Contribution to normal tissue dose from concomitant radiation for two common kV-CBCT systems and one MVCT system used in radiotherapy.

Background and purpose: This study outlines the measured doses for three concomitant imaging modalities used in radiotherapy. Material and methods: Doses were measured using thermo luminescent dosemeters within pelvis and thorax anthropomorphic phantoms for the Varian On Board Imager (OBI), Elekta X-ray Volume Imager (XVI) and Tomotherapy HiArt II systems. Organ sites were selected to include those organs which would be irradiated by the treatment beam during the therapy exposure. Results: Doses for kilovoltage imaging systems are comparable within the pelvis phantom at 20-30 mGy. Thorax phantom doses are lower, especially where user specified protocols are used at 5-10 mGy. Tomotherapy doses are typically less than 10 mGy for both phantoms. Conclusions: Concomitant imaging dose is a small fraction of the therapy dose, however, for a high fraction technique, the imaging dose can become comparable to the therapy dose outside primary target volume. Recommendations for optimisation of imaging in radiotherapy are presented. (C) 2012 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 105 (2012) 139-144