ESTRO 2025 - Abstract Book

S4076

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

1918

Digital Poster Analysis of patient positioning during online adaptive treatment of prostate cancer

Kata Könyves 1 , András Herein 1 , Ádám Gáldi 1,2 , Domonkos Szegedi 1,3 , Zoltán Nagy Takácsi 1,4 , Péter Ágoston 1,4 1 Radiotherapy, National Institute of Oncology, Budapest, Hungary. 2 Doctoral College, Semmelweis University, Budapest, Hungary. 3 Doctoral School of Physical Sciences, University for Technology and Economics, Budapest, Hungary. 4 Department of Oncology, Semmelweis University, Budapest, Hungary Purpose/Objective: Our study aimed to examine whether there is a correlation between the patients’ time lying on the treatment couch and their intra-adaptation position changes during online adaptive radiotherapy (oART) treatment of prostate cancer. We also analysed the adaptation time during our learning curve. Material/Methods: 145 fractions of 29 patients with prostate cancer were evaluated, who received oART on Varian Ethos linear accelerator between December 2023 and October 2024. During oART, two CBCT scans are performed per fraction. The first scan is necessary to assess the patients’ daily anatomy and set up position, to contour the treatment volumes accordingly. During the contouring, some anatomy and position changes might happen. Therefore, a second CBCT scan is required to gather information about the changes, to compare it to the first CBCT in order to have an image guidance for intra-adaptation changes. The vectorial displacement and the time elapsed between the two CBCTs were collected from the Ethos treatment planning system. Results: On average, 13.17 minutes (range: 8.8-19.4 min., SD: 3.69 min.) elapsed between the two CBCT scans (Figure 1.), and the length of the displacement was 0.16 cm (between 0.02 cm and 1.77 cm, SD: 0.15 cm ). There was no correlation between the elapsed time and the length of the displacement (Figure 2.). During the learning curve the change in intra-adaptation time was only minimal, with a slightly decreasing trend.

Figure 1. The time elapsed between the first and the second cone beam CT during online adaptive radiotherapy of prostate cancer. Orange dots represent average time for each patients. Dotted line shows a slight trend in intra adaptation time.