1. A method for extending initial image data of a subject for dose estimation, comprising: obtaining first X-ray image data of the subject for dose calculation, wherein the first X-ray image data has a first field of view; obtaining second X-ray image data for extending the first field of view of the first X-ray image data, wherein the second X-ray image data has a second field of view that is larger than the first field of view; registering the second X-ray image data to the first X-ray image data employing a registration algorithm that includes an adjustable regularization term, which determines a strength of the registration and adjusting the regularization term as a function of distance from the first field if view; extracting a sub-portion of the second X-ray image data that is outside the first field of view; and extending the first field of view based on the second image data by combining the first X-ray image data and the extracted sub-portion, producing extended X-ray image data.