The use of intensity modulated radiation therapy promises to spare organs at risk by applying better
dose distribution on the tumor. The specific challenge of this methods is the exact positioning of
the patient and the localization of the exposured organ. With respect to the filling of rectum and
bladder the prostate can move several millimeters up to centimeters. Therefore, the position of
the prostate should be determinated and corrected daily before irradiation. We used a B-mode
US machine (Ultramark 9, advanced Technology Laboratories, USA) which was calibrated using
an optical tracking system (Polaris, NDI, Can). After correct positioning of the patient in the
simulation room three anatomical markers (apex prostate, prostate lateral sinister/dexter) were
identified and their positions calculated with respect to the coordinate system of the simulator. The
same situation is given in the treatment room. Both, simulator and accelerator are registered by
a simple point-to-point registration using a block with five drilled holes with known coordinates in
the block coordinate system. The block is aligned by means of laser markers. When the patient
is placed on the treatment table, the three anatomical landmarks are located on the US images
and their positions are calculated with respect to the coordinate system of the treatment room.
Applying a point-to-point registration results in a rotation matrix and a translation vector in the
desired coordinate system which can be used for repositioning by translating and rotating the patient
table. Additionally, a fiducial registration error (FRE) is calculated which gives a dimension of the
accuracy the three points were identified. We found an fiducial registration error (FRE) of 2.4 mm
+/- 1.2 mm for the point-to-point registration of the anatomical landmarks. The FRE for the point-to-point registration between the block and the optical tracking system was 0.5 mm +/- 0.2 mm.
According to the US calibration we found an error of 0.8 mm +/- 0.2 mm.