Abstract
Three alternative methodologies to the Computed-Tomography Dose Index for the evaluation of Cone-Beam Computed Tomography dose are compared, the Cone-Beam Dose Index, IAEA Human Health Report No. 5 recommended methodology and the AAPM Task Group 111 recommended methodology. The protocols were evaluated for Pelvis and Thorax scan modes on Varian® On-Board Imager and Truebeam kV XI imaging systems. The weighted planar average dose was highest for the AAPM methodology across all scans, with the CBDI being the second highest overall. A 17.96% and 1.14% decrease from the TG-111 protocol to the IAEA and CBDI protocols for the Pelvis mode and 18.15% and 13.10% decrease for the Thorax mode were observed for the XI system. For the OBI system, the variation was 16.46% and 7.14% for Pelvis mode and 15.93% to the CBDI protocol in Thorax mode respectively.
Highlights
The introduction of kV CBCT for position verification and adaptive therapy has enabled steeper dose gradients and tighter treatment margins for improved treatment outcomes [1]
A 17.96% and 1.14% decrease from the TG-111 protocol to the IAEA and Cone Beam Dose Index (CBDI) protocols for the Pelvis mode and 18.15% and 13.10% decrease for the Thorax mode were observed for the XI system
Since the introduction of CTDI, computed tomography (CT) scanning has developed to include helical CT, multi-slice CT and wide beam Cone-Beam CT where images can be acquired in a single rotation
Summary
The introduction of kV CBCT for position verification and adaptive therapy has enabled steeper dose gradients and tighter treatment margins for improved treatment outcomes [1]. Regular CBCT imaging during a course of radiotherapy delivers additional concomitant dose which may lead to increased risk of secondary malignancies [2,3,4]. Since the introduction of CTDI, CT scanning has developed to include helical CT, multi-slice CT and wide beam Cone-Beam CT where images can be acquired in a single rotation. The relevance of the CTDI as a dose indicator for wide beam scanning has come under question due to underestimation of scatter dose lying outside the 100mm chamber length, CTDI phantoms being of insufficient length to achieve scatter equilibrium and non-uniformity along the beam [8,9,10]
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