Portal dose image verification: formalism and application of the collapsed cone superposition method

A formalism tailored for portal dose image verification is proposed to facilitate the comparison of calculated and measured portal dose distributions. Each portal image is converted into a dose proportional image and normalized to the reference beam calibration dose per monitor unit. The calculated or measured dose to a detector phantom is accordingly normalized so as to enable direct comparison. The collapsed cone kernel superposition method is adapted and evaluated for calculation of portal dose distributions in a water-equivalent detector phantom through comparisons with Monte Carlo calculations and with measurements. The deviation compared with Monte Carlo calculations for 6 and 15 MV was between +0.9% (the 0.9 quantile) and -2.1% (the 0.1 quantile) for a range of investigated geometries. Collapsed cone calculations compared with measurements for clinical fields agreed within [-1.9%, +2.4%] for 15 MV and [-0.9%, +3.2%] for 6 MV for the 0.1 and 0.9 quantiles, respectively. Hence, the absolute portal dose to a detector phantom could be calculated and verified well within the present accuracy requirements for clinical dose calculations.