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

## TL;DR

This study shows that MR-Linac improves prostate cancer SBRT by reducing urethral radiation compared to other techniques, without affecting target coverage.

## Contribution

MR-Linac provides significant urethral dose reduction in prostate SBRT compared to VMAT and Tomotherapy.

## Key findings

- MR-Linac reduced urethral maximum and mean doses by approximately 3.3 Gy compared to VMAT and Tomotherapy.
- Rectal and femoral head doses were also lower with MR-Linac, while bladder and penile bulb doses increased slightly but remained acceptable.
- MR-Linac achieved comparable prostate coverage while offering better organ protection.

## Abstract

Prostate cancer is commonly treated with stereotactic body radiotherapy (SBRT), which delivers high radiation doses in a few sessions. While effective, SBRT poses challenges in protecting nearby organs at risk (OARs), such as the urethra, bladder, and rectum, from excessive radiation exposure. This study compared three planning techniques—MRI-guided radiotherapy using MR-Linac, volumetric modulated arc therapy (VMAT), and Tomotherapy—across 30 patients to evaluate differences in target coverage and OAR dose. All plans achieved comparable coverage of the prostate; however, MR-Linac demonstrated a significant reduction in urethral dose, lowering both maximum and mean doses by approximately 3.3 Gy compared to VMAT and Tomotherapy. Rectal and femoral head doses were also reduced with MR-Linac, while minor increases in bladder and penile bulb doses were observed but remained clinically acceptable. These findings highlight the potential of MR-Linac to improve treatment precision and reduce toxicity risk, supporting its integration into prostate SBRT protocols.

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.