Automated method for estimation of displacement of target position on epid cine images without implanted markers in stereotactic body radiotherapy

The purpose of this study was to develop a computerized method for measuring displacement vectors at a target position on electronic portal imaging device (EPID) cine images in treatment without implanted markers in stereotactic body radiotherapy. Our method was based on a template matching technique with cross-correlation coefficient between the reference portal (RP) image and each consecutive portal (CP) image acquired by EPID. EPID images with 512x384 pixels (pixel size: 0.56 mm) were acquired in a cine mode at a sampling rate of 0.5 frame/sec by using an energy of 4, 6, or 10 MV on two linear accelerators. The displacement vector of the target on each cine image was determined from the position at maximum cross-correlation coefficient between the RP image and CP image. We applied our method to EPID cine images of a lung phantom with a tumor model simulating respiratory motion, and 5 cases with a non-small cell lung cancer and one case of metastasis. For validation of our proposed method, displacements of a target position calculated by our method were compared with those obtained manually by two radiation oncologists. As a result, for lung phantom images, target displacements by our method relatively correlated well with those by the oncologists (r=0.972-0.994). Correlation values for 4 cases ranged from 0.854 to 0.948, while the values for the other 2 cases were 0.609 and 0.644. This preliminary result suggested that our method may be useful for monitoring displacements of target positions without implanted markers in stereotactic body radiotherapy.