2D kV orthogonal imaging with fiducial markers is more precise for daily image guided alignments than soft-tissue cone beam computed tomography for prostate radiation therapy

Purpose The hypothesis is that 2-dimensional kV orthogonal imaging with fiducial markers (kV-FM) and soft-tissue cone beam computed tomography (ST-CBCT) are equally reproducible for daily positional alignments for image guided (IG) intensity modulated radiation therapy (IMRT) for prostate cancer.Methods and materials Ten patients undergoing definitive treatment for prostate cancer with IG-IMRT were imaged daily with kV-FM and ST-CBCT. For each acquired kV and CBCT image, offline alignments to the digitally reconstructed radiograph or planning CT, respectively, were made twice by the same physician to assess intraobserver test-retest reproducibility. The 256 kV and 142 CBCT images were analyzed, and the test-retest analysis was performed again on a subset of images by another physician to verify the results.Results The results demonstrated that kV-FM had better intraobserver test-retest reproducibility in the anterior-posterior (AP; 95% confidence interval [CI] Pearson correlation coefficient [r], 0.987-0.991), left-right (LR; 95% CI r, 0.955-0.969), and superior-inferior (SI; 95% CI r, 0.971-0.980) directions for daily IG alignments compared with ST-CBCT (AP: 95% CI r, 0.804-0.877; LR: 95% CI r, 0.877-0.924; SI: 95% CI r, 0.791-0.869). Errors associated with intraobserver test-retest reproducibility were submillimeter with kV-FM (AP: 0.4 ± 0.7 mm; RL: 0.4 ± 1.0 mm; SI: 0.5 ± 0.7 mm) compared with ST-CBCT (AP: 2.1 ± 2.2 mm; LR: 1.3 ± 1.4 mm; SI: 1.2 ± 1.8 mm). The mean shift differences between kV-FM and ST-CBCT were 0.3 ± 3.8 mm for AP, -1.1 ± 8.5 mm for LR, and -2.0 ± 3.7 mm for SI. Dose-volume histograms were generated and showed that test-retest variability associated with ST-CBCT IG-alignments resulted in significantly increased dose to normal structures and a reduced planning target volume dose in many patients.Conclusions The kV-FM-based daily IG alignment for IMRT of prostate cancer is more precise than ST-CBCT, as assessed by a physician's ability to reproducibly align images. Given the magnitude of the error introduced by inconsistency in making ST-CBCT alignments, these data support a role for daily kV imaging of FM to enhance the precision of external beam dose delivery to the prostate.