ESTRO 36 Abstract Book

S166 ESTRO 36 2017 _______________________________________________________________________________________________

may lead to better palliation and possibly increase survival with the advantage of a short overall treatment time. The superior soft tissue contrast of MRI might improve tumour contouring and MRI is capable of tumour motion quantification. We want to investigate the technical feasibility and safety of MRI-guided SBRT for LAPC. Material and Methods From July 2013 to January 2016, 20 patients with LAPC or medically unresectable pancreatic cancer without distant metastasis were included in this study (Table). A custom made abdominal corset was manufactured to reduce breathing induced tumour motion. Contouring of the gross tumour volume (GTV) and organs at risk (OARs) was performed on 4D treatment planning CT and multiparametric MRI. A GTV-to-PTV margin of 3 mm was applied. We quantified tumour motion with cine MRI. After treatment planning, the static dose distribution was convolved with the cine MRI based motion trajectory to simulate and evaluate the delivered dose to the GTV, PTV, and OARs. SBRT was carried out up to a dose of 24 Gray in 3 fractions in one week. Online position verification was performed with 4D CBCTs, with 4D matching based on gold fiducial markers at the midventilation position. Results All patients underwent uncomplicated endoscopic fiducial marker placement. Tumours and OARS were clearly visible with contrast enhanced CT and multiparametric MRI (Figure). On the 4D planning CT and on the 4D CBCT scans, the fiducial markers were clearly visible. The corset decreased peak-to-peak tumour motion in craniocaudal direction on average from 11.3 to 7.2 mm. With incorporation of the tumour motion trajectory, the dose distribution was blurred and, in this way, the actual delivered dose was simulated. In all patients, an adequate dose distribution was achieved with acceptable dose in the OARs (Table). Position verification based on 4D marker matching was feasible. No grade 3 or higher treatment related toxicity was observed in these patients to date.

Figure: Contouring with the aid of MRI and CT. Red: GTV. Green: pancreatic head. Blue: duodenum.