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Fast Online Daily Replanning for Rotational Correction in Prostate Radiotherapy

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C Kontaxis

C Kontaxis*, G Bol, L Kerkmeijer, J Lagendijk, B Raaymakers, University Medical Center Utrecht, Utrecht, The Netherlands


SU-I-GPD-J-63 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To perform fast online replanning in prostate radiotherapy in order to account for both daily rotations and displacements captured by prostate fiducial markers and investigate the possibility of margin reduction via this regime.

Methods: We simulated a 35-fraction prostate adaptive treatment for 5 subjects based on fiducial marker data available in our clinic. The online rigid rotations and displacements captured by EPID were used to generate the daily anatomy. Our fast online replanning platform MR-Linac Planning System (MRLTP) was utilized to perform daily replanning based on both recorded rotations and translations. The treatment was simulated for various PTV margins, 0 to 8 (clinical) mm in 2 mm increments, in order to assess the replanning pipeline and establish the feasibility of margin reduction. The adaptive regime was compared to a conventional static plan delivered to the daily anatomies which only included the daily rotations.

Results: A new prostate MRLTP plan can be generated in less than 2 minutes. Daily replanning generated very similar dose distributions to the conventional plan (average CTV D99 relative difference: -0.2%) and maintained the desired target coverage among all margins while the conventional plan was increasingly affected (average CTV D99 decrease: 0.5 to 3.1 Gy between 6 and 0 mm). Replanning produced tighter dose distributions resulting in OAR sparing in the high dose region, with Bladder and Rectum V72 being decreased by 7% and 30% respectively on average.

Conclusion: We demonstrate that online replanning for daily rotational compensation based on rigid marker transformations is feasible and can be utilized to decrease the PTV margin, limited mainly by the anatomical deformations and intrafraction motion. We are now working towards fast MR-guided daily replanning for prostate based on 3D deformation fields that will allow safe clinical margin reduction and accurate dose evaluation of the complete patient anatomy.

Funding Support, Disclosures, and Conflict of Interest: This research is financially supported by Elekta AB, Stockholm, Sweden.

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