ESTRO 2020 Abstract book

S443 ESTRO 2020

OC-0713 MR-linac boosts for patients with cervix cancer ineligible for brachytherapy; preliminary experience J. Hes 1 , A. Van Lier 1 , A. De Leeuw 1 , M. Den Hartogh 1 , P. Kroon 1 , E. De Groot-van Breugel 1 , I. Jürgenliemk-Schulz 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective In patients with cervix cancer elective external beam radiotherapy (EBRT) is preferably combined with brachytherapy (BT) boosts. In some cases BT is not feasible, due to anatomical reasons, comorbidity or other patient related factors. In these situations we currently combine elective EBRT with boosts on a conventional (CBCT) linac allowing for total target doses of about 70Gy. A wide PTV margin for the target is necessary to account for interfraction and intrafraction motion. The MR-linac (MRL) (Unity, Elekta AB) with its superior soft tissue contrast and possibility for online treatment planning according to the changing anatomy is a promising alternative. Here we present our first clinical experience with MRL boosts for the above mentioned situation. Material and Methods Recently 3 patients with cervical cancer, ineligible for BT were treated on the MRL. In our institute receiving 3 MRL boost fractions each. For each fraction relevant target and organ delineations have been adapted according to the daily anatomy as seen on MRI. IMRT-plans with 9 beams (7MV FFF) were generated using Monaco TPS (Elekta AB). The CTV to PTV margin was 3mm in all cases. Target dose above 70Gy EQD2 and OAR dose levels (as usually applied in our department for combined elective EBRT + BT boost treatment planning) were aimed for (Table1). All dose levels include the elective EBRT dose (45Gy/25 fractions). For the (stereotactic) MRL boosts, isotoxic treatment planning was performed, i.e. the target dose to the PTV was restricted by hard OAR constraints. The eventual dosimetric effect of intrafraction motion was evaluated on the anatomy as seen on post treatment MRI for each fraction. Treatment response is measured on MRI and by clinical investigation at 3 months after treatment. Morbidity is scored according to the Common Terminology Criteria for Adverse Events Version 4.0.

Results Table1 shows the total doses (EQD2) achieved for targets and OAR for the 3 patients treated so far. All targets received doses ≥ 70Gy EQD2 and all OAR constraints were met. The target structures were under dosed in all three patients, which could be attributed to the unfavorable location between OARs and target (e.g. patient 3) (Figure 1). After recalculation on the post-treatments scans a small dose deviation is seen on CTV (-1.1, +1.6, -2.4Gy EQD2, resp.) compared to the doses anticipated at daily treatment planning. Patient 1 showed a complete remission after 3 months, Follow-up imaging of the other 2 patients has not been performed yet. No acute grade ≥ 3 toxicity was reported.