ESTRO 2023 - Abstract Book

S1615

Digital Posters

ESTRO 2023

Figure 1 - Computed differences (mm and °) between positioning errors detected by stereoscopic x-rays and surface tracking. 173 verifications included. X, y, z correspond to lateral, longitudinal and vertical and rot-x, rot-y and rot-z to pitch, roll and yaw.

Conclusion Surface guidance on intracranial treatments can help reduce the dose related to setup imaging and can identify patient motion during irradiation before verification imaging is triggered, even with immobilization devices in place. X-ray imaging should remain as a means to accurately verify and correct patient position.

PO-1872 Assessment of setup errors and internal motion for rectal cancer using MR-guided radiotherapy

T. Everaert 1 , A. Gutierrez Ruiz 1 , R. Atef Kamel 1 , J. Bodenst Bezuidenhout 1 , J. Smeulders 1 , T. Gevaert 1 , M. De Ridder 1

1 UZ Brussel, Radiotherapy, Brussels, Belgium

Purpose or Objective Short-term preoperative radiotherapy with a simultaneous integrated boost on the tumor with or without concomitant chemotherapy is accepted as standard of care for T3-T4 rectum treatment before total mesorectal excision. The use of MR guided radiotherapy offers better soft tissue visualization, real-time intrafraction motion monitoring and online adaptive planning when needed, potentially leading to the reduction of treatment margins. The aim of this work was to assess the setup errors and internal motion of patients treated for rectal cancer with MR-guided radiotherapy. Materials and Methods Ten patients underwent treatment on the MRIdian® (Viewray) for rectum carcinoma in five fractions of 5 Gy with a simultaneous boost of 6 Gy per fraction on the GTV. In total 44 fractions were analyzed. For every fraction, an extra post treatment MR sequence (TrueFISP) was acquired. Intrafraction patient movement was analyzed via a rigid fusion of pre and post-treatment MR. To calculate internal motion the movement of the mesorectum and GTV was analyzed over the course of the treatment. Results Rigid fusion analysis of the pre- and post-treatment scans shows a mean of the absolute values of 0.94 mm ± 0.98 SD in lateral, 1.26 mm ± 1.31 SD in vertical and 0.75 mm ± 0.62 SD in longitudinal direction. No rotational movement bigger than 1.68° was found. This suggests that patient position during treatment is stable. The internal movements of the mesorectum and GTV comparing simulation to post-treatment MR and pre- to post-treatment MR are shown in table 1. Intrafractional internal motion of the mesorectum and GTV was found consistently lower when compared to interfractional motion (simulation vs post-treatment scan).

Conclusion