ESTRO 2024 - Abstract Book

S4384

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

1 Joint Department of Physics, The Royal Marsden Hospital and The Institute of Cancer Research, Sutton, United Kingdom. 2 Radiation Physics, Department of Hematology, Oncology and Radiation Physics, Skåne University Hospital, Lund, Sweden. 3 Medical Radiation Physics, Department of Translational Medicine, Lund University, Malmö, Sweden. 4 Royal Marsden NHS Foundation Trust, and The Institute of Cancer Research, Sutton, United Kingdom

Purpose/Objective:

With magnetic resonance (MR)-guided radiotherapy, personalization of the treatment is enabled by daily MR-imaging and treatment adaptation. Clinical introduction of advanced motion management strategies, such as gating, have been realised by intra-fraction motion detected on 2D-cine MR-imaging. The technical advancements in motion management in MR-guided radiotherapy pose questions regarding the potential dosimetric gain and how strategies such as multi leaf collimator (MLC)-tracking and gating compare to each other.

This study aimed to investigate the dosimetric impact of using advanced motion management strategies in MR-guided radiotherapy for different patient-specific target motions.

Material/Methods:

The study population consisted of ten prostate cancer patients, included in the HERMES trial (NCT04595019) (ClinicalTrials.gov, Westley et al., 2022), treated using MR-guided radiotherapy on the Elekta Unity MR-linac (Elekta AB, Stockholm, Sweden). Five patients were prescribed 24Gy/2 fractions with intraprostatic boost to 27Gy, delivered in four subfractions, and five patients were prescribed 36.25Gy/5 fractions. Clinical target volume (CTV) to planning target volume (PTV) margin was 3 mm for all patients. Treatment was delivered using intensity modulated radiation therapy with daily online replanning followed by a position correction if required. Patient specific target motion traces in anterior-posterior (AP), left-right (LR) and superior-inferior (AP) were created for each treatment fraction using 2D-cine images acquired during treatment delivery. The produced target motion traces and the daily adapted treatment plan for each treatment fraction were used to reconstruct the delivered dose without motion management with an in-house real-time tracking and adaptation software. The software incorporated a model of the Elekta Unity MLC and was connected to a research version of Monaco treatment planning system (v.6.09.00, Elekta AB, Stockholm, Sweden). Real-time dose reconstruction was performed using an isocenter-shift method and final dose calculation using the GPUMCD-library (Elekta AB, Stockholm, Sweden) (Ahmad et al., 2016, Hissoiny et al., 2011). Reconstructed dose without motion management was compared to the reconstructed dose for treatment delivery simulated with gating and MLC-tracking using the same motion traces. Gating was simulated using a 3 mm gating threshold and 10 second baseline reset, meaning an automatic isocentre shift back to a baseline of zero motion if target motion was outside of the gating window for more than 10 seconds. MLC-tracking was simulated with MLC motion restricted to SI-direction (1D) and without leaf restrictions (3D). Treatment delivery with zero-motion was used as a baseline. The reconstructed fraction dose was multiplied with the number of fractions to enable comparison to clinical dosimetric criteria.

Results: