This example shows how far the prostate may move. The shift is understood with respect to reference conditions (position of prostate during Simulation or CT).

The field is the 90°-field of a four-field conformal prostate boost plan. In this session and for this field, the marker shift is more than 1 cm in both PA and caudo-cranial direction! All previous and later sessions showed much smaller marker shifts. Even within the same session, the following 270° field showed only 5 mm marker shift. This means that the port image contains both inter-fraction movement and intra-fraction movement of the prostate.

To demonstrate the effect, the images are bone matched. During Simulation, the rectum was marked with contrast agent, which fortunately is no standard procedure anymore (very often, the cloud of contrast agent was obscuring a precious gold marker!). Before treatment, the patient was positioned according to lasers, then port images were taken. After online bone matching, the small deltas were corrected and the field was treated.

This patient belonged to the first group of patients with gold markers, for whom the standard procedure of "bone matching" was still followed.

If one assumes that the prostate is where the gold markers are, one can conclude that the target was partly missed during this treatment. After 10 patients, the online matching and correction policy was switched from "bone matching" to "marker matching".