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

Abstract

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.

Figure 1

Intraobserver reproducibility for test-retest of interfractional shifts for daily image guided alignments. Offline alignments to the planning computed tomography were made twice each (test and retest) by 2 radiation oncologists (observers 1 and 2) for each set of daily images for kV orthogonal imaging with fiducial markers and soft-tissue cone beam computed tomography. The test and retest shift (in millimeters) for each image in each axis were plotted against one another so that correlations could be calculated (Table 3).

Figure 2

Errors associated with image guided alignments may lead to decreased planning target volume (PTV) coverage or increased toxicity. Dose-volume histograms (DVHs) were generated from five different intensity modulated radiation therapy plans for each of the 10 patients in the study. The first DVH was from the original treatment plan and the next 3 DVHs were derived by applying the fluence of the original plan to new isocenters with posterior shifts of 0.5, 1, and 2 standard deviations of the intraobserver variability for shifts made with soft-tissue cone beam computed tomography–based image guidance (ie, 1.1, 2.2, and 4.4 mm from the isocenter used for treatment). To generate the fifth DVH, a new intensity modulated radiation therapy plan was generated for the PTV with expanded margins determined by the variability in soft-tissue cone beam computed tomography–based image guidance, as described in the Results section (ie, the standard 5 mm in all dimensions with an additional 5 mm anterior-posterior and left-right and 10 mm superior-inferior). Each of these simulations was run for the 10 patients included in the study, and DVH data were extracted for each contoured structure for each patient. Pooled patient DVH data were plotted for the (A) PTV including circumferential 5 mm margins (n = 10); (B) rectum (n = 10); (C) bladder (n = 10); (D) small bowel (n = 3); and (E) large bowel (n = 2). Two-way analysis of variance was used to test statistically significant differences in the percent structure volume receiving a given radiation therapy dose for plans 2 to 5 relative to plan 1. PTV with 5 mm margins at isocenter was used for treatment.