Comparison of localization performance with implanted fiducial markers and cone-beam computed tomography for on-line image-guided radiotherapy of the prostate

Abstract

Purpose: The aim of this work was to assess the accuracy of kilovoltage (kV) cone-beam computed tomography (CBCT)-based setup corrections as compared with orthogonal megavoltage (MV) portal image-based corrections for patients undergoing external-beam radiotherapy of the prostate.

Methods and materials: Daily cone-beam CT volumetric images were acquired after setup for patients with three intraprostatic fiducial markers. The estimated couch shifts were compared retrospectively to patient adjustments based on two orthogonal MV portal images (the current clinical standard of care in our institution). The CBCT soft-tissue based shifts were also estimated by digitally removing the gold markers in each projection to suppress the artifacts in the reconstructed volumes. A total of 256 volumetric images for 15 patients were analyzed.

Results: The Pearson coefficient of correlation for the patient position shifts using fiducial markers in MV vs. kV was (R2 = 0.95, 0.84, 0.81) in the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions, respectively. The correlation using soft-tissue matching was as follows: R2 = 0.90, 0.49, 0.51 in the LR, AP and SI directions. A Bland-Altman analysis showed no significant trends in the data. The percentage of shifts within a +/-3-mm tolerance (the clinical action level) was 99.7%, 95.5%, 91.3% for fiducial marker matching and 99.5%, 70.3%, 78.4% for soft-tissue matching.

Conclusions: Cone-beam CT is an accurate and precise tool for image guidance. It provides an equivalent means of patient setup correction for prostate patients with implanted gold fiducial markers. Use of the additional information provided by the visualization of soft-tissue structures is an active area of research.

Figure 1

(a) Photograph of the medical linear accelerator imaging platform with portal imaging system marked in yellow and volumetric imaging system in cyan. Currently, implanted gold fiducial markers (b) are used to guide daily set-up correction. The same markers appear distinctly using the kV volumetric imaging system (c) as well as bone and soft-tissue structures.

Figure 1

(a) Photograph of the medical linear accelerator imaging platform with portal imaging system marked in yellow and volumetric imaging system in cyan. Currently, implanted gold fiducial markers (b) are used to guide daily set-up correction. The same markers appear distinctly using the kV volumetric imaging system (c) as well as bone and soft-tissue structures.

Figure 1

(a) Photograph of the medical linear accelerator imaging platform with portal imaging system marked in yellow and volumetric imaging system in cyan. Currently, implanted gold fiducial markers (b) are used to guide daily set-up correction. The same markers appear distinctly using the kV volumetric imaging system (c) as well as bone and soft-tissue structures.

Figure 2

(a) Volumetric cone-beam CT data set captured after patient set-up. The volumetric data set here is 40×40×25.6 cm³ with 1 mm³ voxels and a total imaging dose of 1.4–2.8 cGy. A single coronal slice of the acquisition volume with fiducial markers in FOV is shown in (b). To support soft-tissue matching of the target volume the markers are digitally suppressed in the projections and reconstructed to yield (c).

Figure 2

(a) Volumetric cone-beam CT data set captured after patient set-up. The volumetric data set here is 40×40×25.6 cm³ with 1 mm³ voxels and a total imaging dose of 1.4–2.8 cGy. A single coronal slice of the acquisition volume with fiducial markers in FOV is shown in (b). To support soft-tissue matching of the target volume the markers are digitally suppressed in the projections and reconstructed to yield (c).

Figure 2

(a) Volumetric cone-beam CT data set captured after patient set-up. The volumetric data set here is 40×40×25.6 cm³ with 1 mm³ voxels and a total imaging dose of 1.4–2.8 cGy. A single coronal slice of the acquisition volume with fiducial markers in FOV is shown in (b). To support soft-tissue matching of the target volume the markers are digitally suppressed in the projections and reconstructed to yield (c).

Figure 3

Daily set-up corrections for a typical patient applied in the (a) left/right, (b) anterior/posterior and (c) superior/inferior directions. The (x)'s represents the couch shifts estimated using two orthogonal portal images. The (o)'s represent the shifts based on 3D localization of the center-of-mass of the markers using on-line cone-beam CT while the (diamonds) represent the couch shift estimated by matching the CTV contours to the planning CT.

