Program Information
A Critical Look at CBCT-Based Dose Calculation Accuracy as It Is Applied to Adaptive Radiotherapy
A Bejarano Buele*, N Sperling , E Parsai , University of Toledo Medical Center, Toledo, OH
Presentations
SU-D-206-5 (Sunday, July 31, 2016) 2:05 PM - 3:00 PM Room: 206
Purpose:
Cone-beam CTs (CBCT) obtained from On-Board Imaging Devices (OBI) are increasingly being used for dose calculation purposes in adaptive radiotherapy. Patient and target morphology are monitored and the treatment plan is updated using CBCT. Due to the difference in image acquisition parameters, dose calculated in a CBCT can differ from planned dose. We evaluate the difference between dose calculation in kV CBCT and simulation CT, and the effect of HU-density tables in dose discrepancies
Methods:
HU values for various materials were obtained using a Catphan 504 phantom for a simulator CT (CTSIM) and two different OBI systems using three imaging protocols: Head, Thorax and Pelvis. HU-density tables were created in the TPS for each OBI image protocol. Treatment plans were made on each Catphan 504 dataset and on the head, thorax and pelvis sections of an anthropomorphic phantom, with and without the respective HU-density table. DVH information was compared among OBI systems and planning CT.
Results:
Dose calculations carried on the Catphan 504 CBCTs, with and without the respective CT-density table, had a maximum difference of -0.65% from the values on the planning CT. The use of the respective HU-density table decreased the percent differences from planned values by half in most of the protocols. For the anthropomorphic phantom datasets, the use of the correct HU-density table reduced differences by 0.89% on OBI1 and 0.59% on OBI2 for the head, 0.49% on OBI1 for the thorax, and 0.25% on OBI2 for the pelvis. Differences from planned values without HU-density correction ranged from 3.13% (OBI1, thorax) to 0.30% (OBI2, thorax).
Conclusion:
CT-density tables in the TPS yield acceptable differences when used in partly homogeneous medium. Further corrections are needed when the medium contains pronounced density differences for accurate CBCT calculation. Current difference range (1-3%) can be clinically acceptable.
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