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

Abstract

IntroductionBladder 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.MethodsPatients voided their bladders prior to treatment. Following setup, a daily MRI (MRDAILY) was acquired, and an adaptive workflow was performed. An MRI was acquired during delivery to coincide with treatment completion (MRPOST). Time from MRDAILY to MRPOST was recorded. Offline, target coverage of the clinical target volume (CTV) was visually monitored by comparing MRDAILY and MRPOST images. Images were reviewed by the radiation oncologist (RO) if the CTV was outside of the PTV on the MRPOST. 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, MRPOST 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%.ResultsA 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 MRPOST 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.ConclusionWhole bladder MRIgART with 5 mm isotropic PTV margins with a personalised approach to margin expansion is feasible in achieving acceptable CTV coverage.