Work

Feasibility of reduced PTV margins in magnetic resonance image-guided online adaptive radiotherapy for bladder cancer: a personalised approach

Year: 2026

Type: article

Abstract: Introduction Bladder MRI-guided online adaptive radiotherapy (MRIgART) has enabled a reduction in planning target volume (PTV) margins, reducing normal tissue irradiation. Yet, margin reduction is limited due to workflow duration and intra-fractional bladder variation. This study reports the feasibility of a 5 mm isotropic PTV in whole bladder MRIgART, 55 Gy in 20 fractions daily, and 36 Gy in 6 fractions weekly, with a personalised approach to margin expansion. Methods Patients voided their bladders prior to treatment. Following setup, a daily MRI (MR DAILY ) was acquired, and an adaptive workflow was performed. An MRI was acquired during delivery to coincide with treatment completion (MR POST ). Time from MR DAILY to MR POST was recorded. Offline, target coverage of the clinical target volume (CTV) was visually monitored by comparing MR DAILY and MR POST images. Images were reviewed by the radiation oncologist (RO) if the CTV was outside of the PTV on the MR POST . Under RO discretion, margins were personalised to the minimum margin that would have encompassed the CTV and applied online for subsequent fractions. In retrospective analysis, MR POST CTVs were recontoured, and the adaptive plan recalculated. CTV coverage was deemed acceptable if V95% ≥ 95%. If this objective was not met, the gross tumour volume (GTV) was delineated and the dose reported; coverage was deemed acceptable if GTV V95% ≥ 95%. Results A total of 138 fractions from ten patients were analysed. Median (interquartile range) workflow time was 25.6 (20.8–30.7) minutes. PTV margins were personalised for 31 fractions in three patients. 119 MR POST images were available for analysis. CTV coverage was achieved in 94% (112/119) of fractions. Where the CTV mandatory objectives were missed (6%; 7/119 fractions), GTV coverage was acceptable in 6 of the 7 fractions. Conclusion Whole bladder MRIgART with 5 mm isotropic PTV margins with a personalised approach to margin expansion is feasible in achieving acceptable CTV coverage.

Source: Clinical and Translational Radiation Oncology

DOI: 10.1016/j.ctro.2026.101321