Study of IBA-CC01 response in composite photon beams using Monte Carlo simulations.

Radiotherapy dosimetry in composite and modulated fields remains complex, especially when using small field ionization chambers in the second part of the Alfonso et al. formalism. This study investigates the response of the IBA- CC 01 ionization chamber in machine-specific reference, one static field of, and clinical IMRT step-and-shoot composite fields for a 6 MV flattening-filter-free (FFF) TrueBeam STx ® photon beam. A previously validated BEAMnrc model of the TrueBeam linac was used to generate high-statistics phase-space files, which were then combined with an egs_chamber model of the IBA-CC01 to calculate absorbed dose to water and detector response in static and composite fields. Latent variance was evaluated at three fixed points (central, off-axis, and peripheral) across several IMRT step-and-shoot fields, showing that the detector's latent variance remains below 0.2% and is largely independent of the detector's position. Radiochromic film measurements using Gafchromic EBT4 in a solid water phantom, following AAPM TG-235, validated the Monte Carlo simulation of the plan-class-specific reference fields. For these, off-axis ratio profiles from film and Monte Carlo agree within a few percent in the high-dose region, and a gamma analysis with 3.5%/2.5 mm criteria (global) yielded passing rates of 97% and 95% for cross-planes and in-plane profiles, respectively. Monte Carlo-derived correction factors for the IBA-CC01 in IMRT step-and-shoot composite fields are close to one and, within a mean absolute difference of less than 1.5%, align with the small field correction factors reported in IAEA-AAPM TRS 483 for static fields of similar size. These findings suggest that, for the 6 MV FFF TrueBeam beam and the IMRT step-and-shoot fields examined here, the IBA-CC01 functions effectively as a practical field detector for relative dosimetry and for calculating detector-specific correction factors in composite fields. In contrast, absolute reference dosimetry should still rely on reference-class ionization chambers under conventional reference conditions.