Program Information
The Influence of Cone Beam (CBCT) Scan Isocenter On Doses of Organs at Risk (OAR): A Monte Carlo Study
A Abuhaimed1*, C Martin2 , M Sankaralingam3 , (1) King Abdulaziz City for Science and Technology, Riyadh, Saudi Arabia, (2) University of Glasgow, Glasgow, UK, (3) Beatson West of Scotland Cancer Centre, Glasgow, UK.
Presentations
SU-H4-GePD-J(A)-4 (Sunday, July 30, 2017) 4:30 PM - 5:00 PM Room: Joint Imaging-Therapy ePoster Lounge - A
Purpose: Isocenters of cone-beam (CBCT) scans utilized during image-guided-radiation-therapy (IGRT) depend on the target position. They vary for each patient, although the same size of the field is used. This study investigates the influence of scan isocenters on organs-at-risk (OAR) doses.
Methods: A Monte-Carlo model of kV on-board-imager (OBI) system was developed using EGSnrc/BEAMnrc. Organ doses resulting from four scan protocols (head and neck, thorax, abdomen, and pelvis) were assessed using EGSnrc/DOSXYZnrc for the ICRP–110 adult male and female reference phantoms. Changes in OAR doses were studied for longitudinal shifts of the isocenters by 2.4 cm.
Results: For head scans, as the center of the scan was moved down the body, the brain dose declined from 4-1 mGy, while the dose to the salivary glands, increased from 0.3-5 mGy. Dose to the eye lens was about 0.95 mGy and did not change appreciably with position.For thorax scans, doses to the lung, heart, and breast were in the ranges 6–9, 9–12, and 6–11 mGy, respectively, over most positions, but all fell substantially when the isocenter of the scan was moved above mid of the lungs. However, when the scan isocenter was moved upwards, the thyroid dose increased to 24 mGy. In the abdominal and pelvic regions, the dose to each organ passes through a maximum as the field is moved down, but remains relatively constant (±10%) over distances of 51 cm. The highest doses in (mGy) for males (M) and females (F) for some OAR as the field is moved down are stomach (M–33,F–41), kidneys (M–28,F–35), Liver (M–28,F–37), colon (M–25,F–33), bladder (M–51,F–55), prostate and uterus both 44.
Conclusion: The isocenter of CBCT scans used in IGRT affects OAR doses. Organs may lie partly within the primary x-ray field, and small differences in isocenter sometimes have a major influence on doses received.
Contact Email: