A unified computed tomography dose index (CTDI) which includes all scattered radiation was used to investigate the difference in absorbed dose to the patient between a computed tomography (CT) and a cone beam CT (CBCT). The unified CTDI, denoted CTDI300,w, was measured using a 100 mm pencil ionization chamber at three positions covering 300 mm at each of the five phantom slots in three CTDI body phantoms. The dose length product of a lumbar spine protocol for the modalities was comparable (CT: 153 mGycm, CBCT: 126 mGycm), though the normalized CTDI300,w for the CT was 4.6 times higher than for the CBCT. The comparison between CTDI300,w and the two dose indexes provided by the systems showed a substantial underestimation for both modalities. The most accurate, but also the most inconvenient method to compare absorbed dose is to use the CTDI300,w. Most important is to always use the same dose index.
Larsson et al. (Tue,) studied this question.