Program Information
Beam Quality Variations Within a CTDI Phantom for the Energy Range 100 to 140 KVp
P Papaconstadopoulos1*, N Tomic1 , H Bekerat1 , P Watson2 , E Mainegra-Hing3 , F DeBlois1 , J Seuntjens2 , S Devic1 , (1) McGill University, Jewish General Hospital, Montreal, QC, Canada (2) McGill University, Cedars Cancer Centre, Montreal, QC, Canada (3) National Research Council of Canada, Ottawa, ON, Canada
Presentations
MO-RAM-GePD-I-2 (Monday, July 31, 2017) 9:30 AM - 10:00 AM Room: Imaging ePoster Lounge
Purpose: to investigate beam quality variations within a CTDI phantom and report effective energy changes for low energy x-ray dosimetry.
Methods: a cylindrical CTDI phantom (PMMA) and treatment couch were modeled using the EGSnrc/cavity code. Monte Carlo simulations of a collimated rotating source were performed with initial beam spectra in the range of 100-140 kVp and half value layers (HVLs) ranging from 3 to 9 mm. The initial spectra were calculated using the SpekCalc code. In-phantom spectra were collected within a circular cavity region (radius=1 cm) in three periphery and one central points at the same locations as a CTDI chamber would normally be placed. The effective energies of the collected spectra were calculated and compared to the respective calculations performed in air (without phantom and couch) using the same initial spectra.
Results: the effective energies in the CTDI phantom are in general higher for low beam qualities (100 kVp, 3-5 mm), similar for intermediate energies (120 kVp, 5-7 mm) and lower for high beam qualities (140 kVp, 7-9 mm) compared to the calculations in air. The gradually increasing beam softening effect with input effective energy can be attributed to variations in scattering conditions under a broad beam geometry. The effective energy in the center location appears to be the least sensitive to spectra variations as it is dictated by phantom scattering symmetric from all directions. The impact of the couch is found small but non-negligible, especially for the bottom periphery point.
Conclusion: interesting beam quality variations were observed within a CTDI phantom in the clinical CT energy range, which may lead to either beam hardening or softening depending on the initial spectrum and the in-phantom location. The center location of the phantom appears to be the most suitable for stable low energy x-ray dosimetry.
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