We used a Monte Carlo (MC) model based on Geant4 (GEometry ANd Tracking) to generate dose profiles in the central plane of the CTDI phantom. MC simulations were carried out for three different sizes of z-collimator and different tube voltages (80, 100, or 120 kVp), a tube current of 80 mA, and an exposure time of 25 ms.
We derived optimal weighting coefficients by taking the integral of the radial dose profiles. The first-order linear equation and the quadratic equation were used to fit the dose profiles along the radial direction perpendicular to the central plane, and the fitted profiles were revolved about the Z-axis to compute the mean dose (i.e., total volume under the fitted profiles/the central plane area). The integral computed using the linear equation resulted in the same equation as conventional CTDIW, and the integral computed using the quadratic equation resulted in a new CTDIW (CTDIMW) that incorporates different weightings ("2/3 and 1/3") and the middle dose point instead of the central dose point.
Compared to the results of MC simulations, our new CTDIMW showed less error than the previous CTDIW methods by successfully incorporating the curvature of the dose profiles regardless of acquisition protocols. Our new CTDIMW will also be applicable to the AAPM-ICRU phantom, which has a middle dose point.