ABSTRACT
When a patient is treated, it is just as important that the radiation should be delivered to
the correct anatomical site as it is that the correct dose should be delivered. With the advent of
conformal radiotherapy, the requirements for positional accuracy are increased. Traditionally,
quality assurance of patient positioning has been achieved using industrial x-ray film, but film
needs to be processed, and the information is only available some time after the treatment
fraction has been delivered. In order to provide an instantaneous verification of the patient
setup, electronic portal imaging devices (EPIDs) have been developed that are attached to the
gantry of the linear accelerator.