  In the present paper a general setup for determination of imperfect geometry
of radiotherapeutic devices has been proposed that base on geometric algebra
framework. To account for this imperfect geometry, two methods of a calibration
were presented, consisting of determining for each angular position of a gantry
a correction shift which must be applied to the origin of a laboratory frame of
reference to place it along a radiation axis for this angular position. Closed
form solutions for these corrections are provided.
