Technical note: A PBS source model based independent dose calculation frame on TOPAS MC

We endeavor to present a comprehensive methodology for establishing a versatile Monte Carlo (MC) dose calculation platform that operates independently of the treatment planning system (TPS, e.g., RayStation).

We assumed the source emission plane was at the nozzle exit. To align with the measured data, we optimized the phase space parameters (spot size, angular spread, correlation, energy spread, and nominal energy) and the beam source model's absolute dose (the number of protons per MU) in the source emission plane. Additionally, we devised a method that automatically converts patient plans into executable MC scripts capable of running in the TOPAS MC software.

Our efforts successfully established a library encompassing the source model parameters. The disparities between the measured and simulated beam spot sizes were all below 0.3 mm. Moreover, the differences in the depth-dose curve's distal falloff (R₉₀) were less than 0.1 mm, and the mean point-to-point dose differences were less than 0.7 %. Remarkably, the 3D gamma passing rates (GPRs) for three spread-out Bragg peaks (SOBPs, 3 mm/3% criteria) were 100 %. Furthermore, when comparing TOPAS and TPS MCs for 23 pencil beam scanning (PBS) patient plans, the mean 3D GPRs for 2 mm/2% and 3 mm/3% criteria were 99.96 % and 100 %, respectively.

We have successfully developed a comprehensive MC framework for PBS, employing a well-defined beam source model. The method we presented for building the PBS MC framework holds potential to build a dose verification tool and for scientific research.