Assessment of dose distribution in pancreatic SABR in the presence of a metallic biliary stent using Monte Carlo modeling.

BACKGROUND

Stereotactic ablative radiotherapy (SABR) with dose escalation is increasingly used for pancreatic cancer; however, the dosimetric impact of metallic biliary stents remains poorly characterized. Self-expanding metal stents (SEMS), commonly placed for obstructive jaundice, contain high-density nitinol that produces computed tomography (CT) artifacts, including beam hardening, potentially compromising treatment planning system (TPS) dose accuracy. Precise dose estimation is critical given the proximity of the duodenum and other organs at risk (OARs).

PURPOSE: This study evaluated dose distribution in pancreatic SABR in the presence of a metallic biliary stent using Monte Carlo (MC) simulation with refined stent contouring and material correction.

METHODS

Two patients with SEMS (Cook Evolution, nitinol alloy) treated with SABR (50 Gy in 5 fractions) using volumetric modulated arc therapy (VMAT) were retrospectively analyzed. Plans were generated in Eclipse TPS (v.13.7) using the analytical anisotropic algorithm. MC simulations were performed in PRIMO (PENELOPE-based) with ≤2% uncertainty. A novel contouring technique separated the hollow lumen from the metallic stent wall to correct CT beam hardening artifacts. Nitinol (density 6.45 g/cm3) was incorporated into MC calibration. Comparisons between standard MC, modified MC, and TPS were assessed using percentage agreement (PA), gamma passing rate (GPR; 3%/3 mm), and dose-volume histogram metrics.

RESULTS

Refined contouring reduced modeled stent volumes from 26.1 to 22.8 cm3 and from 19.87 to 18.23 cm3. The standard MC model overestimated planning target volume (PTV) dose by 14.9% and 15.7%, with PTV GPRs of 55.29% and 79.78%, respectively. Approximately 2% dose enhancement was observed at the stent-tissue interface. Compared with the modified MC model, TPS underestimated dose lateral to the stent by up to 12.3% (worst case, Patient B, Plan 2) and overestimated it elsewhere by up to 12.2%. The duodenum showed the largest OAR discrepancies, with TPS up to 9.7% underestimation adjacent to the stent, whereas distant structures demonstrated high agreement.

CONCLUSION

Conventional TPS algorithms exhibit clinically relevant inaccuracies in the presence of metallic biliary stents. Incorporating refined stent contouring and material correction within an MC framework improves dosimetric accuracy and may support safer dose escalation in pancreatic SABR.