Evaluation of In-room Volumetric Imaging Doses for Image-guided Radiotherapy: A Multi-institutional Study

AIMS: We investigated imaging dose and noise under clinical scan conditions at multiple institutions using a simple and unified method, and demonstrated the need for diagnostic reference levels in image-guided radiotherapy (IGRT). MATERIALS AND METHODS: Nine cone-beam and helical computed tomography (CT) scanners (Varian, Elekta, Accuray Inc., and BrainLAB) from seven institutions were investigated in this study. The weighted cone-beam dose index (CBDI(w)) was calculated for head and pelvic protocols using a 100 mm pencil chamber under the conditions used in actual clinical practice at each institution. Cone-beam CT image noise was evaluated using polymethylmethacrylate head and body phantoms with diameters of 16 and 32 cm, respectively. RESULTS: For head and pelvic protocols, CBDI(w) values ranged from 0.94–6.59 and 1.47–20.9 mGy, respectively. Similarly, standard deviation (SD) values ranged from 9.3–34.0 and 26.9–97.4 HU, respectively. The SD values tended to increase with decreasing imaging dose (r = −0.33 and −0.61 for the head and pelvic protocols, respectively). CONCLUSIONS: Among the nine machines, the imaging dose for high imaging dose institutions was approximately 20 mGy to the pelvic phantom, and there was a 14-fold difference in dose compared with the other institutions. These results suggest the need to establish DRLs for IGRT to guide clinical decision-making.