Patient positioning and its influence on dose distribution

In comparison to conventional radiotherapy, the positioning of a patient for BNCT treatment has some unique aspects. In particular, the neutron beam coming from the core of a nuclear research reactor is fixed and horizontal. In particular, a head fixation mask, which is prepared at the patient referral hospital, is included in the CT images. The mask allows reproducible positioning for use during the treatment. Fiducial markers placed on the mask before imaging, provide reference points. The INEEL treatment planning code used in Petten produces a beam angle, and beam line entrance and exit co-ordinates. These are related to the fiducial marker co-ordinates. A spreadsheet, named COSINE, developed at Petten produces positioning co-ordinates from the data produced by the rtt MC code. These co-ordinates are related to a positioning frame, which allows the marking of the beams on the mask. In order to have reliable treatment data, the influence of small deviations of angle or target point on dose distribution must be known. To demonstrate this, a number of beams have been calculated with the Petten beam, with slight variations and compared with an approved plan. (author)