Figure 3

Daily set-up corrections for a typical patient applied in the (a) left/right, (b) anterior/posterior and (c) superior/inferior directions. The (x)'s represents the couch shifts estimated using two orthogonal portal images. The (o)'s represent the shifts based on 3D localization of the center-of-mass of the markers using on-line cone-beam CT while the (diamonds) represent the couch shift estimated by matching the CTV contours to the planning CT.

Figure 3

Daily set-up corrections for a typical patient applied in the (a) left/right, (b) anterior/posterior and (c) superior/inferior directions. The (x)'s represents the couch shifts estimated using two orthogonal portal images. The (o)'s represent the shifts based on 3D localization of the center-of-mass of the markers using on-line cone-beam CT while the (diamonds) represent the couch shift estimated by matching the CTV contours to the planning CT.

Figure 4

Two-dimensional correlations of the applied MV shifts vs. the predicted kV cone-beam CT shifts (a-c), the soft-tissue based shifts (d-e) for all 15 patients. The cone-beam only shifts (fiducial markers vs. soft-tissue) are shown in (g-i). The shift directions were in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions.

Figure 4

Two-dimensional correlations of the applied MV shifts vs. the predicted kV cone-beam CT shifts (a-c), the soft-tissue based shifts (d-e) for all 15 patients. The cone-beam only shifts (fiducial markers vs. soft-tissue) are shown in (g-i). The shift directions were in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions.

Figure 4

Two-dimensional correlations of the applied MV shifts vs. the predicted kV cone-beam CT shifts (a-c), the soft-tissue based shifts (d-e) for all 15 patients. The cone-beam only shifts (fiducial markers vs. soft-tissue) are shown in (g-i). The shift directions were in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions.

Figure 4

Two-dimensional correlations of the applied MV shifts vs. the predicted kV cone-beam CT shifts (a-c), the soft-tissue based shifts (d-e) for all 15 patients. The cone-beam only shifts (fiducial markers vs. soft-tissue) are shown in (g-i). The shift directions were in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions.

Figure 4

Two-dimensional correlations of the applied MV shifts vs. the predicted kV cone-beam CT shifts (a-c), the soft-tissue based shifts (d-e) for all 15 patients. The cone-beam only shifts (fiducial markers vs. soft-tissue) are shown in (g-i). The shift directions were in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions.

Figure 4

Two-dimensional correlations of the applied MV shifts vs. the predicted kV cone-beam CT shifts (a-c), the soft-tissue based shifts (d-e) for all 15 patients. The cone-beam only shifts (fiducial markers vs. soft-tissue) are shown in (g-i). The shift directions were in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions.

Figure 4

Two-dimensional correlations of the applied MV shifts vs. the predicted kV cone-beam CT shifts (a-c), the soft-tissue based shifts (d-e) for all 15 patients. The cone-beam only shifts (fiducial markers vs. soft-tissue) are shown in (g-i). The shift directions were in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions.

Figure 4

Two-dimensional correlations of the applied MV shifts vs. the predicted kV cone-beam CT shifts (a-c), the soft-tissue based shifts (d-e) for all 15 patients. The cone-beam only shifts (fiducial markers vs. soft-tissue) are shown in (g-i). The shift directions were in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions.

Figure 4

Two-dimensional correlations of the applied MV shifts vs. the predicted kV cone-beam CT shifts (a-c), the soft-tissue based shifts (d-e) for all 15 patients. The cone-beam only shifts (fiducial markers vs. soft-tissue) are shown in (g-i). The shift directions were in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions.

Figure 5

Bland-Altman error analysis for MV portal image based shifts vs. CBCT fiducial marker based shifts (a-c) and soft-tissue matching (d-f) as well as shifts based on cone-beam soft-tissue vs. cone-beam fiducial markers (g-i) for all 15 patients and all fractions in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions. The current clinical action levels of +/−3mm are drawn for reference (dot-dash line). The (x)'s represent the difference between the predicted shifts based on the CofM of the three fiducial markers measured in kV and MV plotted as a function of the average couch shift. The analysis is repeated for the CTV soft-tissue matching (d, e, f) as denoted by the (o)'s. The final analysis (diamonds) compares the CTV shift to the kV shift (g, h, i).

