Daily dosimetric variation between image-guided volumetric modulated arc radiotherapy and MR-guided daily adaptive radiotherapy for prostate cancer stereotactic body radiotherapy

Aim To evaluate differences between MR-guided daily-adaptive RT (MRgRT) and image-guided RT (IGRT) with or without fiducial markers in prostate cancer (PCa) stereotactic body radiotherapy (SBRT) in terms of dose distribution on critical structures. Material and methods Two hundred treatment sessions in 40 patients affected by low and intermediate PCa were evaluated. The prescribed dose was 35 Gy in 5 fractions delivered on alternate days. MRgRT patients (10) were daily recontoured, re-planned, and treated with IMRT technique. IGRT patients without (20) and with (10) fiducials were matched on soft tissues or fiducials and treated with VMAT technique. Respective CBCTs were retrospectively delineated and the prescribed plan was overlaid for dosimetric analysis. The daily dose for rectum, bladder, and prostate was registered. Results MRgRT resulted in a significantly lower rate of constraints violation as compared to IGRT without fiducials, especially for rectum V28Gy, rectum V32Gy, rectum V35Gy, rectum Dmax, and bladder Dmax. IGRT with fiducials reported high accuracy levels, comparable to MRgRT. MRgRT and IGRT with fiducials reported no significant prostate CTV underdosage, while IGRT without fiducials was associated with occasional cases of prostate CTV under dosage. Conclusion MR-guided daily-adaptive SBRT seems a feasible and accurate strategy for treating prostate cancer with ablative doses. IGRT with the use of fiducials provides a comparable level of accuracy and acceptable real-dose distribution over treatment fractions. Future study will provide additional data regarding the tolerability and the clinical outcome of this new technological approach.