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

Authors

Bethany Williams, Saleh Sandoughdaran, Joan Chick, Alex Dunlop, Trina Herbert, Robert Huddart, Adam Mitchell, Simeon Nill, Shaista Hafeez, Helen A McNair

Published in

Clinical and translational radiation oncology. Volume 62. Pages 101321. Epub Oct 10, 2026.

Abstract

Bladder MRI-guided online adaptive radiotherapy (MRIgART) has enabled a reduction in planning target volume (PTV) margins, reducing normal tissue irradiation. Yet, further 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.
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%.
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.
Whole bladder MRIgART with 5 mm isotropic PTV margins with a personalised approach to margin expansion is feasible in achieving acceptable CTV coverage.

PMID:
42926875
Bibliographic data and abstract were imported from PubMed on 25 Oct 2026.

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