Evaluation of CBCT Imaging Dose Metrics for QA on an X-Ray–Based Adaptive Radiotherapy Platform
Abstract
Purpose
To compare the traditional Computed Tomography Dose Index (CTDI) and the Cone-Beam-Dose-Index (CBDI), an alternative metric that quantifies the average dose in the central region of a cone-beam CT (CBCT) scan, for imaging dose measurements on bore-based linacs as part of routine quality assurance (QA) for an online x-ray–based adaptive radiotherapy program (ART).
Methods
CBCT imaging dose was quantified on three Ethos systems equipped with Hypersight technology for three clinical imaging protocols: Pelvis, Pelvis Large, and Head. Measurements were performed using a 100mm pencil ionization chamber (X2-CT Sensor, RaySafe, Sweden) positioned at central and peripheral locations within a 32cm diameter cylindrical acrylic phantom. The Head protocol used a removable 16cm diameter insert. Imaging was performed using default tube voltage and exposure settings, and a fixed slice collimation width of 230.5mm, which is standard for ART plans at our institution. Dose was calculated using two methodologies: the CTDIw and CBDIw. The average and range of dose values calculated across all machines were reported for each imaging protocol and calculation method and compared with vendor-reported values, which were obtained using an alternative methodology recommended in IAEA Report #5.
Results
The average CTDIw measurements across all systems were 4.49mGy (4.39mGy-4.66mGy) for Pelvis, 6.85mGy (6.66mGy-7.13mGy) for Pelvis Large, and 0.91mGy (0.89-0.93mGy) for Head, where the range is shown in parentheses. Relative to vendor-reported values, these measurements were lower by -49.8%, -49.5% and -55.2%, respectively. CBDIw measurements were 10.35mGy (10.11mGy-10.75mGy) for Pelvis, 15.49mGy (14.67mGy-16.44mGy) for Pelvis Large, and 2.13mGy (2.09mGy-2.15mGy) for Head, differing from vendor-reported values by +15.8%, +14.2% and +4.9%, respectively.
Conclusion
CBDIw provided imaging dose estimates closer to vendor-reported values than CTDIw for each investigated CBCT protocol. CBDIw may be a more suitable metric than CTDIw for routine CBCT imaging dose QA of bore-based linacs.