Figure 5

Bland-Altman error analysis for MV portal image based shifts vs. CBCT fiducial marker based shifts (a-c) and soft-tissue matching (d-f) as well as shifts based on cone-beam soft-tissue vs. cone-beam fiducial markers (g-i) for all 15 patients and all fractions in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions. The current clinical action levels of +/−3mm are drawn for reference (dot-dash line). The (x)'s represent the difference between the predicted shifts based on the CofM of the three fiducial markers measured in kV and MV plotted as a function of the average couch shift. The analysis is repeated for the CTV soft-tissue matching (d, e, f) as denoted by the (o)'s. The final analysis (diamonds) compares the CTV shift to the kV shift (g, h, i).

Figure 5

Bland-Altman error analysis for MV portal image based shifts vs. CBCT fiducial marker based shifts (a-c) and soft-tissue matching (d-f) as well as shifts based on cone-beam soft-tissue vs. cone-beam fiducial markers (g-i) for all 15 patients and all fractions in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions. The current clinical action levels of +/−3mm are drawn for reference (dot-dash line). The (x)'s represent the difference between the predicted shifts based on the CofM of the three fiducial markers measured in kV and MV plotted as a function of the average couch shift. The analysis is repeated for the CTV soft-tissue matching (d, e, f) as denoted by the (o)'s. The final analysis (diamonds) compares the CTV shift to the kV shift (g, h, i).

Figure 5

Bland-Altman error analysis for MV portal image based shifts vs. CBCT fiducial marker based shifts (a-c) and soft-tissue matching (d-f) as well as shifts based on cone-beam soft-tissue vs. cone-beam fiducial markers (g-i) for all 15 patients and all fractions in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions. The current clinical action levels of +/−3mm are drawn for reference (dot-dash line). The (x)'s represent the difference between the predicted shifts based on the CofM of the three fiducial markers measured in kV and MV plotted as a function of the average couch shift. The analysis is repeated for the CTV soft-tissue matching (d, e, f) as denoted by the (o)'s. The final analysis (diamonds) compares the CTV shift to the kV shift (g, h, i).

Figure 5

Bland-Altman error analysis for MV portal image based shifts vs. CBCT fiducial marker based shifts (a-c) and soft-tissue matching (d-f) as well as shifts based on cone-beam soft-tissue vs. cone-beam fiducial markers (g-i) for all 15 patients and all fractions in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions. The current clinical action levels of +/−3mm are drawn for reference (dot-dash line). The (x)'s represent the difference between the predicted shifts based on the CofM of the three fiducial markers measured in kV and MV plotted as a function of the average couch shift. The analysis is repeated for the CTV soft-tissue matching (d, e, f) as denoted by the (o)'s. The final analysis (diamonds) compares the CTV shift to the kV shift (g, h, i).

Figure 5

Bland-Altman error analysis for MV portal image based shifts vs. CBCT fiducial marker based shifts (a-c) and soft-tissue matching (d-f) as well as shifts based on cone-beam soft-tissue vs. cone-beam fiducial markers (g-i) for all 15 patients and all fractions in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions. The current clinical action levels of +/−3mm are drawn for reference (dot-dash line). The (x)'s represent the difference between the predicted shifts based on the CofM of the three fiducial markers measured in kV and MV plotted as a function of the average couch shift. The analysis is repeated for the CTV soft-tissue matching (d, e, f) as denoted by the (o)'s. The final analysis (diamonds) compares the CTV shift to the kV shift (g, h, i).

