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Initial Investigation of Four-Dimensional (4D) Dose Calculation Based On 4D-MRI

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D Du

D Du*, F Han , Z Zhou , J Lamb , J Neylon , Y Yang , P Hu , P Lee , A Raldow , D Low , M Cao , UCLA School of Medicine, Los Angeles, CA


TU-C3-GePD-J(A)-5 (Tuesday, August 1, 2017) 10:30 AM - 11:00 AM Room: Joint Imaging-Therapy ePoster Lounge - A

Purpose: Four-dimensional (4D) dose calculation based on 4DCT has limited application to abdominal tumors due to poor soft-tissue contrast of 4DCT. The purpose of this pilot study was to investigate the feasibility of 4D dose calculation using 4DMRI images to estimate the delivered dose on abdominal target and organs at risk (OARs) under influence of respiratory-induced motion.

Methods: Three patients with abdominal tumors treated with SBRT were scanned using a novel self-gated 4DMRI sequence on an integrated MRI guided radiotherapy machine. The temporal probability of each phase was derived from the respiratory signal generated from 4DMRI. ITV was generated based on GTV manually contoured on every phase of the 4DMRI and used for initial treatment planning with electron densities obtained from simulation CT. The initial plans were then re-calculated on each phase MRI and the resultant dose was weighted with respect to the phase temporal probability and accumulated on the planning MRI by deformable image registration. The total composite 4D doses were then compared with those of the initial plans.

Results: Large superior-inferior motion of target (ranged 1.1-1.7cm) was observed from 4DMRI. Discrepancies in target coverage were observed between initial static and 4D doses with Dmin for GTV increased by 13% for the 4D dose in one of the patients which correlated with the observed respiratory temporal pattern. Changes in dose to OARs were also observed although clinically insignificant.

Conclusion: The feasibility of implementing 4D dose calculation with 4DMRI was demonstrated and the initial results showed that respiratory motion may impact target or OARs dose and warranted a future study with a large group of sample size.

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