Isocentric rotational performance of the Elekta Precise Table studied using a USB-microscope. 
The isocentric three-dimensional performance of the Elekta Precise Table was investigated.  A pointer was attached to the radiation head of the accelerator and positioned at the geometric rotational axis of the head.  A USB-microscope was mounted on the treatment tabletop to measure the table position relative to the pointer tip.  The table performance was mapped in terms of USB-microscope images of the pointer tip at different table angles and load configurations.  The USB-microscope was used as a detector to measure the pointer tip positions with a resolution down to 0.01 mm.  A new elastic model of the treatment table was developed.  This model describes the performance of the treatment table quite well except from some deviations due to backlash effects.  Geometric and elastic features are described through six parameters.  These parameters are calculated using the linear least squares fitting technique.  A new method to ensure optimal positioning of the table relative to the accelerator is presented.  This method cannot eliminate systematic errors completely.  To eliminate systematic errors we suggest that geometric and elastic models of the table and accelerator gantry arm are incorporated in the dose planning system.