Speaker
Description
Purpose: To evaluate dose distribution accuracy in pancreatic stereotactic ablative radiotherapy (SABR) in patients with self-expanding metal biliary stents using Monte Carlo (MC) simulation, incorporating correct stent materials and an improved contouring technique to account for CT artefacts.
Methods: Two anonymised clinical cases with Nitinol biliary stents were analysed. VMAT treatment plans were created using Eclipse treatment planning system (TPS) v13.7 with the analytical anisotropic algorithm. Two plans were generated per patient: one prioritising PTV coverage and one prioritising organs at risk (OAR) sparing. MC simulations were performed using PRIMO software with three configurations: (1) standard MC without modifications, (2) MC with a new model incorporating correct Nitinol material assignment (HU 3500) and corrected internal stent contents (HU 1000 representing bile fluid rather than artefactual HU -500). A novel contouring technique separating the stent wall from internal contents was implemented. Dose distributions were compared using percentage agreement (PA) and gamma passing rate (GPR) analysis with 3%/3mm criteria.
Results: Comparison between MC standard and the new MC model demonstrated significant differences in PTV dose distribution, with GPR of 55.29% and 43.74% for Patients A and B respectively, indicating the importance of material correction. Comparison between the new MC model and TPS revealed clinically significant discrepancies, with PTV GPR ranging from 43.74% to 55.29%. OARs demonstrated higher agreement (GPR 93-100%). The new contouring approach reduced stent volume from 9.9 cm³ to 6.8 cm³, providing more anatomically accurate representation.
Conclusions: Standard TPS calculations demonstrate significant dose distribution inaccuracies in pancreatic SABR patients with biliary stents. The proposed MC model with correct material assignment and improved contouring technique significantly improves dosimetric accuracy. Without addressing CT beam hardening artefacts and stent material properties, MC simulation alone does not improve upon TPS calculations. These findings have important implications for dose escalation strategies in pancreatic cancer radiotherapy.