Session Item

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.

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. 

On average the time slot needed at the treatment machine was 34 minutes. The process of acquiring a CBCT, evaluating and adjusting the contours, creating the new plan and verifying the CTV on the CBCT scan took on average 20 minutes. Including delivery and post treatment verification this was 26 minutes. The system-generated target volumes needed user adjustments in 50% of fractions. Target volumes on second pretreatment CBCT were all covered with the 95% isodose line. The number of monitor units (MU) of the adapted plan never deviated more than 15% from the number of MU of the reference plan. Patient compliance with respect to bladder filling and total treatment time was excellent.

First clinical experience with CBCT-based online adaptive radiotherapy shows it is feasible and suitable for hypo fractionated rectal cancer treatment.