Monte Carlo-calculated patient organ doses from kV-cone beam CT in image-guided radiation therapy

This study evaluated patient specific organ doses from kV-cone beam computed tomography (kVCBCT) for the Varian on-board imager (OBI) and the Elekta x-ray volumetric imager (XVI) used in image-guided radiation therapy using Monte Carlo (MC) simulations. The beam modeling for both kV-CBCT systems was performed with the EGSnrc/BEAMnrc user-code. The patient dose distributions were calculated from the modeled kV-CBCT beams by using planning CT data sets for five anatomical regions of head, 'head and neck', chest, abdomen, and pelvis. Two default acquisition modes, full scan mode for chest, abdomen, and pelvis, and half scan mode for head and 'head and neck', were used for both kV-CBCT systems. The MC-calculated dose distributions were converted into absorbed doses by Farmer chamber measurements in body-and head-type phantoms. A bodytype phantom (30 cm diameter and 51 cm length) and a head-type phantom (16 cm diameter and 33 cm length) were irradiated with the full scan mode and the half scan mode, respectively. Finally, the patient specific organ doses were quantitatively evaluated from dose-volume histograms. The mean organ doses for soft tissue in chest, abdomen, and pelvis were 1.52-4.13 cGy, 1.30-2.56 cGy, and 2.323.81 cGy for OBI, and 0.82-1.60 cGy, 0.66-1.04 cGy, and 0.97-1.41 cGy for XVI, respectively. In the full scan mode, organ doses for XVI were one-half to one-third of those for OBI. The organs located at the anterior surfaces like heart and testes were higher doses than central regions. Meanwhile, the mean doses for soft tissue in the head and 'head and neck' were 0.09-0.43 cGy and 0.09-0.45 cGy for OBI, and 0.13-0.39 cGy and 0.09-0.31 cGy for XVI, respectively. In the half scan mode, the higher doses were observed at the posterior surface for OBI and at the anterior surface for XVI, depending on the scan direction.