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Feasibility Study of Using a Dual-Energy Cone Beam CT (DECBCT) in Proton Therapy Treatment Planning

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H Peng

H Peng1*, T Kanehira2 , S Takao2 , T Matsuura2 , K Umegaki3 , H Shirato2 , L Xing1 , (1) Stanford University, School of Medicine, Stanford, CA, (2) Department of Radiation Oncology, Graduate School of Medicine, Sapporo, Hokkaido, (3) Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido.


SU-F-J-213 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The aim of this study is to evaluate the feasibility of using a dual-energy CBCT (DECBCT) in proton therapy treatment planning to allow for accurate electron density estimation.

Methods: For direct comparison, two scenarios were selected: a dual-energy fan-beam CT (high: 140 kVp, low: 80 kVp) and a DECBCT (high: 125 kVp, low: 80 kVp). A Gammex 467 tissue characterization phantom was used, including the rods of air, water, bone (B2-30% mineral), cortical bone (SB3), lung (LN-300), brain, liver and adipose. For the CBCT, Hounsfield Unit (HU) numbers were first obtained from the reconstructed images after a calibration was made based on water (=0) and air materials (=-1000). For each tissue surrogate, region-of-interest (ROI) analyses were made to derive high-energy and low-energy HU values (HUhigh and HUlow), which were subsequently used to estimate electron density based on the algorithm as previously described by Hunemohr N., et al. Parameters k1 and k2 are energy dependent and can be derived from calibration materials.

Results:While for the dual-energy FBCT, the electron density is found be within +/-3% error relative to the values provided by the phantom vendor: -1.8% (water), 0.03% (lung), 1.1% (brain), -2.82% (adipose), -0.49% (liver) and -1.89% (cortical bones). While for the DECBCT, the estimation of electron density exhibits a relatively larger variation: -1.76% (water), -36.7% (lung), -1.92% (brain), -3.43% (adipose), 8.1% (liver) and 9.5% (cortical bones).

Conclusion: For DECBCT, the accuracy of electron density estimation is inferior to that of a FBCT, especially for materials of either low-density (lung) or high density (cortical bone) compared to water. Such limitation arises from inaccurate HU number derivation in a CBCT. Advanced scatter-correction and HU calibration routines, as well as the deployment of photon counting CT detectors need be investigated to minimize the difference between FBCT and CBCT.

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