ESTRO 2024 - Abstract Book

S4003

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

toxicity such as nausea, vomiting, and anorexia, due to higher doses in the organs at risk (OARs). To address this, various techniques have been developed to manage respiratory motion during pancreatic cancer treatment, such as expiration gating (EXP GAT). In EXP GAT, the tumor is only irradiated during the expiration phases of the breathing (mostly phase 30-70). In 2021, we implemented this gating technique for patients with borderline resectable pancreatic cancer. This study investigated the impact of EXP GAT and FB on target volume coverage and dose to surrounding OARs during treatment and compared EXP GAT with FB.

Material/Methods:

In this study, we included 15 patients with borderline resectable pancreatic cancer, treated with neoadjuvant chemoradiotherapy between November 2021 and July 2022. Before the 4D planningCT, all patients received 2-3 liquid bioXmark markers (Nanovi A/S, Lyngby, Denmark). Patients underwent two 4DCT scans to account for respiratory motion with the Respiratory Gating for Scanners (RSGC) system (Varian, medical systems), one was performed with intravenous contrast. Prescribed radiotherapy dose was 36 Gy in 15 daily fractions of 2.4 Gy, combined with weekly gemcitabine 1,000 mg/m2. Patients were treated with EXP GAT, and prior to each radiotherapy fraction, patients were positioned on the Truebeam (Version 2.7, Varian medical systems), with online setup on an expiratory gated cone beam CT scan. The primary objective was to compare the efficacy of EXP GAT with FB with a specific focus on assessing the impact on PTV volume and radiotherapy doses to stomach and duodenum. The marker motion was quantified in three directions: cranio-caudal (CC), right-left (RL), and ventro-dorsal (VD), throughout a full respiratory cycle. The gross tumor volume (GTV) and the internal target volume (ITV) were contoured for both EXP GAT and FB treatment by the radiation oncologist, with the addition of 5 mm clinical target volume (CTV) and 10 mm PTV. The stomach and duodenum were also delineated on both the FB and EXP GAT scans. For each patient, a FB treatment plan and EXP GAT treatment plan was executed. Dosimetric parameters analyzed for the stomach and duodenum included the D1cc, V25Gy, and Dmean. Statistical tests such as the paired T-test and Wilcoxon signed-rank test were employed in data analysis. The mean age of the 15 patients who received neoadjuvant chemoradiotherapy was 69 years. The analysis of marker movement in three dimensions showed that the average motion in the CC direction was the greatest, measuring 1.0 cm (range 0.35 - 1.8 cm). Subsequently, the motion in the VD direction was 0.29 cm and in RL direction 0.24 cm (see Figure 1). Nine out of 15 patients had less than 1.00 cm motion in CC. When comparing the PTV volumes between the EXP GAT and FB treatment plans, the majority of patients demonstrated an increase in PTV volume in the FB plan, with an average volume increase of 43.02 cm³. Dosimetrically, no significant differences were found in the D1cc for the stomach or duodenum between the EXP GAT and FB plans. However, statistically significant differences were observed in V25Gy and Dmean for both organs (p < 0.001). There was no significant correlation found between marker motion and the dose to the stomach. For the duodenum, there was a significant correlation observed between marker motion and V25Gy. Figure 2 effectively illustrated the impact of marker motion, suggesting that a defined threshold of 45% indicated a low likelihood of toxicity. If the motion of the markers was more than 0.9 cm in FB, all patients exceeded the defined threshold of 45% for the V25Gy of the duodenum. Results: