Assessment of single-spot TOPAS MC simulation for proton pencil beam scanning in homogeneous and heterogeneous media and quantification of dosimetric impact of CT metal artifact on IMPT and VMAT plans

Abstract

Proton pencil beam scanning (PBS) radiation therapy has become thriving worldwide. The accuracy of proton dose calculation that based on proper modeling for each single scanning spot as well as accurate CT image data is crucial. The goal of this work is first to investigate the lateral dose profile (LAT) for analytical proton beam algorithm (PBA) in Eclipse compared to the Monte Carlo (MC) simulation using the tool for particle simulation (TOPAS) with double Gaussian (DG) and triple Gaussian (TG) modeling. Second, to evaluate the dosimetric impact of CT metal artifact for intensity modulated proton therapy (IMPT) and volumetric modulated arc therapy (VMAT). The Sumitomo proton PBS nozzle was constructed based on the manufacturer blueprints using TOPAS MC code. This model was implemented to reproduce the integral depth-dose (IDD) curves and LAT and was validated with the commissioning data. The TOPAS simulated data in homogeneous and heterogeneous media were utilized to model LAT dose distribution with DG and TG fitting for investigating the long-range scattering. Furthermore, treatment plans of IMPT and VMAT for the original CT (plan_ori), CT with 0 HU replacement on effected area (planori0) and CT with MAR method to suppress the artifact (plan_MARs) for prostate cancer patients with hip prosthesis, head and neck (HN) patients with dental metallic implants and spine patients with spinal prosthetic implants were compared and analyzed. The comparison between TOPAS simulation and TPS calculated and measured commissioning data on IDD range parameters and LAT with FWHM showed good agreement. The doses on LAT demonstrated slight discrepancies compared to TPS data on the tails since TPS using DG modeling underestimates the long-range scattering on LAT dose distribution, therefore, TG indicated better prediction. As for the dosimetric influence of CT metal artifact, VMAT plans has smaller dosimetric impact than IMPT plans. The beam arrangement to avoid the artifact area for proton plans is important. Besides, the volumes of target and metal implant, as well as the distance between target and metal showed close correlation on the dosimetric impact of CT metal artifact.