Session Item

The bladder is a mobile, deformable radiotherapy target.  Online adaptive radiotherapy strategies enable inter-fraction bladder variation to be addressed.  Intra-fraction variation also needs consideration as the bladder volume increases over time.   In this study we assess intra-fraction margin required for an online MR guided adaptive bladder radiotherapy workflow delivered within 20 minutes.  

Patients recruited prospectively to receive whole bladder (CTV) radiotherapy, 55Gy in 20 fractions on the 1.5T MR-linac (Unity Elekta AB, Stockholm, Sweden) system within a single centre clinical research and ethics committee approved study (NCT03727698) were evaluated.  All patients were treated with an empty bladder, with on-line recontouring and optimisation at each fraction (Adapt-to-Shape, ATS workflow) based on daily T2W session MRI.  For treatment the PTV was defined as CTV with an anisotropic expansion of 1.5cm superiorly and anteriorly, 1cm posteriorly, and 0.5cm laterally and inferiorly.   Retrospectively 56 fractions from 7 patients were randomly selected.  CTV coverage was assessed on T2W MRI acquired at approximately 20 minutes (CTV MR_20mins) of the clinical workflow with PTV defined as CTV with isotropic 4mm expansion (PTV4mm) and CTV with isotropic 5mm expansion (PTV5mm).    CTV MR_20mins coverage was assessed i) visually to determine if would have been deemed clinically acceptable for treatment, and ii) by re-calculating the estimated dose delivered on recontoured CTV (Monaco v5.40.01 Elekta AB, Stockholm, Sweden).  Target dose coverage was deemed acceptable if 95% of CTV MR_20mins received >95% of the prescribed dose.

Mean time between daily T2W session MRI and the second T2W MRI on which assessment was made was 19.1 minutes.  PTV4mm encompassed CTV MR_20mins in 22/56 (39%) fractions.  PTV5mm encompassed CTV MR_20mins in 37/56 (66%) fractions.  Of the 19 fractions where PTV5mm did not encompass CTV MR_20mins, D99 median (range) was 52.3Gy (33.0 - 53.4Gy) and CTV MR_20mins  V95% median (range) was 99% (85.3 - 99.9%).   52/56 (93%) fractions met target volume constraints with PTV5mm.   Of the fractions not meeting target volume constraints with PTV5mm, Adapt-to-Position (ATP) workflow i.e., virtual couch shift after ATS would have encompassed CTV MR_20mins in 2/4 fractions.  Both fractions not meeting target constraints occurred in the same patient.

Bladder target coverage can be maintained with PTV margin reduction to 5mm when utilising a full online adaptive bladder radiotherapy workflow delivered in 20 minutes.