Feasibility of CycleGAN enhanced low dose CBCT imaging for prostate radiotherapy dose calculation

Daily cone beam computed tomography (CBCT) imaging during the course of fractionated radiotherapy treatment can enable online adaptive radiotherapy but also expose patients to a non-negligible amount of radiation dose. This work investigates the feasibility of low dose CBCT imaging capable of enabling accurate prostate radiotherapy dose calculation with only 25% projections by overcoming under-sampling artifacts and correcting CT numbers by employing cycle-consistent generative adversarial networks (cycleGAN). Uncorrected CBCTs of 41 prostate cancer patients, acquired with ∼350 projections (CBCT_org), were retrospectively under-sampled to 25% dose images (CBCT_LD) with only ∼90 projections and reconstructed using Feldkamp-Davis-Kress. We adapted a cycleGAN including shape loss to translate CBCT_LDinto planning CT (pCT) equivalent images (CBCT_{LD_GAN}). An alternative cycleGAN with a generator residual connection was implemented to improve anatomical fidelity (CBCT_{LD_ResGAN}). Unpaired 4-fold cross-validation (33 patients) was performed to allow using the median of 4 models as output. Deformable image registration was used to generate virtual CTs (vCT) for Hounsfield units (HU) accuracy evaluation on 8 additional test patients. Volumetric modulated arc therapy plans were optimized on vCT, and recalculated on CBCT_{LD_GAN}and CBCT_{LD_ResGAN}to determine dose calculation accuracy. CBCT_{LD_GAN}, CBCT_{LD_ResGAN}and CBCT_orgwere registered to pCT and residual shifts were analyzed. Bladder and rectum were manually contoured on CBCT_{LD_GAN}, CBCT_{LD_ResGAN}and CBCT_organd compared in terms of Dice similarity coefficient (DSC), average and 95th percentile Hausdorff distance (HD_avg, HD₉₅). The mean absolute error decreased from 126 HU for CBCT_LDto 55 HU for CBCT_{LD_GAN}and 44 HU for CBCT_{LD_ResGAN}. For PTV, the median differences ofD_98%,D_50%andD_2%comparing both CBCT_{LD_GAN}to vCT were 0.3%, 0.3%, 0.3%, and comparing CBCT_{LD_ResGAN}to vCT were 0.4%, 0.3% and 0.4%. Dose accuracy was high with both 2% dose difference pass rates of 99% (10% dose threshold). Compared to the CBCT_org-to-pCT registration, the majority of mean absolute differences of rigid transformation parameters were less than 0.20 mm/0.20°. For bladder and rectum, the DSC were 0.88 and 0.77 for CBCT_{LD_GAN}and 0.92 and 0.87 for CBCT_{LD_ResGAN}compared to CBCT_org, and HD_avgwere 1.34 mm and 1.93 mm for CBCT_{LD_GAN}, and 0.90 mm and 1.05 mm for CBCT_{LD_ResGAN}. The computational time was ∼2 s per patient. This study investigated the feasibility of adapting two cycleGAN models to simultaneously remove under-sampling artifacts and correct image intensities of 25% dose CBCT images. High accuracy on dose calculation, HU and patient alignment were achieved. CBCT_{LD_ResGAN}achieved better anatomical fidelity.