This presentation is concerned primarily with certain irradiation technics evolved over the past eleven years of clinical radiotherapeutic experience with the 2-million-volt Van de Graaff electrostatic x-ray generator. Field shaping may be considered to have two objectives: that of conforming the shape of the beam of radiation to the known tumor volume and its possible extension and that of protecting adjacent normal areas by exclusion from the beam. In the absence of motion between patient and beam, the field may be given any desired shape depending upon the clinical problem and the philosophy of the radiotherapist. Accessory protective devices in such stationary-field therapy are usually modifications in the outline or dose distribution of the original field. Applied to 360° rotation of the patient, a fixed field is usually considered as necessarily symmetrical about the axis of rotation. The use of an accessory protective device in this situation requires in general that it also be shaped and rotate. This more intricate situation is illustrated by the set-up used in the rotational treatment of carcinoma of the thyroid. Here a basic rectangular field is shaped by appropriate absorbers so that the treatment beam includes the entire thyroid bed and lymphatics from the angle of the jaw down to the anterior mediastinum at the level of the carina. The field is shaped to reduce the dose to uninvolved regions such as teeth, cerebellum, and lungs. This shaping, however, does not protect the spinal cord. The accessory spinal cord protector, a lead rod 5∕8 inch in diameter, is shaped to conform to the cervical spinal curvature. In the film the spinal cord protector is shown in the beam and rotating synchronously with the patient so that its x-ray shadow falls exactly on the cord for all angular positions. Mechanically this is accomplished with the aid of a small selsyn motor mounted on the face-plate of the x-ray generator. The motor is synchronized with the rotating treatment chair. The patient and cord protector require approximately three to four revolutions for a daily tumor dose of 175 r; the spinal cord dose is reduced to approximately 50 per cent of the tumor dose. Black and white film is used to show the shadow cast by the continuously rotating cord protector within the fixed, shaped thyroid field. The second part of the movie illustrates the concept of variable field shaping during rotation. The example is from a case of carcinoma of the urachus with extension to the urinary bladder. The cancer, lying on the parietal peritoneal surface of the anterior abdominal wall, extended as a narrow band from the umbilicus to the bladder dome. Here it spreads to infiltrate the anterior and superior surfaces of the bladder. The usual multiportal or rotational technics applied to give a tumor dose of 6,000 r would result in an undesirably and unnecessarily high dosage to the small intestine.