Dose differences between patients treated with MR-only, CT-only, or MR-CT fusion radiotherapy for prostate cancer

Magnetic Resonance (MR)-only radiotherapy has been clinically implemented but its dose impact has not been assessed in clinical practice. This study evaluated the volume and dose differences between patients treated with CT-only, MR-CT fusion, or MR-only prostate radiotherapy pathways.

Four hundred fifty-four patients from a single centre were treated using MR-only (n = 96), CT-only (n = 248), or MR-CT (n = 110) pathways. Patients were selected for different pathways based solely on geographic location and treatment date. Patients were contoured by the same group of delineators and were planned for 60 Gy in 20 fractions by an automated planning algorithm. Clinical Target Volume (CTV), bladder, penile bulb and rectum volumes, and clinical dose constraints were compared with Kurshkal-Wallis tests, adjusted for multiple testing with a P<.05 significance level.

Median MR-only CTVs were 5 cm3 smaller than CT-only (P=.004). Penile bulb Dmean was 12.0 Gy (CT-only), 9.1 Gy (MR-CT), and 5.9 Gy (MR-only, P<.001), with 79.0%, 90.9%, and 95.8% of patients within constraint. Rectum D2 cm3 was 57.4 Gy (CT-only), 57.6 Gy (MR-CT), and 56.5 Gy (MR-only, P<.001), with 35.1%, 20.9%, and 56.2% of patients within rectum V60 Gy constraint.

The MR-only pathway produced significant reductions of 13% in CTV volume, 51% in penile bulb Dmean, and 2% in rectum D2 cm3 compared to CT-only.

The dose benefit from MR-only has been assessed in clinical practice, demonstrating significant reductions in penile bulb and rectum doses compared to both CT-only and MR-CT pathways. This suggests the MR-only pathway is required to provide the full benefit of MR contouring to reduce toxicities from prostate radiotherapy.