Beam-Shapers Oriented by Gravity in Rotational Therapy

For the general case of irregularly shaped tumors, the requirements of uniform dose to tumor, minimum dose to surrounding tissue, and protection of radiosensitive organs are better fulfilled in rotational therapy, if the beam shape changes continuously during the rotation. The value of such technics also depends on the number of installations that can use them and on the simplicity of their application. The following technics are applicable to rotating cobalt-60 units and rotating accelerators. In addition, they are simple and inexpensive, because the beam-shapers are oriented by gravity. Shaping of Beam Sides In the rotational treatment of any irregular and deep tumor, the integral dose to healthy tissue becomes minimum if the external rays of the beam are continuously tangent to the tumor surface. This can be achieved by two identical radiation absorbers suspended in the beam. The two shafts of suspension are parallel to the axis of rotation and symmetrical to the central ray of the beam. During the rotation each shaper remains parallel to itself. A weight attached to each shaper's end sets its permanent orientation without entering into the beam. Diagram 1 shows the application of the above technic in the frequent case of elongated tumor. The geometric arrangement is shown in a vertical cross section for seven positions of the rotating source. During the rotation the beam width varies to just cover the tumor for all positions. Figure 1 is a multiple exposure photograph of the actual accessory in several positions of rotation. The position of the shapers is adjustable. They are cylindrical2 because the tumor is supposed to be so. In the general case of irregular tumors the shapers will be noncylindrical, and correction filters will have to be added if the average beam width varies significantly from one cross section to another. Diagram 2 gives the resulting distribution of dose in the case shown in Diagram 1, for which the source to axis of rotation distance is 75 cm, the cobalt-source diameter equals 2 cm, the maximum width of the field is 11 cm and the axes of the elliptical cross section of each cylindrical lead shaper are 10 and 5.5 cm. Protection of Sensitive Organs The reduction of dose to one or more radiosensitive organs located inside or in the vicinity of the tumor is often desirable. The protection of each organ is assigned to an individual protector. All protectors are attached to a common holder. The holder rotates freely about a shaft which is parallel to the axis of rotation and meets the central ray of the beam. Each protector is similar to the corresponding organ in size and shape. The geometric arrangement of the protectors around the shaft of suspension is similar to the arrangement of the organs around the axis of rotation. The similarity ratio equals the ratio of source-shaft distance to source-axis distance, which is the magnification ratio of the projection of protectors to the corresponding organs.