MR-Linac-Based SBRT for Prostate Cancer: Dosimetric Benefits for Urethral Sparing Compared to VMAT and Tomotherapy

Background: Stereotactic body radiotherapy (SBRT) for prostate cancer delivers high doses in few fractions but poses challenges in sparing adjacent organs at risk (OARs), particularly the intra-prostate urethra, bladder, rectum and penile bulb. Magnetic resonance-guided radiotherapy (MRgRT) using MR-Linac offers superior soft-tissue visualization and daily adaptive planning, potentially reducing OAR dose while maintaining target coverage. This study aimed to compare dose-volume parameters among MR-Linac (ML), volumetric modulated arc therapy (VMAT), and Tomotherapy (HT) plans for prostate SBRT. Methods: Thirty patients with localized prostate cancer were retrospectively analyzed. For each patient, three plans were generated: ML, VMAT and HT, using identical prescription and planning objectives. Dose-volume histogram (DVH) metrics were evaluated for clinical target volume (CTV), planning target volume (PTV), and OARs. Statistical comparisons were performed using non-parametric Friedman's Test with post hoc Bonferroni test, with significance set at a p < 0.05. Results: CTV coverage was comparable across all modalities. ML achieved significantly higher PTV Dmin and near-maximum doses compared to VMAT and HT. Notably, ML provided substantial urethral sparing, reducing Dmax and Dmean by approximately 3.3 Gy compared to both VMAT and HT (p < 0.001). Rectal dose metrics were also lower with ML, while bladder and penile bulb doses showed minor increases (<3.5 Gy), considered clinically negligible. Femoral head doses were reduced in ML plans. Conclusions: MR-Linac planning for prostate SBRT offers meaningful dosimetric advantages, particularly in intra-prostate urethra urethral dose reduction, without compromising target coverage. These findings support incorporating MR-guided adaptive workflows into SBRT protocols to enhance OAR protection and potentially reduce treatment-related toxicity.