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An Intra-Institutional Study of Cone-Beam CT Dose for Image-Guided Radiation Therapy

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N Knutson

N Knutson1,2*, L Rankine1 , M Reilly1 , H Li1 , (1)Washington University School of Medicine, Saint Louis, MO, (2) Present Address: Mount Sinai Roosevelt Hospital, New York, NY,


SU-E-J-19 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: 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.

Methods: 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.

Results: 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%.

Conclusion: 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.

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