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Fast Volumetric MRI On An MRI-Linac Enables On-Line QA On Dose Deposition in the Patient

S Crijns

S Crijns1*, M Glitzner1, B Denis de Senneville1,2, C Kontaxis1, M Maenhout1, G Bol1, J Lagendijk1, B Raaymakers1, (1) University Medical Center Utrecht, Utrecht, The Netherlands, (2) Mathematical Institute of Bordeaux, University of Bordeaux, Talence Cedex, France


TU-G-CAMPUS-J-5 (Tuesday, July 14, 2015) 4:30 PM - 5:00 PM Room: Exhibit Hall

Purpose: The introduction of the MRI-linac in radiotherapy brings MRI-guided treatment with daily plan adaptions within reach. This paradigm demands on-line QA. With its ability to perform continuous volumetric imaging in an outstanding soft-tissue contrast, the MRI-linac promises to elucidate the dose deposition process during a treatment session. Here we study for a prostate case how dynamic MRI combined with linac machine parameters and a fast dose-engine can be used for on-line dose accumulation.

Methods: Prostate imaging was performed in healthy volunteer on a 1.5T MR-scanner (Philips, Best, NL) according to a clinical MR-sim protocol, followed by 10min of dynamic imaging (FLASH, 4s/volume, FOV 40x40x12cm³, voxels 3x3x3mm³, TR/TE/α=3.5ms/1.7ms/5°). An experienced radiation oncologist made delineations, considering the prostate CTV. Planning was performed on a two-compartment pseudoCT (air/water density) according to clinical constraints (77Gy in PTV) using a Monte-Carlo (MC) based TPS that accounts for magnetic fields. Delivery of one fraction (2.2Gy) was simulated on an emulator for the Axesse linac (Elekta, Stockholm, SE). Machine parameters (MLC settings, gantry angle, dose rate, etc.) were recorded at 25Hz. These were re-grouped per dynamic volume and fed into the MC-engine to calculate a dose delivered for each of the dynamics. Deformations derived from non-rigid registration of each dynamic against the first allowed dose accumulation on a common reference grid.

Results: The DVH parameters on the PTV compared to the optimized plan showed little changes. Local deformations however resulted in local deviations, primarily around the air/rectum interface. This clearly indicates the potential of intra-fraction adaptations based on the accumulated dose. Application in each fraction helps to track the influence of plan adaptations to the eventual dose distribution. Calculation times were about twice the delivery time.

Conclusion: The current result paves the way to perform on-line treatment delivery QA on the MRI-linac in the near future.

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