A test object for evaluation of portal films.

A test object has been designed for evaluation of the image quality of portal films in high energy photon radiation therapy. It consists of a pattern of notched polyvinyl chloride cylinders, fastened to a plastic sheet and immersed in water during exposure. In a pilot experiment, films produced with the test object were evaluated by a panel of observers. The results indicate that the use of the test object simulates the clinical application of portal films well. It is concluded that the test object can simplify studies of the efficacy of various methods to produce and view portal films.