The partial acquisition mode, which scans the patient over 200°, is commonly used in the clinic for cone-beam CT (CBCT) scans, for example it is widely used for image guided radiation therapy (IGRT) procedures and for dental scans. This study aimed to investigate suitability of using size-specific dose estimates (SSDE) conversion factors derived for the full acquisition mode, which covers a region of interest over 360°, for partial scans.

A previously validated Monte Carlo (MC) model was employed to simulate a kVimaging system, on-board imager (OBI), mounted on a Varian linear accelerator (TrueBeam), and to assess imaging doses [1]. Two scan protocols used for head and body scans were studied with 120 kV. SSDE factors for both protocols were assessed under different conditions using the full and partial modes. Three field sizes were used for each protocol, where the longitudinal extension in the z-axis was increased from 8 cm to 24 cm, in increments...

As shown in Figures 1 - 3, the SSDE factors were affected by the acquisition mode. For the head protocol, factors of the full scans were higher than those for the partial scans by up to 6% for small diameters, and the variations dropped for large diameters to <1%. However,the reverse was true for the body protocol. Factors of the partial scans were higher than those for the full scans. The variations increased with the phantom diameter, from8% for small diameters to 17% for large...

The SSDE factors were influenced by the acquisition mode, particularly for the body scans. Use of the full scans factors for partial scans would affectaccuracyofestimation ofpatientsdoses by 3% and 11%, on average, for the head and body, respectively.

1The National Centre for Applied Physics, King Abdulaziz City for Science and Technology, Riyadh, Saudi Arabia. 2Department of Clinical Physics and Bioengineering, University of Glasgow, Glasgow, UK. 3Department of Biomedical Physics, King Faisal Specialist Hospital & Research Centre, Riyadh, Saudi Arabia.

[1]Abuhaimed, A., Martin, C. J., Sankaralingam, M., Gentle, D. J. & Mcjury, M. An assessment of the efficiency of methods for measurement of the computed tomography dose index (CTDI) for cone beam (CBCT) dosimetry by Monte Carlo simulation. 2014, Phys Med Biol, 59, 6307-26. [2]Abuhaimed, A., Martin, C. J., & Demirkaya, O. Influence of cone beam CT (CBCT) scan parameters on size specific dose estimate (SSDE): A Monte Carlo study. 2018, submitted to Phys Med Biol.