Shadow tray

   Book for Physicists  »  Quality Assurance  »  Quality Control and Quality Assurance for Teletherapy Equipment  »  Checks on standard linear accelerators  »  Shadow tray

 
Individual shielding trays are used with pre-mounted lead blocks. Such systems are based on all the lead trays fitting accurately into the shadow tray holder. If there is a transferability between machines it is necessary that trays should fit in all machines. For checking this a reference lead tray should be kept. The tray should have a cross which should coincide with the centre of the beam (not necessarily be the centre of the tray). The correct alignment of the tray consists of comparing the shadow of this cross with the crosswires. It is also important that blocks should be correctly aligned at the horizontal position of the gantry and they should not move relative to the centre of the beam when the collimator is rotated. To check this a straight sided lead block is necessary to be mounted on a second reference tray so that one corner of the block is exactly coincides with the axis of the beam. At the horizontal gantry by rotating the collimator the position of the block corner compared to the centre of the beam. Occasionally films should be taken at each orientation of the collimator to verify the relationship between the lead tray and the radiation centre of the field.