ESTRO 2020 Abstract book

S444 ESTRO 2020

Conclusion It is feasible to give EBRT boosts with the MRL in case BT boosts are not feasible in cervix cancer patients. With the MRL we achieve higher doses to the target than with CBCT- linacs, but compared to BT we miss the specific high dose volumes and the achieved target doses are lower. We strive to further optimize our treatment planning and fractionation scheme to improve the balance between target and OAR dose. OC-0714 Feasibility of online adaptive MR-guided radiotherapy for rectal cancer on a 1.5T MR-linac. M. Intven 1 , S. De Mol van Otterloo 1 , S. Mook 1 , P. Doornaert 1 , R. Tijssen 1 1 UMC Utrecht, Radiation Oncology Department, Utrecht, The Netherlands Purpose or Objective The aim of this study was to assess the feasibility of an online adaptive workflow on a 1.5T MR-linac for patients with rectal cancer. Shape and volume of the clinical target volume (CTV) for radiotherapy in rectal cancer are influenced by bladder and rectal filling. Online adaptation of these daily variations may enable a reduction of the currently large treatment planning target volume (PTV) margins. This abstract reports on the clinical feasibility of daily online replanning in terms of technical feasibility, target coverage, treatment time, and patient compliance. Material and Methods Patients were treated on a 1.5T MR-linac (Unity, Elekta AB, Stockholm Sweden). At each fraction a 3D T2 weighted (T2w) MR-sequence was acquired on which the CTV was delineated after a deformable registration of the contours from the pre-treatment imaging. PTV margins for these first rectal cancer patients on the MR-linac were, conservatively, based on the standard PTV margins used in our clinic for conventional linac treatments, which were 1 cm isotropically around the mesorectum and 8 mm around the elective nodal regions. Based on the new contours a full online replanning was done after which a new 3D T2w MR-sequence was acquired for position verification purposes. Treatment delivery was done using a 5-field Intensity Modulated Radiotherapy (IMRT) technique during which another two 3D T2w scans were made. Another 3D T2w sequence was made after treatment delivery (figure 1). The duration of each step in the workflow was monitored by one of the RTTs.

Results Twenty-five patients with rectal cancer were treated with 25 Gy in 5 fractions. 18 (72%) of patients was male and medium age was 63.0 (range 43.5-81.3). Most patients had intermediate-risk stage disease located in middle or distal rectum. 118 (94%) of the total 125 planned fractions were delivered on the MR-linac. Two fractions (1.6%) were delivered on a conventional accelerator due to logical reasons and three fractions (2.4%) due to technical problems with the MR-linac. Two fractions (1.6%) were not delivered at all due to an acute infectious disease of the patient. No fractions were aborted, missed or canceled due to intolerance of the patient. Median time per fraction, defined as the duration of time between the patient leaving and re-entering the dressing room, was 48 minutes. Minimal and maximal time per fraction was 32 and 72 minutes. The latter due to a technical failure. Rest of the timings is summarized in table 1. A CTV coverage of >99% was achieved every fraction. In 2.5% of fractions PTV coverage was below the target of 97% and the V26.75 Gy was above 5 cc in 0,8%. However, both violations were accepted after visual inspection of the treatment plan.