The present invention provides a significantly improved diagnostic tool and procedure for radiation therapy in or near body cavities accessible through existing orifices. This invention makes use of a flexible catheter and balloon. The balloon is inflated by fluid communication through the catheter. This invention makes use of several preferable embodiments which provide radiopaque references, seals, control of movement, and drainage. The radiopaque references are provided by means of radiopaque lines painted on the balloon's interior or exterior surface, by means of the use of radiopaque fluid to inflate the balloon, or by means of a slidable clip made of radiopaque material which is positioned at the patient's orifice. Sealing is provided by means of the slidable clip which also permits adjustment of the balloon size and shape. Control of movement is achieved through the use of a small balloon usually located near the patient's orifice. Drainage, often required when performing this procedure in the bladder, is achieved by the use of a Foley-type catheter.