A DOUBLE-HEADED FIXED BEAM TELETHERAPY MACHINE FOR LOW COST MEGAVOLTAGE THERAPY.

Abstract

Compared to kilovoltage radiation, megavoltage radiation has important advantages for external radiation therapy, but the much higher costs of the machines and treatment rooms have prevented more general use. Figure 1 shows an arrangement which greatly lowers the costs of megavoltage therapy. Two identical treatment rooms are served from one double-headed machine mounted in the wall between the two rooms. The teletherapy source, either cobalt 60 or cesium 137, is in the middle in the “off” position and can be moved into either head by a steel cable operated manually by a control wheel. Automatic return of the source to the “off” position at the end of the treatment time is provided by a spring. While one patient is treated in one room, the next patient is set up in the adjoining room, an arrangement similar to Failla's layout for the Memorial Hospital 4 g radium bomb. The radiation beam is always directed straight down. Its size and shape are adjusted with shielding blocks placed on a tray below each head. The obvious disadvantage of this arrangement is that the radiation beam cannot be angled or rotated. However, our large experience with movable megavoltage machines indicates that the majority of radiation therapy patients can be satisfactorily treated with “straight-down” beams. The lack of mobility of the head in our design is compensated for by the possibility of using field sizes up to 45 × 30 cm at 70 cm source distance. With such large fields, most tumors and lymph node areas can be treated in a single field. In our experience, this is most desirable for therapy machines, since it is difficult to join adjacent fields properly. In principle, movable teletherapy machines can also be equipped with large fields, but the difficulties of room protection and beam collimation have limited the field size in most commercially available teletherapy machines to much smaller dimensions. The low cost of the arrangement shown in Figure 1 is due to the following features: 1. The simplicity of the machine and of the controls. 2. The elimination of a special stand for the head because it is supported by the wall between the treatment rooms. 3. The use of shielding blocks on a tray for collimation and beam-shaping. 4. The ease of room protection since, with the machine installed on the lowest floor, only 20 in. of concrete for the walls and 15 in. of concrete for the ceiling are required for satisfactory protection with a cobalt-60 source up to 2,000 c. 5. The possibility of using low-curie teletherapy sources since with a double-headed machine, no treatment time is lost while a patient is set up, and thus approximately twice as many can be treated as with standard single-headed teletherapy machines of the same source activity. The suggested arrangement appears especially suitable for hospitals which already have one rotational megavoltage machine and desire to treat more patients with megavoltage radiation.