SU‐FF‐I‐49: Daily Patient Dose From Kilo‐Voltage Cone‐Beam Computed Tomography: A Comparison Between XVI and OBI

Abstract

Purpose: To make a comprehensive set of absolute dose measurements from the X‐ray Volumetric Imager (XVI®, Elekta Medical Systems) and the On‐Board Imager (OBI®, Varian Medical Systems) on cylindrical phantoms using a calibrated ion chamber. Method and Materials: Two cylindrical (acrylic) phantoms with diameters of 18 cm (head phantom) and 30 cm (body phantom) were used for all measurements. Each phantom has a hole in the center and 2 cm below the phantom surface. For the XVI unit, the four manufacturer‐supplied protocols were measured, namely, “head & neck”, “prostate”, “pelvis”, and “pelvis — large”. The total mAs settings were also varied for each protocol to verify the dose linearity. For the OBI unit, full bow‐tie, half bow‐tie, and no bow‐tie filters were used in combination with the two scanning modes, namely, “full‐fan” and “half‐fan”. Results: For the XVI unit, the (isocenter) doses range between 0.1 and 2.2 cGy for the “head & neck” and “prostate” protocol, respectively, and agree well with the “Nominal Scan Dose” supplied by the manufacturer. The measured doses were highly linear with the total mAs settings. For the OBI unit, the (isocenter) doses range between 0.9 and 8.5 cGy. In the full bow‐tie with the “full‐fan” mode, the head phantom central dose was 8.5 cGy. This is significantly larger than a similar protocol scan measured with the XVI unit (“head & neck”). Half value layers were between 4.6 to 7.0‐mm‐Al for the two x‐ray units. Conclusion: A comprehensive series of dose measurements were made on the XVI and OBI CBCT imagers. It was observed that the OBI unit generally results in higher dose with up to 8.5 cGy measured with the head phantom. More studies are required to determine the increase in image quality expected from the increase in patient dose from the OBI unit.

Low-dose cone-beam CT (LD-CBCT) reconstruction for image-guided radiation therapy (IGRT) by three-dimensional dual-dictionary learning

“…CBCT imaging would be repeatedly applied to a patient during the IGRT treatment course for over 25 times in common. It raises a great concern that repeated CBCT scans deliver too much dose to the patient at 3 ∼ 5 cGy per scan and 90 ∼ 150 cGy if scanned daily for Varian on-board imager [4] and the isocenter doses ranged between 0.1 and 2.2 cGy per scan for the Elekta X-ray volumetric imager [5]. In addition, CBCT delivery dose from a Varian Trilogy accelerator was accurately investigated by Monte Carlo simulations and their results have shown that doses from a typical head and neck CBCT scan to eye, spinal cord, brain and bone could be up to 8 cGy, 6 cGy, 5 cGy and even 25 cGy, respectively for Varian on-board imager [6].…”