In a paper appearing in Radiology for May 1960, on “Synchronous Field Shaping in Rotational Megavolt Therapy,” two geometric technics for improving dose distribution in the patient were suggested. It is our object, in this second contribution, to examine the same subject more quantitatively and to discuss the influence of some geometric factors on the distribution of ionization. All the isodose curves shown below were determined in the following manner: A horizontal x-ray beam, produced by a 2-Mev Van de Graaff generator, was directed toward a Masonite phantom standing on a therapy table and rotating about a vertical axis 125 cm. from the source. The distribution of ionization was recorded on x-ray film sandwiched in transverse cross sections of the phantom. Synchronous Protection With the synchronous protection technic, the irradiation of a sensitive organ can be reduced while some or all of the surrounding tissue is treated. The application of this procedure to protection of the spinal cord is illustrated in Figure 1. The patient rotates around a vertical axis which meets the neck cross section at C. The absorber, P, protects the cord, A, against the radiation emitted by source S. Suppose that P and A are rotating with the same angular velocity about O and C, respectively, and that PO was parallel to AC initially. Then the cord, A, will remain under the x-ray shadow of the protector, P, for any value of the angle ϑ of rotation. This technic was applied to a Masonite cylinder 20 cm. in diameter, with the field wide enough to cover the whole width of the cylinder at all times. A small selsyn-driver was driven mechanically by the rotating table and was electrically connected to a similar selsyn-follower, which in turn rotated the protector, P. In this way, the protector P rotated in synchronism with the phantom. The protectors and distances employed are indicated in Figure 2. The value of the protection in each case is shown by the depth of the upside-down bell formed if the corresponding curve of Figure 2 is rotated about the vertical axis. For the curve C of maximum protection, a non-moving vertical lead bar, of 1.2 × 6 cm. cross section, was placed at the vertical central plane of the beam, with the 6 cm. dimension parallel to the beam. Thus, a cylinder of protection, 2 cm. in diameter, was formed around the axis of rotation of the Masonite cylinder. Since the transmission of direct radiation through 6 cm. of lead is negligible, all the dosage in the protected region is due to scattered radiation. Usually, in practice, the width of a rotational field is much smaller than 20 cm. This reduces the scattered radiation, with corresponding improvement of the protection. The curves A, B, and C correspond to the unfavorable combination of wide field and small diameter protector