ESTRO 36 Abstract Book

S79 ESTRO 36 2017 _______________________________________________________________________________________________

on gantries for proton therapy that have a different geometry and did not use a bow-tie filter. The performance of an a priori scatter correction algorithm was in this study compared for the first time on CBCT systems for photon vs. proton therapy gantries. Material and Methods The a priori scatter correction algorithm used a plan CT (pCT) and raw CB projections. The projections were acquired with On-Board Imagers of a Varian photon therapy Clinac and of a Varian proton therapy ProBeam system. The projections were initially corrected for beam hardening followed by reconstruction using the RTK back projection Feldkamp-Davis-Kress algorithm (rawCBCT). Manual, rigid and deformable registrations were applied using Plastimatch on the pCT to the rawCBCT. The resulting images were forward projected onto the same angles as the raw CB projections. The two projections sets were then subtracted from each other, Gaussian and median filtered, and then subtracted from the raw projections and finally reconstructed to a scatter corrected CBCT. To evaluate the algorithm, water equivalent path length (WEPL) maps were calculated from anterior to posterior on different reconstructions of the data sets (CB projections and pCT). Initially we evaluated CB projections of an Alderson phantom acquired on the Clinac system before comparing CB projections of the same CatPhan phantom acquired on both the Clinac and the ProBeam systems. Results In the analysis of the Clinac projections of the Alderson phantom, the scatter correction resulted in sub-mm mean WEPL difference from the rigid registration of the pCT, considerably smaller than what was achieved with the regular Varian CBCT reconstruction algorithm (Figure 1). The largest improvement was for the half-fan (below neck) scans. With the Catphan phantom the rawCBCT was very similar to the Varian reconstruction, due to a refitting of beam hardening curve. When comparing reconstructions of photon to proton gantry CB projections (Figure 2) we found that the a priori scatter correction improved the mean WEPL difference while preserving image quality (the number of countable line pairs) for both gantry types. The photon gantry showed less WEPL difference, however used a higher pulse current per acquisition ( 2.00 mAs), compared to the proton gantry (1.4 mAs). The complete scatter correction is performed within three minutes on a desktop with NVidia graphics.

Conclusion We have shown that an a priori scatter correction algorithm for CB projections improves CBCT image quality on both photon- and proton therapy gantries, potentially opening for CBCT-based image/dose-guided proton therapy. OC-0159 Dual energy CBCT increases soft tissue CNR ratio and image quality compared to standard CBCT in IGRT M. Skaarup 1 , D. Kovacs 1 , M.C. Aznar 1 , J.P. Bangsgaard 1 , J.S. Rydhög 1 , L.A. Rechner 1 1 The Finsen Center - Rigshospitalet, Clinic of Oncology, Copenhagen, Denmark Purpose or Objective We investigate a method for enhancing soft tissue contrast to noise ratio (CNR) and clinical image quality of cone- beam computed tomography (CBCT) by using a dual energy CBCT protocol. Material and Methods Nine patients were scanned using a standard CBCT protocol of either 100 or 125 kVp and a DE-CBCT protocol of two separate scans of 80 and 140 kVp respectively. Other scan parameters were identical and total radiation dose was kept at a similar level for both protocols. Virtual monochromatic dual energy (VMDE) images were reconstructed using a linear mix of the 80 and 140 kVp scan. The weight, with which the two images were combined, was calculated based on known attenuation coefficients of two basis materials at a specific monochromatic energy. A linear combination of these can be used to express the attenuation coefficients of the 80 and 140 kVp scan at that same monochromatic energy. To find the optimal virtual reconstruction energy for soft tissue imaging, multiple reconstructions were done for energies in the range 40- 180 keV. CNR measurements were performed on both standard CBCT and VMDE images for a number of different tissue combinations, e.g. contrast between tumour-fat, tumour- surrounding tissue, muscle-fat, rectum-surrounding tissue, parotid-fat, seminal vesicle-surrounding tissue and lung-heart (see figure 1 for an example). In addition, 5 experienced observers conducted a blinded ranking between VMDE images (reconstructed at 55, 65, 75 and 100 keV) and the standard CBCT images, i.e. five image series per patient. For each combination of image series the observers were asked to compare the images side-by- side, focusing on soft tissue image quality as well as