Characterization of 2D Ionization Chamber Array (ICA) for IMPT Plan Specific Quality Assurance (PSQA) By Monte Carlo (MC) Simulations
Abstract
Purpose
MC simulations were used to accurately determine the water-equivalent thickness (WET) of the buildup housing, the effective point of measurement (EPOM), and the proton- and chamber-specific factor fQ, the MC-derived component of the beam quality correction factor (kQ,Qo), for a commercial ICA (PTW Octavius 1500XDR) in monoenergetic and spread-out Bragg peak (SOBP) proton beams
Methods
Parallel 10×10 cm² proton beams with nominal energies of 70, 150, and 250 MeV, and an SOBP beam (max energy = 200MeV, modulation width = 5 cm), were simulated using PHITS. The housing buildup WET and ICA EPOM were determined from relative shifts in percent depth-doses (PDD) simulated (1) with housing buildup only and (2) including the air chamber geometry, relative to reference PDDs in water. fQ was calculated at combined relative uncertainty (0.5%) using the full ICA geometry as the ratio of dose to a water disk (0.025 cm × 1 cm²) at 2 cm depth to the dose to air in the chamber cavity positioned at the same WET depth. Nuclear interactions, secondary electron, positron, and photon transport were simulated using INCL and EGS5 models at 10 keV cut-off for all transported particles.
Results
The housing buildup WET was 1.4 ± 0.17 mm, and the ICA EPOM was 8.9 mm. Calculated fQ values were 1.11, 1.10, 1.08 for 70, 150, 250 MeV monoenergetic beams.
Conclusion
The MC-derived EPOM agreed with literature experimental values within 1 mm. fQ varied by up to 3% across the clinical proton energy range. For mixed-energy IMPT fields, cross-calibration of the ICA at a single energy may introduce dose uncertainties up to 3% in measured 2D dose distributions. Cross-calibration in a medium proton energy is recommended. This residual energy-dependence should be considered when defining gamma index analysis criteria.