Investigation of the beam width and profile of kilovoltage CBCT using different measurement techniques and analysis of the dosimetric effects of beam parameters.

Abstract

Background: The aim of this study is to investigate the beam width and beam profile of kilovoltage cone beam computed tomography (kV CBCT) using different measurement techniques on an O-ring linear accelerator. The effect of the imaging beam on the dosimetric parameters was analysed.

Materials and methods: The uncertainty of field size adjustment, the dependence of beam width on field size, and the effect of deflection from the isocenter on the beam profile were investigated by ionization chamber detector matrices. The 2D beam profile of the CBCT was analysed by relative ionization chamber measurements.

Results: The average setup uncertainties of the field sizes were 0.3 mm ± 0.02 mm. The dependence of beam width on field size investigation revealed that the largest discrepancies occurred for small field sizes, which are important in determining computed tomography dose index (CTDI) values of the kV CBCT. The beam width deviation between measured and vendor-based data was larger than 1 mm below 40 mm field of view. The pelvis protocol demonstrated the smallest CTDI value difference of 2.3%, yet presented the largest effective dose deviation of 0.12 mSv.

Conclusions: The measured CTDI coefficients were higher than predicted by the manufacturer for all cases. The currently internationally recommended CTDI measurement protocols for CBCT contain no reference to the determination of the beam width as a basic element of the calculations. Based on our measurement results, the beam width parameters affect CTDI: therefore, it would be advisable to apply this type of correction.