Figure 5

Bland-Altman error analysis for MV portal image based shifts vs. CBCT fiducial marker based shifts (a-c) and soft-tissue matching (d-f) as well as shifts based on cone-beam soft-tissue vs. cone-beam fiducial markers (g-i) for all 15 patients and all fractions in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions. The current clinical action levels of +/−3mm are drawn for reference (dot-dash line). The (x)'s represent the difference between the predicted shifts based on the CofM of the three fiducial markers measured in kV and MV plotted as a function of the average couch shift. The analysis is repeated for the CTV soft-tissue matching (d, e, f) as denoted by the (o)'s. The final analysis (diamonds) compares the CTV shift to the kV shift (g, h, i).

Figure 5

Bland-Altman error analysis for MV portal image based shifts vs. CBCT fiducial marker based shifts (a-c) and soft-tissue matching (d-f) as well as shifts based on cone-beam soft-tissue vs. cone-beam fiducial markers (g-i) for all 15 patients and all fractions in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions. The current clinical action levels of +/−3mm are drawn for reference (dot-dash line). The (x)'s represent the difference between the predicted shifts based on the CofM of the three fiducial markers measured in kV and MV plotted as a function of the average couch shift. The analysis is repeated for the CTV soft-tissue matching (d, e, f) as denoted by the (o)'s. The final analysis (diamonds) compares the CTV shift to the kV shift (g, h, i).

Figure 5

Bland-Altman error analysis for MV portal image based shifts vs. CBCT fiducial marker based shifts (a-c) and soft-tissue matching (d-f) as well as shifts based on cone-beam soft-tissue vs. cone-beam fiducial markers (g-i) for all 15 patients and all fractions in the (a, d, g) left/right, (b, e, h) anterior/posterior and (c, f, i) superior/inferior directions. The current clinical action levels of +/−3mm are drawn for reference (dot-dash line). The (x)'s represent the difference between the predicted shifts based on the CofM of the three fiducial markers measured in kV and MV plotted as a function of the average couch shift. The analysis is repeated for the CTV soft-tissue matching (d, e, f) as denoted by the (o)'s. The final analysis (diamonds) compares the CTV shift to the kV shift (g, h, i).

Figure 6

Plot of differences between applied MV shift and predicted shift based on kV fiducial markers. The 3D differences are shown in (a). The shaded ellipsoid represents the 95% confidence interval. A histogram of the differences for each cardinal direction is shown for (b) left/right (c) anterior/posterior and (d) superior/inferior.

Figure 6

Plot of differences between applied MV shift and predicted shift based on kV fiducial markers. The 3D differences are shown in (a). The shaded ellipsoid represents the 95% confidence interval. A histogram of the differences for each cardinal direction is shown for (b) left/right (c) anterior/posterior and (d) superior/inferior.

Figure 7

Plot of differences between applied MV shift and predicted shift based on CTV. The 3D differences are shown in (a). The shaded ellipsoid represents the 95% confidence interval. The ellipsoid appears rotated about the x-axis. This indicates the shift differences in y, and z are not independent. A histogram of the differences for each cardinal direction is shown for (b) left/right (c) anterior/posterior and (d) superior/inferior.

Figure 7

Plot of differences between applied MV shift and predicted shift based on CTV. The 3D differences are shown in (a). The shaded ellipsoid represents the 95% confidence interval. The ellipsoid appears rotated about the x-axis. This indicates the shift differences in y, and z are not independent. A histogram of the differences for each cardinal direction is shown for (b) left/right (c) anterior/posterior and (d) superior/inferior.

Figure 8

Plot of differences between the predicted shift based on kV markers and that based on CTV contours. The 3D differences are shown in (a). The shaded ellipsoid represents the 95% confidence interval. The ellipsoid appears rotated about the x-axis. Again this indicates the shift differences in y, and z are not independent. A histogram of the differences for each cardinal direction is shown for (b) left/right (c) anterior/posterior and (d) superior/inferior.

Figure 8

Plot of differences between the predicted shift based on kV markers and that based on CTV contours. The 3D differences are shown in (a). The shaded ellipsoid represents the 95% confidence interval. The ellipsoid appears rotated about the x-axis. Again this indicates the shift differences in y, and z are not independent. A histogram of the differences for each cardinal direction is shown for (b) left/right (c) anterior/posterior and (d) superior/inferior.