Abstract
Background and purposeDaily online adaptation of the clinical target volume (CTV) using MR-guided radiotherapy enables margin reduction of the planning target volume (PTV). This study describes the implementation and initial experience of MR-guided radiotherapy on the 1.5T MR-linac and evaluates treatment time, patient compliance, and target coverage, including an initial assessment of margin reduction. Materials and methodsPatients were treated on a 1.5T MR-linac (7MV, FFF). At each fraction a 3D T2 weighted (T2w) MR-sequence was acquired on which the CTV was adapted after a deformable registration of the contours from the pre-planning CT scan. Based on the new contours a full online replanning was done after which a new 3D T2w MR-sequence was acquired for position verification. A 5 field Intensity Modulated Radiotherapy (IMRT) plan was delivered. ResultsForty-three patients with rectal cancer were treated with 25 Gy in 5 fractions of which 18 with reduced margins. In total, 204 of 215 fractions were delivered on the MR-linac all of which obtained a clinically acceptable treatment plan. Median in-room time per fraction was 48 min (interquartile range 8). No fractions were canceled or interrupted because of patient intolerance. CTV coverage after margin reduction was good on all post-treatment scans but one due to passing gas. ConclusionMR-guided radiotherapy using daily full online recontouring and replanning on a 1.5T MR-linac for rectal cancer is feasible and currently takes about 48 min per fraction.
Highlights
Background and purposeDaily online adaptation of the clinical target volume (CTV) using MR-guided radiotherapy enables margin reduction of the planning target volume (PTV)
As this study aims to estimate the clinical feasibility of MR-guided radiotherapy (MRgRT) in the general rectal cancer population, it was decided that all patients referred for short course radiotherapy, i.e. 5 times 5 Gy in accordance with the Dutch guidelines, were eligible for treatment on a 1.5T MR-linac (Elekta Unity, Elekta AB, Stockholm, Sweden)
Forty-three rectal cancer patients were selected for treatment on the MR-linac between November 2018 and March 2020
Summary
Online adaptation of the clinical target volume (CTV) using MR-guided radiotherapy enables margin reduction of the planning target volume (PTV). This study describes the implementation and initial experience of MR-guided radiotherapy on the 1.5T MR-linac and evaluates treatment time, patient compliance, and target coverage, including an initial assessment of margin reduction. At each fraction a 3D T2 weighted (T2w) MR-sequence was acquired on which the CTV was adapted after a deformable registration of the contours from the pre-planning CT scan. Results: Forty-three patients with rectal cancer were treated with 25 Gy in 5 fractions of which 18 with reduced margins. Conclusion: MR-guided radiotherapy using daily full online recontouring and replanning on a 1.5T MRlinac for rectal cancer is feasible and currently takes about 48 min per fraction.
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