Radiation protection considerations for endocavitary X ray units.

Application of a contact X ray therapy unit for endocavitary irradiation of selected rectal carcinomas is gaining acceptance as a treatment of choice. The patient is placed on a proctoscopic table in the surgical suite and the anus is anesthetized and dilated to allow insertion of a 3 cm diameter proctoscope designed to admit the X ray tube. The measured half-value layer for 50 kVp X rays filtered by 1.0 mm of aluminum was determined to be 0.65 mm of aluminium. The dose rate at the end of the 4.0 cm source-skin-distance (SSD) cone was measured to be on the order of 1000 rad/minute. Radiation surveys performed for several treatment geometries indicate that exposure rate levels in the environs immediately around the patient can be quite high. Radiation measurements were made for the following locations: where the radiotherapist stands holding the X ray tube; at the side of the patient where the technologist or any supporting personnel might stand; and at the patient's posterior surface in the direction where the anesthesiologist might stand. Radiation levels ranged from 0.001 to 1.0 roentgen (R) per hour and depend primarily on the depth and angle of the X ray tube inserted into the rectum.