Computerized method for measurement of displacement vectors of target positions on EPID cine images in stereotactic radiotherapy

The purpose of this study was to develop a computerized method for measurement of displacement vectors of target position on electronic portal imaging device (EPID) cine images in a treatment without implanted markers. Our proposed method was based on a template matching technique with cross-correlation coefficient between a reference portal (RP) image and each consecutive portal (CP) image acquired by the EPID. EPID images with 512×384 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 10MV on linear accelerators. The displacement vector of the target on each cine image was determined from the position in which took the maximum cross-correlation value between the RP image and each 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, displacement vectors of a target position calculated by our method were compared with those determined manually by two radiation oncologists. As a result, for lung phantom images, target displacements by our method correlated well with those by the oncologists (r=0.972 - 0.994). Correlation values for 4 cases ranged from 0.854 to 0.991, but the values for the other two cases were 0.609 and 0.644. This preliminary result suggested that our method may be useful for monitoring of displacement vectors of target positions without implanted markers in stereotactic radiotherapy.