Purpose: To demonstrate feasibility of planning small animal irradiation using cone beam computed tomography (CBCT) data as input to a Monte Carlo (MC) based treatment planning system (TPS). Methods: The BEAMnrc/EGSnrc code was used to model 225 kVp photon beams produced by a small animal irradiator (XRAD 225Cx, Precision X‐ray). Homogenous phantom plugs (Gammex RMI 467, Middleton, WI) of known material and physical density were scanned using the XRAD 225Cx CBCT system to associate unique CBCT units to the corresponding material density. Additionally, the known physical densities were then correlated to appropriate atomic compositions to create a material data set library. This enabled correlating phantom CBCT units to a material/density matrix used as input to DOSXYZnrc MC computations. A 4×4×4 cm3 multi slab phantom in homogeneous (all solid water) and heterogeneous (lung block between water slabs) configurations were CBCT scanned using 80 kVp and 0.3 mAs setting. MC dose calculation was performed for 10 mm diameter circular field. Ultimately, 10 mm circular fields in homogenous and heterogeneous phantom configuration were used to compare absolute dose computed by MCTPS and film measurement. The CBCT based MC model was finally demonstrated using an animal CBCT scan. Results: The gamma map of CBCT based MC calculation and film measurement at 1cm depth plane in solid water medium shows excellent agreement of 98.7%within selected region of interest using 3%/0.5 mm gamma criteria. The absolute dose comparison between measurement and simulation in homogenous and heterogeneous phantom is within 1%. In a small animal the CBCT based MC model qualitatively demonstrated higher dose to bone relative to dose deposited to soft tissues. Conclusion: The CBCT based MC model is a valuable tool for improved dose calculation accuracy for small animal treatment planning in orthovoltage energy range.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging