Room: Exhibit Hall | Forum 1
Purpose: The position of CBCT scans used for image-guided radiation therapy (IGRT) depends on the patient treatment. The aim of this study is to investigate the influence of the position on effective dose (E).
Methods: A validated Monte Carlo model of an on-board-imager (OBI) kV-system was used to assess E resulting from four scan protocols (Head, thorax, abdomen, and pelvis) employed in the clinic for IGRT. E for each protocol was estimated as an average of values estimated for the ICRP reference adults male and female phantoms. Each phantom was divided into four regions, each of which was scanned by the corresponding protocol. Initially, the scan isocenter was set at the center of each region, and E assessed at this position was considered as a reference value. Subsequently, the isocentre was shifted by Â±5 cm over the z-axis in equal increments, with x and y positions being kept at the center (0,0,z).
Results: Variations by factors of 0.75-1.20 were found for the head, largely due to contributions from thyroid dose when the isocentre was shifted towards the thorax region. Since lung and breast spread over a large area of the chest, their values give lower variations in E, with a factor of 0.80-1.05, for thorax scans. For abdomen scans, E declined from a factor of 1.22 to 0.87 when the isocenter was moved down of the body due to the reduction in colon and stomach doses. Changes in the contribution from the urinary bladder at the upper part of the pelvis region and the gonads at bottom compensated for one another giving variations by factors of only 0.75-1.02, for pelvis scans.
Conclusion: Position of the scan isocenter should be taken into consideration when assessment of patient dose resulting from some applications that depend on acquiring scans at different isocenter positions.