Several methods have been proposed by different authors and recommended with the associations to specify the beam quality. We have repeated measurements on some combinations of ionization chambers and phantom materials including to get the measurements. To calculate using a ±2% variation of TPR value, the variation of MV number are 18.5 % (6MV) and 40.2 %(18MV). The variation of dose in that situation was a spread of 0.5∼1.08 % in a mean coefficient of variance. We have to use a “MV number” for a proper choice of mass collision stopping power ratios and perturbation factors used in the conversion of ionization measurements into absorbed dose, and have to choose the “quality index” like the ratio of ionizations for quality assurance of the beam quality.
