For the purpose of investigation, Monte Carlo simulation was used and experimental measurements were performed. CBCT dental unit was simulated using Monte Carlo N-Particle transport code (MCNP® 6.1.1beta) and model verification was performed using PMMA head CTDI phantom and radiochromic films (Figure 2) as described in previously published study [5].

For calculation of organ doses and for the assessment of effective doses computational anthropomorphic phantom was used („Zubal“) (Figure 3) [6, 7].

Most often clinical protocols involving different exposure geometries including sizes and positions of fields of view (FOV) and different tube current-exposure time products (mAs) were simulated (Tables 1 and 2). Simulations were performed for both, 180^o and 360^o of X-ray tube rotation and in all cases for the X-ray tube voltage of 90 kV.

DAP was measured using DAP-meter (VacuTec, Germany). CBCT dose indices were calculated using doses obtained by simulation. In case of CBCTDI₁, simulations were performed in 15 points along the diameter line. Measured DAP and calculated CBCT dose indices were compared to effective doses for every exposure geometry and analysed.

A strong linear correlation between DAP and effective dose exists for all FOVs (Figure 4). As it has been shown in previously published study [5], better correlation was found in cases of FOVs positioned on maxilla (Figure 4b) which can be explained by strong influence of distance of the FOV position from thyroid gland. In case of CBCT dose indices, correlation between these dose indices and effective dose is weaker (Figure 5).

As described earlier, obtaining values of CBCT dose indices requires more time and more complex measuring and calculating procedure when compared to obtaining values of DAP. Since dose index used to represent the dose to the patient should be easily measurable [8], based on the results of this study, we propose DAP to be used as dose index in case of dental CBCT examinations.