ESTRO 2024 - Abstract Book

S5536

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

ESTRO 2024

Keywords: Paediatric, cone beam CT, audiovisual distraction

1326

Proffered Paper

Does intra-fraction motion of pelvic lymph nodes depend on localization in online MR-guided RT?

Mylène Noordeloos, Kim Antonissen, Karolin Ezendam, Eva Bongers, Paul Jeene, Danny Schuring

Radiotherapiegroep, Department of Radiation Oncology, Arnhem/Deventer, Netherlands

Purpose/Objective:

Since June 2021 we have been performing SBRT for lymph node oligometastases using online adaptive MR-guided radiotherapy on a 1.5 T MR-linac. This treatment offers superior soft tissue contrast and the possibility for online plan adaptation. This might allow for reduced PTV margins and organ-at-risk (OAR) dose, or even further hypofractionation. The online adaptive workflow however results in an increased time-on-table, which might lead to a larger intrafraction variation compared to conventional treatments. Mobility of these target volumes might also vary depending on their location. The purpose of this study was to quantify the intrafraction motion of lymph node oligometastases during the online treatment procedure and correlate this to the location of the lymph nodes.

Material/Methods:

From June 2021 until September 2023, 47 patients with pelvic lymph node oligometastases were treated on a 1.5 T MR-Linac (Elekta Unity, Crawley, UK) with a prescription dose of 5x7 Gy to the PTV, aiming to escalate the dose in the CTV. Daily MR-guided online adaptive RT was performed using an adapt-to-shape (ATS) workflow in which the CTV is rigidly copied to and translated on the MR acquired just prior to treatment (the session scan), and OARs in the vicinity of the target are adapted. A new treatment plan is then created based on the daily anatomy of the patient. A second MR (PV scan) was then acquired just prior to treatment to verify that the lymph node is still within the PTV. During beam delivery, the position of the lymph node is monitored using a cine-MR sequence with transversal, sagittal and coronal slice through the lymph node. Finally, after delivering the treatment beams, a third MR (post scan) was acquired to verify the position of the CTV at the end of the treatment. On average, the online treatment took 18 minutes from the end of the session MR to the end of beam delivery. Intrafraction motion during the online treatment was determined by registering the post scan on the session MR for each treatment fraction. The resulting shift in the lateral, longitudinal and anterior-posterior direction, as well as the 3D displacement was recorded. Each lymph node was classified to be either mesorectal, paraaortic, parailiac, or an obturator node. Pairwise statistical testing was performed to investigate if the intrafraction motion is significantly different for the different node localizations. Finally, the percentage of fractions with an intrafraction motion exceeding a PTV margin of 1, 2 and 3 mm was determined for the four different node positions.

Results: