ESTRO 2024 - Abstract Book

S4376

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

In addition, continuous monitoring with SGRT during expiration breath-hold abdominal SBRT treatments resulted in a better intrafraction control than using a reflecting block.

Keywords: SGRT, SBRT, EBH

2741

Digital Poster

Validation of 2D shifts for 3D motion in MR-guided radiotherapy of prostate cancer

Tim Everaert, Jacques Bodenstein Bezuidenhout, Jens Van Loon, Benjamin Vanspeybroeck, Adrian Gutierrez Ruiz, Sven Van Laere, Thierry Gevaert, Mark De Ridder

UZ Brussel, Radiotherapy, Brussels, Belgium

Purpose/Objective:

Hypofractionation is increasingly used to treat prostate cancer. In our department, clinically localized prostate cancer is treated in 5 fractions of 7.2 Gy to the PTV and 8 Gy to the CTV together with a simultaneous integrated boost of 8.4 Gy to the GTV. Online adaptive workflow using a magnetic resonance (MR)-Linac system (MRIdian®, Viewray, USA) is performed for every single fraction. The built-in MR system makes it possible to perform intrafraction motion monitoring in the sagittal plane. 2D table shifts, longitudinal and vertical, can be performed to keep the target (gating) structure within its boundary just before and during irradiation. These target shifts can be caused by factors such as bladder filling or pelvic muscle relaxation. The aim of this study is to quantify the 3D motion of the prostate and to validate the 2D table shifts and its impact on target coverage.

Material/Methods:

To quantify the 3D motion of the prostate, a comparison of the CTV between a pre- and post-treatment MR was performed. The pre-treatment MR was obtained from the online adaptive workflow and the post-treatment MR was acquired directly after irradiation. The CTV was contoured on the pre- and post-treatment MR by a physician. The MR’s were registered by matching the origins of the two images. Afterwards, the post-treatment contours were transferred to the pre-treatment scan. 3D CTV motion was quantified by a vector gathered from the subtraction of the contour centroids of the pre- and post-treatment CTV. The target coverage was calculated by projection of the adaptive planned dose on the pre- and post-treatment CTV.

Results:

In total 10 patients were analyzed. Only fractions were a 2D shift was performed were considered. This gave us 39 fractions. In total 37 2D shifts were performed before beam on and 10 during irradiation. 2D shifts were performed if the gating structure moved compared to the predefined boundary after visual inspection. Some patients had multiple shifts per fraction. The performed absolute 2D shifts had an average value of 2.05 mm (±0.99) in vertical