Computerized estimation of patient setup errors in portal images based on localized pelvic templates for prostate cancer radiotherapy

Abstract

We have developed a computerized method for estimating patient setup errors in portal images based on localized pelvic templates for prostate cancer radiotherapy. The patient setup errors were estimated based on a template-matching technique that compared the portal image and a localized pelvic template image with a clinical target volume produced from a digitally reconstructed radiography (DRR) image of each patient. We evaluated the proposed method by calculating the residual error between the patient setup error obtained by the proposed method and the gold standard setup error determined by consensus between two radiation oncologists. Eleven training cases with prostate cancer were used for development of the proposed method, and then we applied the method to 10 test cases as a validation test. As a result, the residual errors in the anterior-posterior, superior-inferior and left-right directions were smaller than 2 mm for the validation test. The mean residual error was 2.65 ± 1.21 mm in the Euclidean distance for training cases, and 3.10 ± 1.49 mm for the validation test. There was no statistically significant difference in the residual error between the test for training cases and the validation test (P = 0.438). The proposed method appears to be robust for detecting patient setup error in the treatment of prostate cancer radiotherapy.

Fig. 1.

An illustration of the overall scheme of our proposed method for estimating patient setup error in a portal image based on a localized pelvic template of an individual patient undergoing prostate cancer radiotherapy.

Fig. 2.

An illustration of the reconstruction of a digitally reconstructed radiography (DRR) image from a planning CT image based on a ray casting method [13], where sampling points are obtained on a ray.

Fig. 3.

(a) A mean DRR image in the anterior-posterior view, and (b) a mean pelvic template image.

Fig. 4.

(a) A mean DRR image in the lateral view, and (b) a mean pelvic template image.

Fig. 5.

(a) An original DRR image of a patient, (b) the corresponding Sobel-filtered image (green) with four feature regions (pink) detected by the anatomical feature templates and (c) a localized pelvic template of the same patient.

Fig. 6.

An example of localized pelvic templates for an AP and lateral views with reductions of 100% to 40% relative size.

Fig. 7.

Relationship between the relative size for the localized pelvic template image and the residual error in Euclidean distance obtained by the proposed method.

Fig. A1.

Illustrations of (a) a portal image with a measuring scale, (b) an initial irradiation field and (c) a final irradiation field.

Fig. A2.

Standard measuring scale point template image used for determination of the center of the irradiation field.

Fig. A3.

Illustrations of the reduction of scale points in a portal image by inpainting of the scale points with a mean filter: (a) an original portal image, in which a measuring scale was superimposed, (b) a measuring scale binary template and (c) the portal image with reduced scale points.