ESTRO 2025 - Abstract Book

S772

Clinical - Gynaecology

ESTRO 2025

Conclusion: Normalization to point A with graphical optimization provides the best coverage with adequate normal tissue sparing in brachytherapy applications with altered geometry.

Keywords: Cervix, brachytherapy, optmisation

208

Digital Poster Online adaptive radiotherapy for postoperative patients with endometrial cancer. Liselotte A ten Asbroek- Zwolsman, Lisanne GM Zwart, Hans Ligtenberg, Judith J Dasselaar Radiotherapy, Medisch Spectrum Twente, Enschede, Netherlands

Purpose/Objective: For patients with advanced stage endometrial cancer, external beam radiotherapy may be recommended as adjuvant treatment. The target areas for this postoperative treatment include CTV-t (parametrial tissues, surgical area, and the top of vagina) and CTV-n (elective lymph nodes) based on FIGO staging for endometrial cancer. Due to anatomical variations, such as rectum and bladder filling, significant daily variation in the target areas is possible. To correct for these variations, CTV-PTV margins are used, resulting in a high dose to the surrounding organs at risk (OARs). Online adaptive radiotherapy (OART) can be used to correct for these daily variations. The aim of this study is to determine the feasibility of OART in postoperative patients with endometrial cancer. In addition, we examined whether OART has added value and offers dosimetric benefits in comparison with image-guided radiation therapy (IGRT). Material/Methods: Twenty patients who received postoperative radiotherapy for endometrial cancer between January 2022 and December 2023 were included in this retrospective study. For each patient, the OART workflow was simulated in the emulator for five weekly CBCTs. For CTV-t, a CTV-PTV margin of 1.5 cm in all directions was used, except for a CTV PTV margin of 1.0 cm in caudal direction. For CTV-n, a CTV-PTV margin of 0.5 cm was used. The duration of the OART workflow and the number of manual adjustments of the propagated CTV were measured. In addition, the doses in the PTV and OARs using the OART workflow were compared with the IGRT workflow. Results: One hundred OART fractions were simulated. CTVs were manually adjusted in 22 of the 100 fractions. The delivered PTV dose increased for each fraction for the adaptive workflow (mean 99.6% (SD 0.9%)) compared with the IGRT workflow (mean 96.4% (SD 0.2%)) (figure 1). A significant dose reduction of 4.2% was seen for bladder (P < 0.01). Bowel bag and rectum dose were not significantly different (P 0.7 and 0.09) between both plans (figure 2). On average, OART workflow took 8 minutes (SD 39 seconds), which was on average 3 minutes and 30 seconds longer compared with the IGRT workflow.