Dose-length-product determination on cone beam computed tomography through experimental measurements and dose-area-product conversion

The dosimetry of cone beam computed tomography (CBCT) is not fully elaborated yet, and some of these systems presents dose-area-product (DAP) values after an examination rather than, as in the case of traditional CT, the doselength- product (DLP). The purpose of this study was to provide a reproducible and straight-forward method for DLP measurements on CBCT, as well as to validate a tool for estimating DLP for a CBCT system in terms of accuracy. A prototype conversion tool for estimating DLP, using the DAP value, was provided by the vendor of a CBCT system which currently display only DAP. The DAP to DLP conversion tool was validated using five protocols for extremity imaging. DLP was measured using a 30 cm ionization chamber and 30 cm long cylindrical PMMA-phantom. DLP, the integrated absorbed dose within the ionization chamber, was measured through central and peripheral measurements in the phantom in order to calculate the weighted DLP, DLPW,CBCT. Comparisons between DLPW,CBCT and estimated DLP, showed that the conversion tool was accurate within 10%, with a mean average error of 6.1% for all measured protocols. The variation between repeated measurements was small, making the method highly reproducible. In conclusion, in this study a simple method for determining DLP on CBCT was presented, and it was validated that the conversion tool can present the delivered dose in terms of DLP with high accuracy. The measured DLP, as well as the DLP estimated by the conversion tool, is suitable for quality control and relative dose comparisons between protocols, but its’ relation to the DLP of CT systems should be investigated further in order to relate to patient dose.