Exhibit Hall | Forum 4
Purpose: Varian Halcyon is a relatively new treatment delivery system that is seeing widespread clinical adoption. There are limited independent dose verification algorithms that support Varian Halcyon and the IAEA do not provide phase-space files (PSF) that describe Halcyon’s radiation field. The purpose of this study is to adapt existing PSF for 6MV, flattening filter free (6FFF) photon beams to Varian Halcyon photon beams and develop a virtual source model (VSM) to be used in dose verification algorithms.
Methods: The particle fluence from existing PSF for Varian 6FFF beams was modified to match measured Halcyon dose profiles. Beam hardening based on off-axis position was applied to match measured Halcyon depth-dose profiles. The modified PSF particle density, energy spectrum, particle direction, and correlations between characteristics were computed to generate a VSM. The VSM was implemented in a Monte Carlo (MC) dose verification algorithm. Depth-dose and off-axis profiles and dosimetric leaf gaps (DLG) were compared against water phantom measurements to validate the VSM. MC dose profiles were compared against Treatment Planning System (TPS) dose profiles using gamma index analyses.
Results: Depth-dose and off-axis beam profiles are within 1% of measurements for a 10x10 cm2 field. DLG values simulated with MC vs measured values are 0.22 vs 0.32 mm, 0.40 vs 0.42 mm, and -0.53 vs -0.47 mm for the distal, proximal, and combined multi-leaf collimator layers, respectively. Five clinical test cases were simulated using the MC VSM and were compared with TPS doses. Gamma index values across the cases have a mean passing rate of 98.2%.
Conclusion: We have created a VSM for Varian Halcyon by modifying Varian 6FFF PSF. Simulated depth-dose and off-axis beam profiles using the VSM match closely with measurements. The VSM exhibits excellent performance in clinical test cases, indicating great potential for clinical adoption.
Funding Support, Disclosures, and Conflict of Interest: James R. Castle and Xue Feng are employed by Carina Medical LLC. Quan Chen is a shareholder of Carina Medical LLC.
Monte Carlo, Dosimetry, Computer Software
TH- External Beam- Photons: Computational dosimetry engines- Monte Carlo