Tehran University of Medical Sciences Frontiers in Biomedical Technologies 2345-5837 0 0 2026 09 26 1243 1243 Sasan Tizro (1MSc in Radiobiology and Radiation Protection, Department of Medical Physics, Babol University of Medical Science ,Babol,Iran. ) (2 Radiology Center, Salamat Hospital, guilan University of Medical Science, Guilan,,Iran.) Ali Shabestani Monfared Cancer Research Center, Health Research Institute, Babol University of Medical Sciences, Babol, Iran Danial Fazilat-Panah Cancer Research Center, Babol University of Medical Sciences, Babol, Iran Fatemeh Zahra Nosrati Cancer Research Center, Babol University of Medical Sciences, Babol, Iran Davoud Jahansouz Non-Communicable Disease Institute, Mazandaran University of Medical Sciences, Mazandaran, Iran 2025 03 17 2025 09 24 Purpose: Accurate target delineation is essential for radiotherapy planning in glioblastoma. While CT provides the electron density information required for dose calculation, MRI offers superior soft-tissue contrast and may improve tumor delineation. Materials and Methods: Computed Tomography (CT) is routinely used for radiotherapy treatment planning; however, Magnetic Resonance Imaging (MRI) provides superior soft-tissue contrast. This study evaluated the improvement of treatment planning in glioblastoma patients by fusing MRI and CT images, with a focus on changes in Clinical Target Volume (CTV) and radiation dose to the eye region. Thirty patients with histologically confirmed glioblastoma underwent both CT and MRI scans. Two plans were generated per patient: MRI–CT fused (dual-modality) and CT-only (single-modality). Demographics, tumor location, CTV, and eye dosimetry (maximum dose, mean dose, D0.03cc, dose–volume histogram metrics) were collected. CT and MRI protocols, registration, and fusion procedures were performed. Statistical comparisons the Wilcoxon signed-rank test, with significance set at P ≤ 0.05 and 95% Confidence Intervals (CI). Results: Image fusion significantly reduced CTV in both phases (initial: 443.45±190.64 vs 200.19±133.67 cm³; mean difference −243.27 cm³ [95% CI −316.82, −169.71], P<0.001; boost: 265.72±140.72 vs 84.28±57.48 cm³; mean difference −181.44 cm³ [95% CI −232.92, −129.96], P<0.001). Eye doses were consistently lower with fusion: left eye Dmean (initial: −225.06 cGy, P=0.006; boost: −68.05 cGy, P=0.007), left eye Dmax (initial: −620.44 cGy, P=0.010; boost: −142.14 cGy, P=0.015), right eye Dmean (initial: −268.73 cGy, P=0.006; boost: −89.08 cGy, P=0.013), right eye Dmax (initial: −662.06 cGy, P=0.004; boost: −135.52 cGy, P=0.024). Across endpoints, 70–93% of cases favored CT–MRI plans. Conclusion: In 3D-CRT for glioblastoma, CT–MRI fusion improves anatomical specificity, enabling smaller CTVs and meaningfully lower ocular doses. These findings support routine MRI fusion for safer, more conformal planning. MRI–CT fusion enhances target delineation in glioblastoma but may increase eye dose; careful organ-at-risk sparing is warranted. https://fbt.tums.ac.ir/index.php/fbt/article/view/1243 https://fbt.tums.ac.ir/index.php/fbt/article/download/1243/583