Analysis of dose reconstruction techniques in pre-treatment QA of large-field RapidArc technique with a 2D detector...

This study aims to evaluate the impact of two dose reconstruction techniques, standard and composite, on gamma passing rates (GPRs) during pre-treatment quality assurance of large-field pelvic RapidArc plans using the Octavius 4D system with a 1500 2D ionization chamber array. The investigation focuses on quantifying the effect of reconstruction methodology across varying gamma criteria and normalization modes. Ten large-field pelvic RapidArc verification plans for pelvic radiotherapy cases were analysed using both standard reconstructions based on a single measurement and composite reconstructions generated by merging two longitudinally shifted measurements in the Octavius 4D system. Measurements were performed on a Varian TrueBeam linac using 6 MV beams, and dose comparisons were conducted in verisoft software. The composite reconstruction was generated by merging two longitudinally shifted measurements to extend the detector's effective field of view. Gamma analyses were performed under both global and local normalization at multiple criteria (1 mm/2%, 2 mm/2%, 2 mm/3%, 3 mm/2%, and 3 mm/3%). Statistical significance between reconstruction methods was assessed using paired tests (p < 0.05). Under global normalization, 3D GPRs exceeded 95% for all criteria, with negligible differences (< ±2%) between standard and composite reconstructions (p > 0.05). Local normalization exhibited greater sensitivity, showing up to -11.6% lower GPRs for the composite method at the most stringent criterion (1 mm/2%), though these differences diminished (<2%) at 3 mm/3%. Composite reconstructions consistently produced volumetric and transverse-plane GPRs that were comparable to or higher than those from single-plane analyses, confirming stable dose agreement across planes. The composite reconstruction method provides dosimetric accuracy equivalent to the standard approach while enabling full-field verification for large field pelvic RapidArc fields. Its use significantly enhances QA efficiency without compromising clinical reliability. For standard practice, composite reconstruction combined with global normalization and 3 mm/3% gamma criteria offers an optimal balance between accuracy and practicality in large-field pre-treatment QA.