SU-E-J-19: An Intra-Institutional Study of Cone-Beam CT Dose for Image-Guided Radiation Therapy




To determine the variability of Cone-Beam CT Dose Index (CB-CTDI) across multiple on-board imaging (OBI) systems within a single institution, and compare this to manufacturer provided data.


The CB-CTDI was measured on three Trilogy and three TrueBeam Varian OBI systems, for six different clinically used scan protocols. Measurements were taken using a 10 cm long CT ionization chamber in either a 16 cm (head-simulating) or 32 cm (body-simulating) diameter, acrylic, cylindrical, 15 cm long CTDI phantom. We assessed the variation in CB-CTDI between the OBI systems and compared our measured values to the data provided by the manufacturer.


The standard error in the CB-CTDI measured for all protocols was found to be within ±2% and ±5% of the mean for TrueBeam and Trilogy, respectively. For all head scan protocols, the measured TrueBeam values were lower than the manufacturer's reported values, with a maximum difference of 13.9% and an average difference of 11%. For the body scan protocols, the TrueBeam measured values were 3% and 13% greater than the manufacturer's reported values for two out of three protocols, and 38% lower than reported for the third protocol. In total, 7/18 CB-CTDI measurements fell within the manufacturers specified range (±10%). Across all scans the Truebeam machines were found to have a lower CB-CTDI than Trilogy, particularly the head scan protocols, which show decreases of up to 30% .


The intra-institutional variation of CB-CTDI was found to be clinically acceptable at less than 5%. For the TrueBeam OBI system, over half of the measured scans failed to fall with in the manufactured quoted range of 10%, however, all measured values were within 15% of the manufacturer's reported values. For accurate assessment and reporting of imaging dose to radiotherapy patients, our results indicate a need for standardization in CB-CTDI measurement technique.