Author: Ergun E. Ahunbay, Abdul Kareem Parchur, Eric S. Paulson, Ilaria Rinaldi, Angelo Schiavi, Li Zhao 👨🔬
Affiliation: Sapienza University of Rome, Department of Radiation Oncology (Maastro), GROW School for Oncology, Maastricht University Medical Centre+, Department of Radiation Oncology, Medical College of Wisconsin 🌍
Purpose: Secondary dose calculations (SDC) facilitate patient-specific quality assurance for online adaptive intensity-modulated proton therapy (IMPT). However, there is currently a lack of commercial SDC tools for IMPT. The GPU-accelerated fast Monte Carlo (MC) code, FRED, has demonstrated promising results for IMPT SDC. This study aimed to evaluate and compare the accuracy of FRED IMPT MC dose calculations in phantoms with maximal tissue heterogeneities.
Methods: A heterogeneous digital phantom (20 x 20 x 40 cm^3) composed of water was constructed with a cylindrical insert (3 cm diameter and 15 cm length) positioned anterior to a spherical target (5 cm diameter). A single pencil beam scanning field passed through the insert. Four materials with consistent chemical compositions were assigned to the cylindrical inserts in RayStation (v2023B) and FRED (v3.75): air, lung tissue, bone, and titanium. Identical beam-scanning datasets were used to create beam models in RayStation and FRED. A sampling history of 104 ions/spot was used for MC optimization with a statistical uncertainty of 0.5% for the MC dose calculation in the RayStation. The number of primaries per pencil was set to 106 in the FRED. The depth and lateral dose profiles were assessed, and dose distributions at material-water interfaces were compared.
Results: FRED demonstrated excellent agreement with RayStation in terms of depth dose and lateral dose profile comparisons for air, lung, bone, and titanium. The passing rates for the 1%/2 mm γ-index (10% dose threshold) were 93.63%, 94.86%, 96.07%, and 95.99%. For the 2%/2 mm γ-index (10% dose threshold), the passing rates were 98.04%, 99.44%, 99.68%, and 98.57%, respectively.
Conclusion: This study demonstrated excellent agreement between the MC dose calculation in RayStation and FRED for four heterogeneous materials. Our findings support the use of FRED as an SDC tool in IMPT for patients with anatomies comprising heterogeneous tissues.