A new formula for converting dose-area product to effective dose in dental cone-beam computed tomography

The purpose of this study was to determine a dedicated conversion formula between dose-area product (DAP) and effective dose (E) for dental CBCT, which incorporates X-ray beam energy as well as geometric factors.

CBCT exposures of an adult phantom were simulated using PCXMC 2.0 (STUK, Finland). Fifty-seven fields of view (FOV) were included, ranging from Ø4x4cm to Ø21x19cm. Six tube voltages (70 kV to 120 kV), eight combinations of Al (2.5 to 10 mm) and Cu (0 to 0.5 mm) filtration, and four source-isocentre distances (35 to 65 cm) were used, resulting in 10 896 simulated scan protocols. In addition, 10 944 random combinations of scan parameters within the aforementioned ranges were simulated, resulting in 21 840 scan protocols that were used for fitting a formula using multiple linear regression with 8 independent variables. Finally, 2304 random scan protocols were simulated as validation data to evaluate the formula's generalizability.

E/DAP ranged between 0.031 µSv/mGy.cm² and 0.294 µSv/mGy.cm², with a mean of 0.150 µSv/mGy.cm². Due to extensive clustering of the E/DAP, three formulas were determined according to FOV categories (Small: <100 cm², Medium: 100-400 cm², Large: >400 cm²). The resulting formulas showed mean absolute errors of 7.9%, 4.0% and 3.6%, respectively, for the validation data.

The new formulas allow for a straightforward, yet accurate, estimation of the effective dose for dental CBCT based on DAP. Further fine-tuning of the model could be achieved by incorporating, for example, bowtie filtration and off-axis beam geometries.