Room: AAPM ePoster Library
Purpose: One of the reported drawbacks in Computed Tomography (CT) based adaptive radiotherapy is the possible inaccuracy of dose calculation directly on Cone Beam CTs (CBCTs), thus deformable image registration with its intrinsic uncertainties has been used. This study investigates if CBCT quality increased to a point where it can be used for direct dose calculation.
Methods: Experiments were conducted using iterative CBCTs on a Halcyon™ 3.0 linear accelerator (Varian Medical Systems) acquired with the default Pelvis protocol (125kVp). Planning CTs (simCT) were recorded on a Siemens Somatom® Edge Plus (Siemens Healthcare) with the PelvisRT protocol (120kVp). Plans were created in the treatment planning system Eclipse™ (Varian Medical Systems) applying standard IMRT/VMAT clinical protocols. The HU to density calibration curves were created using the electron density phantom Model 062MA (CIRS).
Following phantoms were used to investigate various treatment sites: CIRS Model 801-P for prostate, CT Torso Phantom CTU-42 (Kyoto Kagaku) for larynx and brain, and the female Alderson radiation therapy phantom (RSD) for breast and lung.
Next, the simCT was exchanged with the CBCT and the plans were recalculated. The dose distributions were further analysed in Verisoft® (PTW) using a local 3D gamma criterion of 2%/2mm and a passing rate of =95%. Additionally, doses on a CBCT reconstruction with improved scatter correction and HU assignment were compared to the previous results.
Results: All cases fulfilled the passing criteria using both reconstructions. The range using the commercial Halcyon CBCT reconstruction is 95.0%-100.0%. The prototype algorithm increased the results in the breast case, which showed the lowest passing rates leading to a narrower range of 96.7%-100.0%.
Conclusion: Iterative CBCTs showed good agreement in the dose distributions when compared to simCTs. The improved scatter correction and HU assignment in the reconstruction lead to an improvement of 2.4 percentage points in the breast case.
Funding Support, Disclosures, and Conflict of Interest: Both authors are full-time employees of Varian Medical Systems