Dosimetric Evaluation of MR-Guided IMRT and VMAT Techniques for Early-Stage Glottic Cancer: A Pilot Comparative Planning Study

Background and Objectives: Early-stage glottic cancer requires highly precise radiotherapy due to laryngeal motion and the proximity of critical structures. Magnetic resonance-guided radiotherapy (MRgRT) enables real-time visualization and motion management, potentially improving dose conformity while reducing treatment margins. This pilot comparative planning study compared the dosimetric characteristics of MR-guided intensity-modulated radiotherapy (IMRT) delivered on the Unity MR-linac with volumetric modulated arc therapy (VMAT) plans generated for the TrueBeam STx and Halcyon systems. Materials and Methods: Ten patients with T1–T2N0 glottic squamous cell carcinoma were included. For each patient, treatment plans prescribing 42.5 Gy in five fractions were generated using all three systems. Planning target volume (PTV) coverage, conformity index (CI), homogeneity index (HI), organ-at-risk (OAR) doses, monitor units, and beam-on times were compared using Friedman and post hoc Wilcoxon signed-rank tests. Results: Unity achieved superior dose conformity (CI 0.97 ± 0.01 vs. 0.78 ± 0.15 for TrueBeam STx and 0.58 ± 0.19 for Halcyon; p < 0.001) and the highest mean dose, while PTV coverage (D95% and D98%) was comparable to TrueBeam STx and higher than Halcyon. Reduced treatment margins resulted in significantly smaller PTV volumes (11.09 ± 3.87 cm3 vs. 17.43 ± 4.75 cm3; p = 0.0001). Halcyon demonstrated the most homogeneous dose distribution (HI 7.88 ± 3.14), whereas TrueBeam STx showed intermediate homogeneity (HI 12.81 ± 3.90). Most OAR dose metrics were comparable among techniques, although selected laryngeal structures and the esophagus differed significantly. Beam-on time was longest for Unity. Conclusions: MR-guided IMRT provided superior dose conformity while maintaining target coverage comparable to TrueBeam STx with clinically acceptable OAR sparing. Prospective clinical studies are required to determine whether these dosimetric advantages translate into improved patient outcomes.