ESTRO 2022 - Abstract Book

S1477

Abstract book

ESTRO 2022

Purpose or Objective To assess whether three and single fraction Pancreas SBRT can be delivered within defined PTV coverage targets and organs- at-risk (OAR) constraints on an MR-Linac. Materials and Methods 8 pancreas SABR patients were planned with 39Gy/3# (BED10=90) and 25Gy/1# (BED10=88) with a minimum dose coverage objective of PTV V100% ≥ 60% (CTV=GTV+2mm, PTV=CTV+3mm). OAR constraints were established from national guidelines and published research (see tables). All plans were done on the ViewRay MRIdian® platform (ViewRay®, USA, 2021) using a TRUFI MRI with an accompanying planning CT for electron density information. ~24 IMRT beams were arranged in a pseudo- arc formation avoiding entrance through patients’ arms and couch sides. Beam on time and treatment delivery (beam on time plus time for gantry/MLC mechanical motion) time were noted. The impact of a daily non-adaptive workflow was assessed by rigid registration of the plans on the treatment fraction MRIs. Assessment was done following an IGRT match and with GTV, PTV and OARs re-contoured to determine the predicted dose if plans were delivered without adaptation on each treatment day. Results All plans generated were able to meet the minimum dose coverage objective and OAR constraints (see tables). The median PTV V100 coverage for 39Gy/3# and 25Gy/1# was 75.7% (60.6-91.6%) and 66.1 (60.1-84.2%) respectively. The median treatment delivery times were 15.2min (12.5-21.7min) and 27.8min (21.0-33.2min) for 39Gy/3# and 25Gy/1# respectively. The predicted doses generated from the treatment fraction MRIs showed potential for PTV under-coverage compared to the planned dose with OARs doses exceeding tolerance, therefore daily adaptive recontouring and planning was essential.

Conclusion The study results support proceeding with a Phase 1 trial of three and single fraction Pancreas SBRT as all dose coverage and OAR constraints can be met, as well as all treatments can be delivered in a reasonable timeframe. Given that the pancreas is adjacent to radiosensitive OARs and that there is potential for exceeding dose constraints if the treatment is delivered as originally planned, this supports the use of adaptive planning prior to each treatment fraction to ensure that the treatment is delivered safely. There is on-going work to show the dosimetric impact of a daily online adaptive workflow.

PO-1678 Monitor unit range check as an alternative for secondary dose calculation in online MR-guided RT

D. Schuring 1 , R. Kierkels 1 , G. Hilgers 1 , I. Potters 1 , M. den Hartogh 2 , P. Jeene 2 , K. Surmann 1

1 Radiotherapiegroep, Department of Medical Physics, Arnhem/Deventer, The Netherlands; 2 Radiotherapiegroep, Department of Radiation Oncology, Arnhem/Deventer, The Netherlands Purpose or Objective Currently there are no secondary dose calculations available that can accurately calculate dose in a magnetic field for an Elekta Unity system, the use of an monitor unit (MU)-range check was investigated as an alternative approach for detecting larger errors and outliers in treatment plans. This check is part of a comprehensive QA framework where QA procedures are integrated in the entire MR-linac workflow to have an efficient and effective method for detecting errors. Materials and Methods A treatment procedure was developed for the hypofractioned treatment of prostate cancer patients on a Unity MR-linac (Elekta AB, Stockholm) using 5 fractions of 7.25 or 7.00 Gy. Treatment planning was performed in Monaco 5.40 (Elekta AB, Stockholm) using a class solution with strict guidelines on IMRT parameters and variation of planning objectives. Checks on features essential for the dose calculation and optimization, such as density overrides and adaptation type, were integrated