Preclinical feasibility and validation of MRI thermometry dedicated to interstitial photodynamic therapy of glioblastoma

PURPOSE: Interstitial photodynamic therapy (iPDT) is an emerging loco-regional treatment for glioblastoma based on intracerebral illumination delivered through stereotactically implanted optical fibers. Despite encouraging oncological results, treatment-related morbidity remains significant, and the contribution of unintended light-induced thermal effects has never been experimentally investigated. This study aimed to preclinically characterize temperature changes during illumination using MRI-based thermometry. METHODS: Fifty-seven minutes of illumination were performed in ex vivo calf brain model at room temperature baseline. The temperature was monitored using the proton resonance frequency shift (PRFS) method on a 1.5 T MRI system. Thermometric measurements were first validated against fiber-optic temperature probes during single diffusing fiber illumination. Spatio-temporal thermal distributions were then evaluated for three diffusing fiber configurations with inter-fiber spacings of 2.0, 1.0, and 0.6 cm. Macroscopic assessment of thermal tissue damage was performed after illumination. RESULTS: MRI thermometry showed excellent agreement with fiber optic temperature probes. Cumulative heating strongly depended on inter-fiber spacing. At the barycenter of the diffusing part of the three fibers, the mean ± SD final temperature estimated was 43.7 ± 0.7°C at 2.0 cm spacing, 46.6 ± 5.9°C at 1.0 cm spacing, and 53.5 ± 10.2°C at 0.6 cm spacing. No macroscopic thermal tissue damage was observed. CONCLUSION: MRI thermometry provides a robust and translatable tool to quantify thermal effects during iPDT. Diffusing fiber spacing critically influences temperature distribution, supporting the use of thermometric monitoring to enhance procedural safety. Future clinical thermometric data could be implemented into treatments planning systems to refine fiber placement and enable patient-specific strategies aimed at maximizing therapeutic efficacy while minimizing the risk of thermal injury.