Applicability evaluation of the TRS-483 protocol for the determination of small-field output factors using different...

To evaluate the applicability of TRS-483 output correction factors (CFs) for small-field output factors (OFs) using different multi-leaf collimators (MLC) and field-shaping types.

All measurements were performed on TrueBeam, TrueBeam STx, and Halcyon using 6 MV flattening filter-free energy. Four detectors, including CC01, CC04, microDiamond, and EDGE, were used. Nominal field sizes ranging from 1 × 1 to 4 × 4, and 10 × 10 cm² were used to measure small-field OFs at source-to-axis distance of 100 cm with a 0° gantry angle in a 3D water phantom. Further, the field-shaping types were defined using jaw collimator or MLC (five different configurations). A field size of 10 × 10 cm² was used as the reference for calculation of OFs obtained as ratio of detector readings (OF_det). The percentage difference and coefficient of variation of OF_det and OF_det corrected by applying CF were compared for each field size and configuration.

For OF_det corrected by applying CF, the ranges of percentage difference and coefficient of variation in all configurations for ≥ 2 × 2 cm² fields were reduced from 1.2-2.2 to 0.8-1.3 percentage points (%pt) and from 0.5-1.0 to 0.4-0.7%, respectively. For 1 × 1 cm² field, the ranges of percentage difference and coefficient of variation were reduced from 3.3-5.7 to 1.2-2.2 %pt and from 2.2-3.7 to 0.8-1.1%, respectively.

The CFs described in TRS-483 dosimetry protocol have broad applicability in reducing OF variations between detectors under different MLC and field-shaping types.