ESTRO 2021 Abstract Book

S482

ESTRO 2021

OC-0618 Feasibility CBCT-based online adaptive 5x5Gy radiotherapy for neoadjuvant rectal cancer treatment.

M. Frank 1 , R. de Jong 1 , J. Visser 1 , N. van Wieringen 1 , J. Wiersma 1 , D. Geijsen 1 , A. Bel 1 1 Amsterdam UMC location AMC, Radiation Oncology, Amsterdam, The Netherlands

Purpose or Objective Online adaptive radiotherapy has the potential to reduce toxicity for patients because it enables margin reduction around the clinical target volume (CTV). This study describes and evaluates for the first time a Conebeam CT (CBCT)-based online adaptive workflow for rectal cancer in the neo adjuvant setting with respect to timing of different steps in the process, plan quality, target coverage and patient compliance. Materials and Methods Fifteen consecutive 5 x 5Gy rectal cancer patients were treated on a ring-based linear accelerator (Ethos, Varian, Palo Alto, USA). Relatively small margins of 5 mm were applied to the CTV (consisting of mesorectum, pre sacral space and elective lymph nodes) in AP and LR direction. For the cranial and caudal borders a margin of 8 mm in CC direction was applied (Figure 1). The Ethos operates as an integrated platform for both treatment planning and delivery. A reference plan was generated based on a single planning CT. After setting up the patient the adaptive procedure started with the acquisition of a CBCT. The reference CT scan was registered to the CBCT using deformable registration creating a synthetic CT scan. With the support of artificial intelligence and structure guided deformation contours were adapted by the system to match the anatomy on the CBCT (Figure 1). If necessary, these contours were adjusted. Subsequently a new plan was generated on the synthetic CT scan. A second and third CBCT were acquired to check target coverage of this new plan just before and after treatment delivery, respectively. Treatment was delivered using volumetric modulated arc treatment (VMAT). For this introduction phase, a team consisting of 2 RTTs, a physician and a physicist was present at the linear accelerator for each procedure. RTTs were in charge of running the system. All steps in this process were timed.

Results On average the time slot needed at the treatment machine was 34 minutes. The process of acquiring a CBCT,