status in cells. In this study, we aimed to investigate the feasibility of sodium MR imaging at 7T for the preliminary evaluation of radiotherapeutic efficacy for intracranial tumors. MATERIALS AND METHODS: Data were collected from 16 patients (12 men and 4 women, 24?75?years of age) with 22 intracranial tumors who were treated with stereotactic radiation therapy using CyberKnife at our institution between December 1, 2016, and August 15, 2019. Sodium MR imaging was performed at 7T before and 48?hours, 1?week, and 1?month after CyberKnife radiation therapy. Tissue sodium concentration (TSC) was calculated and analyzed based on manually labeled regions of tumors. RESULTS: Ultra-high-field sodium MR imaging clearly showed the intratumoral signal, which is significantly higher than that of normal tissue (t = 5.250, P <.001)., but the edema zone has some influence. The average TSC ratios of tumor to CSF in the 22 tumors, contralateral normal tissues, edema zones, frontal cortex, and frontal white matter were 0.66 (range, 0.23?1.5), 0.30 (range, 0.15?0.43), 0.58 (range, 0.25?1.21), 0.25 (range, 0.17?0.42), and 0.30 (range, 0.19?0.49), respectively. A total of 12 tumors in 8 patients were scanned at 48?hours, 1?week, and 1?month after treatment. The average TSC at 48?hours after treatment was 0.06 higher than that before treatment and began to decrease at 1?week. The TSC ratios of 10 continued to decline and 2 tumors increased at 1 month, respectively. Tumor volume decreased by 2.4%?99% after 3?months. CONCLUSIONS: Changes in the TSC can be quantified by sodium MR imaging at 7T and used to detect radiobiologic alterations in intracranial tumors at early time points after CyberKnife radiation therapy.