SU-E-J-26: Automated Estimation Method of Patient Setup Errors Using Simulated Portal Images for Prostate Cancer...

We developed a novel automated estimation method for patient setup errors based on simulated and real portal images for prostate cancer radiotherapy.

The estimation of patient setup errors in this study was based on a template matching technique with a cross-correlation coefficient and Sobel filter between the real portal image and localized pelvic template of reference image, which were DRR (digitally reconstructed radiography) images and simulated portal images. The simulated portal image was derived by projecting a CT image according to an inverse exponential power law of x-ray attenuation for a water-equivalent path length of each voxel of the CT image on each ray from a source to each pixel on the EPID (electric portal imaging device). A localized pelvic template of each patient in AP (anterior-posterior) or lateral view was automatically extracted from the DRR or simulated portal images by cropping a rectangular region, which was determined by using the mean pelvic template and four anatomical feature points. We applied the proposed method to three prostate cancer cases, and evaluated it using the residual error between the patient setup error obtained by proposed method and the gold standard setup error determined by two radiation oncologists.

The average residual errors of the patient setup error for the DRR and simulated portal images were 0.79 and 1.26 mm in the left-right (LR) direction, 3.17 and 2.05 mm in the superior-inferior (SI) direction, 1.69 and 5.82 mm in the anterior-posterior (AP) direction, 3.84 and 6.94 mm in Euclidean distance (ED), respectively. If we used the simulated portal image for LR and SI directions and the DRR image for AP direction, the Euclidean distance was 3.22 mm.

The proposed method has a potential to correctly estimate patient setup errors for prostate cancer radiotherapy.