Using routine portal images to detect internal organ motion in prostate cancer patients; a simulation

Geometrical uncertainties in external radiotherapy consist for a large part of set-up errors and internal organ motion. Set-up deviations can be derived from portal images acquired during the treatment. The gray shades in those images reflect the irradiated radiological thicknesses and bony structures are therefore highly visible. Determining the position of the bony anatomy relative to the reference situation yields the set-up deviation. In routine portal images of pelvic fields, the most visible objects besides bony structures are gas pockets in the rectum. These gas pockets might indicate the position of the ventral rectum wall, which is expected to have a good correlation with the prostate position because of their close proximity. In Fig. 1 the idea behind this correlation is visualized. Kroonwijk et al already pointed out that gas pockets in portal images can reveal internal organ motion.[1] They did however not specify how this could be used in practice. In this abstract, a method for using the imaged gas pockets for quantitative determination of the rectum wall and prostate position is proposed. Compared to other methods which make use of e.g. radio-opaque markers or multiple CT data, this method is non-invasive and little extra work is required; portal images are already routinely made for set-up verification in our institute.